- 05 Nov, 2022 3 commits
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Peter Zijlstra authored
GCC fancies inlining memcpy(), and because it cannot prove the destination is page-aligned (it is) it ends up generating atrocious code like: 19e: 48 8b 15 00 00 00 00 mov 0x0(%rip),%rdx # 1a5 <relocate_restore_code+0x25> 1a1: R_X86_64_PC32 core_restore_code-0x4 1a5: 48 8d 78 08 lea 0x8(%rax),%rdi 1a9: 48 89 c1 mov %rax,%rcx 1ac: 48 c7 c6 00 00 00 00 mov $0x0,%rsi 1af: R_X86_64_32S core_restore_code 1b3: 48 83 e7 f8 and $0xfffffffffffffff8,%rdi 1b7: 48 29 f9 sub %rdi,%rcx 1ba: 48 89 10 mov %rdx,(%rax) 1bd: 48 8b 15 00 00 00 00 mov 0x0(%rip),%rdx # 1c4 <relocate_restore_code+0x44> 1c0: R_X86_64_PC32 core_restore_code+0xff4 1c4: 48 29 ce sub %rcx,%rsi 1c7: 81 c1 00 10 00 00 add $0x1000,%ecx 1cd: 48 89 90 f8 0f 00 00 mov %rdx,0xff8(%rax) 1d4: c1 e9 03 shr $0x3,%ecx 1d7: f3 48 a5 rep movsq %ds:(%rsi),%es:(%rdi) Notably the alignment code generates a text reference to code_restore_code+0xff8, for which objtool raises the objection: vmlinux.o: warning: objtool: relocate_restore_code+0x3d: relocation to !ENDBR: next_arg+0x18 Applying some __assume_aligned(PAGE_SIZE) improve code-gen to: 19e: 48 89 c7 mov %rax,%rdi 1a1: 48 c7 c6 00 00 00 00 mov $0x0,%rsi 1a4: R_X86_64_32S core_restore_code 1a8: b9 00 02 00 00 mov $0x200,%ecx 1ad: f3 48 a5 rep movsq %ds:(%rsi),%es:(%rdi) And resolve the problem, however, none of this is important code and a much simpler solution still is to force a memcpy() call: 1a1: ba 00 10 00 00 mov $0x1000,%edx 1a6: 48 c7 c6 00 00 00 00 mov $0x0,%rsi 1a9: R_X86_64_32S core_restore_code 1ad: e8 00 00 00 00 call 1b2 <relocate_restore_code+0x32> 1ae: R_X86_64_PLT32 __memcpy-0x4 Signed-off-by:Peter Zijlstra (Intel) <peterz@infradead.org>
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Peter Zijlstra authored
Boris (and the robot) reported that objtool grew a new complaint about unreachable instructions. Upon inspection it was immediately clear the __weak zombie instructions struck again. For the unweary, the linker will simply remove the symbol for overriden __weak symbols but leave the instructions in place, creating unreachable instructions -- and objtool likes to report these. Commit 4adb2368 ("objtool: Ignore extra-symbol code") was supposed to have dealt with that, but the new commit 9f2899fe ("objtool: Add option to generate prefix symbols") subtly broke that logic by created unvisited symbols. Fixes: 9f2899fe ("objtool: Add option to generate prefix symbols") Reported-by:
Borislav Petkov <bp@alien8.de> Reported-by:
kernel test robot <lkp@intel.com> Signed-off-by:
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Peter Zijlstra authored
When moving a symbol in the symtab its index changes and any reloc referring that symtol-table-index will need to be rewritten too. In order to facilitate this, objtool simply marks the whole reloc section 'changed' which will cause the whole section to be re-generated. However, finding the relocs that use any given symbol is implemented rather crudely -- a fully iteration of all sections and their relocs. Given that some builds have over 20k sections (kallsyms etc..) iterating all that for *each* symbol moved takes a bit of time. Instead have each symbol keep a list of relocs that reference it. This *vastly* improves build times for certain configs. Reported-by:
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- 01 Nov, 2022 10 commits
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Peter Zijlstra authored
In order to avoid known hashes (from knowing the boot image), randomize the CFI hashes with a per-boot random seed. Suggested-by:
Kees Cook <keescook@chromium.org> Signed-off-by:
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Peter Zijlstra authored
Add the "cfi=" boot parameter to allow people to select a CFI scheme at boot time. Mostly useful for development / debugging. Requested-by:
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Peter Zijlstra authored
Implement an alternative CFI scheme that merges both the fine-grained nature of kCFI but also takes full advantage of the coarse grained hardware CFI as provided by IBT. To contrast: kCFI is a pure software CFI scheme and relies on being able to read text -- specifically the instruction *before* the target symbol, and does the hash validation *before* doing the call (otherwise control flow is compromised already). FineIBT is a software and hardware hybrid scheme; by ensuring every branch target starts with a hash validation it is possible to place the hash validation after the branch. This has several advantages: o the (hash) load is avoided; no memop; no RX requirement. o IBT WAIT-FOR-ENDBR state is a speculation stop; by placing the hash validation in the immediate instruction after the branch target there is a minimal speculation window and the whole is a viable defence against SpectreBHB. o Kees feels obliged to mention it is slightly more vulnerable when the attacker can write code. Obviously this patch relies on kCFI, but additionally it also relies on the padding from the call-depth-tracking patches. It uses this padding to place the hash-validation while the call-sites are re-written to modify the indirect target to be 16 bytes in front of the original target, thus hitting this new preamble. Notably, there is no hardware that needs call-depth-tracking (Skylake) and supports IBT (Tigerlake and onwards). Suggested-by:Joao Moreira (Intel) <joao@overdrivepizza.com> Signed-off-by:
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Peter Zijlstra authored
Add the location of all __cfi_##name symbols (as generated by kCFI) to a section such that we might re-write things at kernel boot. Notably; boot time re-hashing and FineIBT are the intended use of this. Signed-off-by:
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Peter Zijlstra authored
When code is compiled with: -fpatchable-function-entry=${PADDING_BYTES},${PADDING_BYTES} functions will have PADDING_BYTES of NOP in front of them. Unwinders and other things that symbolize code locations will typically attribute these bytes to the preceding function. Given that these bytes nominally belong to the following symbol this mis-attribution is confusing. Inspired by the fact that CFI_CLANG emits __cfi_##name symbols to claim these bytes, use objtool to emit __pfx_##name symbols to do the same when CFI_CLANG is not used. This then shows the callthunk for symbol 'name' as: __pfx_##name+0x6/0x10 Signed-off-by:Yujie Liu <yujie.liu@intel.com> Link: https://lkml.kernel.org/r/20221028194453.592512209@infradead.org
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Peter Zijlstra authored
When code is compiled with: -fpatchable-function-entry=${PADDING_BYTES},${PADDING_BYTES} functions will have PADDING_BYTES of NOP in front of them. Unwinders and other things that symbolize code locations will typically attribute these bytes to the preceding function. Given that these bytes nominally belong to the following symbol this mis-attribution is confusing. Inspired by the fact that CFI_CLANG emits __cfi_##name symbols to claim these bytes, allow objtool to emit __pfx_##name symbols to do the same. Therefore add the objtool --prefix=N argument, to conditionally place a __pfx_##name symbol at N bytes ahead of symbol 'name' when: all these preceding bytes are NOP and name-N is an instruction boundary. Signed-off-by: -
Peter Zijlstra authored
Due to how gelf_update_sym*() requires an Elf_Data pointer, and how libelf keeps Elf_Data in a linked list per section, elf_update_symbol() ends up having to iterate this list on each update to find the correct Elf_Data for the index'ed symbol. By allocating one Elf_Data per new symbol, the list grows per new symbol, giving an effective O(n^2) insertion time. This is obviously bloody terrible. Therefore over-allocate the Elf_Data when an extention is needed. Except it turns out libelf disregards Elf_Scn::sh_size in favour of the sum of Elf_Data::d_size. IOW it will happily write out all the unused space and fill it with: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND entries (aka zeros). Which obviously violates the STB_LOCAL placement rule, and is a general pain in the backside for not being the desired behaviour. Manually fix-up the Elf_Data size to avoid this problem before calling elf_update(). This significantly improves performance when adding a significant number of symbols. Signed-off-by:
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Peter Zijlstra authored
In order to facilitate creation of more symbol types, slice up elf_create_section_symbol() to extract a generic helper that deals with adding ELF symbols. Signed-off-by:
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Peter Zijlstra authored
This is a full revert of commit: f1389181 ("kallsyms: Take callthunks into account") The commit assumes a number of things that are not quite right. Notably it assumes every symbol has PADDING_BYTES in front of it that are not claimed by another symbol. This is not true; even when compiled with: -fpatchable-function-entry=${PADDING_BYTES},${PADDING_BYTES} Notably things like .cold subfunctions do not need to adhere to this change in ABI. It it also not true when build with CFI_CLANG, which claims these PADDING_BYTES in the __cfi_##name symbol. Once the prefix bytes are not consistent and or otherwise claimed the approach this patch takes goes out the window and kallsym resolution will report invalid symbol names. Therefore revert this to make room for another approach. Reported-by:
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Peter Zijlstra authored
Lukas reported someone fat fingered the CONFIG_ symbol; fix er up. Fixes: 5d821386 ("x86/retbleed: Add SKL return thunk") Reported-by:
Lukas Bulwahn <lukas.bulwahn@gmail.com> Signed-off-by:
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- 25 Oct, 2022 1 commit
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Dan Carpenter authored
The first argument of WARN() is a condition, so this will use "addr" as the format string and possibly crash. Fixes: 3b6c1747 ("x86/retpoline: Add SKL retthunk retpolines") Signed-off-by:
Dan Carpenter <dan.carpenter@oracle.com> Signed-off-by:
Dave Hansen <dave.hansen@linux.intel.com> Link: https://lore.kernel.org/all/Y1gBoUZrRK5N%2FlCB@kili/
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- 22 Oct, 2022 1 commit
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Ingo Molnar authored
There's a conflict between the call-depth tracking commits in x86/core: ee3e2469 ("x86/ftrace: Make it call depth tracking aware") 36b64f10 ("x86/ftrace: Rebalance RSB") eac828ea ("x86/ftrace: Remove ftrace_epilogue()") And these fixes in x86/urgent: 883bbbff ("ftrace,kcfi: Separate ftrace_stub() and ftrace_stub_graph()") b5f1fc31 ("x86/ftrace: Remove ftrace_epilogue()") It's non-trivial overlapping modifications - resolve them. Conflicts: arch/x86/kernel/ftrace_64.S Signed-off-by:
Ingo Molnar <mingo@kernel.org>
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- 21 Oct, 2022 1 commit
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Chang S. Bae authored
When an extended state component is not present in fpstate, but in init state, the function copies from init_fpstate via copy_feature(). But, dynamic states are not present in init_fpstate because of all-zeros init states. Then retrieving them from init_fpstate will explode like this: BUG: kernel NULL pointer dereference, address: 0000000000000000 ... RIP: 0010:memcpy_erms+0x6/0x10 ? __copy_xstate_to_uabi_buf+0x381/0x870 fpu_copy_guest_fpstate_to_uabi+0x28/0x80 kvm_arch_vcpu_ioctl+0x14c/0x1460 [kvm] ? __this_cpu_preempt_check+0x13/0x20 ? vmx_vcpu_put+0x2e/0x260 [kvm_intel] kvm_vcpu_ioctl+0xea/0x6b0 [kvm] ? kvm_vcpu_ioctl+0xea/0x6b0 [kvm] ? __fget_light+0xd4/0x130 __x64_sys_ioctl+0xe3/0x910 ? debug_smp_processor_id+0x17/0x20 ? fpregs_assert_state_consistent+0x27/0x50 do_syscall_64+0x3f/0x90 entry_SYSCALL_64_after_hwframe+0x63/0xcd Adjust the 'mask' to zero out the userspace buffer for the features that are not available both from fpstate and from init_fpstate. The dynamic features depend on the compacted XSAVE format. Ensure it is enabled before reading XCOMP_BV in init_fpstate. Fixes: 2308ee57 ("x86/fpu/amx: Enable the AMX feature in 64-bit mode") Reported-by:
Yuan Yao <yuan.yao@intel.com> Suggested-by:
Dave Hansen <dave.hansen@intel.com> Signed-off-by:
Chang S. Bae <chang.seok.bae@intel.com> Signed-off-by:
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- 20 Oct, 2022 4 commits
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Maxim Levitsky authored
clear_cpu_cap(&boot_cpu_data) is very similar to setup_clear_cpu_cap() except that the latter also sets a bit in 'cpu_caps_cleared' which later clears the same cap in secondary cpus, which is likely what is meant here. Fixes: 47125db2 ("perf/x86/intel/lbr: Support Architectural LBR") Signed-off-by:
Maxim Levitsky <mlevitsk@redhat.com> Signed-off-by:
Kan Liang <kan.liang@linux.intel.com> Link: https://lkml.kernel.org/r/20220718141123.136106-2-mlevitsk@redhat.com
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Peter Zijlstra authored
Different function signatures means they needs to be different functions; otherwise CFI gets upset. As triggered by the ftrace boot tests: [] CFI failure at ftrace_return_to_handler+0xac/0x16c (target: ftrace_stub+0x0/0x14; expected type: 0x0a5d5347) Fixes: 3c516f89 ("x86: Add support for CONFIG_CFI_CLANG") Signed-off-by:
Mark Rutland <mark.rutland@arm.com> Tested-by:
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Peter Zijlstra authored
Remove the weird jumps to RET and simply use RET. This then promotes ftrace_stub() to a real function; which becomes important for kcfi. Signed-off-by:
Thomas Gleixner <tglx@linutronix.de> Signed-off-by:
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Borislav Petkov authored
Fix ./include/trace/events/xen.h:28:31: warning: ‘enum paravirt_lazy_mode’ \ declared inside parameter list will not be visible outside of this definition or declaration which turns into a build error: ./include/trace/events/xen.h:28:50: error: parameter 1 (‘mode’) has incomplete type 28 | TP_PROTO(enum paravirt_lazy_mode mode), \ due to enum paravirt_lazy_mode being visible only under CONFIG_PARAVIRT. Just pull it up where it is unconditionally visible. Signed-off-by:Borislav Petkov <bp@suse.de> Signed-off-by:
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- 18 Oct, 2022 2 commits
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Babu Moger authored
AMD systems support zero CBM (capacity bit mask) for cache allocation. That is reflected in rdt_init_res_defs_amd() by: r->cache.arch_has_empty_bitmaps = true; However given the unified code in cbm_validate(), checking for: val == 0 && !arch_has_empty_bitmaps is not enough because of another check in cbm_validate(): if ((zero_bit - first_bit) < r->cache.min_cbm_bits) The default value of r->cache.min_cbm_bits = 1. Leading to: $ cd /sys/fs/resctrl $ mkdir foo $ cd foo $ echo L3:0=0 > schemata -bash: echo: write error: Invalid argument $ cat /sys/fs/resctrl/info/last_cmd_status Need at least 1 bits in the mask Initialize the min_cbm_bits to 0 for AMD. Also, remove the default setting of min_cbm_bits and initialize it separately. After the fix: $ cd /sys/fs/resctrl $ mkdir foo $ cd foo $ echo L3:0=0 > schemata $ cat /sys/fs/resctrl/info/last_cmd_status ok Fixes: 316e7f90 ("x86/resctrl: Add struct rdt_cache::arch_has_{sparse, empty}_bitmaps") Co-developed-by:Stephane Eranian <eranian@google.com> Signed-off-by:
Babu Moger <babu.moger@amd.com> Signed-off-by:
James Morse <james.morse@arm.com> Reviewed-by:
Reinette Chatre <reinette.chatre@intel.com> Reviewed-by:
Fenghua Yu <fenghua.yu@intel.com> Cc: <stable@vger.kernel.org> Link: https://lore.kernel.org/lkml/20220517001234.3137157-1-eranian@google.com
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Borislav Petkov authored
Currently, the patch application logic checks whether the revision needs to be applied on each logical CPU (SMT thread). Therefore, on SMT designs where the microcode engine is shared between the two threads, the application happens only on one of them as that is enough to update the shared microcode engine. However, there are microcode patches which do per-thread modification, see Link tag below. Therefore, drop the revision check and try applying on each thread. This is what the BIOS does too so this method is very much tested. Btw, change only the early paths. On the late loading paths, there's no point in doing per-thread modification because if is it some case like in the bugzilla below - removing a CPUID flag - the kernel cannot go and un-use features it has detected are there early. For that, one should use early loading anyway. [ bp: Fixes does not contain the oldest commit which did check for equality but that is good enough. ] Fixes: 8801b3fc ("x86/microcode/AMD: Rework container parsing") Reported-by:Ștefan Talpalaru <stefantalpalaru@yahoo.com> Signed-off-by:
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- 17 Oct, 2022 18 commits
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Zhang Rui authored
Today, core ID is assumed to be unique within each package. But an AlderLake-N platform adds a Module level between core and package, Linux excludes the unknown modules bits from the core ID, resulting in duplicate core ID's. To keep core ID unique within a package, Linux must include all APIC-ID bits for known or unknown levels above the core and below the package in the core ID. It is important to understand that core ID's have always come directly from the APIC-ID encoding, which comes from the BIOS. Thus there is no guarantee that they start at 0, or that they are contiguous. As such, naively using them for array indexes can be problematic. [ dhansen: un-known -> unknown ] Fixes: 7745f03e ("x86/topology: Add CPUID.1F multi-die/package support") Suggested-by:
Len Brown <len.brown@intel.com> Signed-off-by:
Zhang Rui <rui.zhang@intel.com> Signed-off-by:
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Zhang Rui authored
CPUID.1F/B does not enumerate Package level explicitly, instead, all the APIC-ID bits above the enumerated levels are assumed to be package ID bits. Current code gets package ID by shifting out all the APIC-ID bits that Linux supports, rather than shifting out all the APIC-ID bits that CPUID.1F enumerates. This introduces problems when CPUID.1F enumerates a level that Linux does not support. For example, on a single package AlderLake-N, there are 2 Ecore Modules with 4 atom cores in each module. Linux does not support the Module level and interprets the Module ID bits as package ID and erroneously reports a multi module system as a multi-package system. Fix this by using APIC-ID bits above all the CPUID.1F enumerated levels as package ID. [ dhansen: spelling fix ] Fixes: 7745f03e ("x86/topology: Add CPUID.1F multi-die/package support") Suggested-by:
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Zhang Rui authored
The coretemp driver supports up to a hard-coded limit of 128 cores. Today, the driver can not support a core with an ID above that limit. Yet, the encoding of core ID's is arbitrary (BIOS APIC-ID) and so they may be sparse and they may be large. Update the driver to map arbitrary core ID numbers into appropriate array indexes so that 128 cores can be supported, no matter the encoding of core ID's. Signed-off-by:
Guenter Roeck <linux@roeck-us.net> Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20221014090147.1836-3-rui.zhang@intel.com
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Nathan Chancellor authored
A recent change in LLVM made CONFIG_EFI_STUB unselectable because it no longer pretends to support -mabi=ms, breaking the dependency in Kconfig. Lack of CONFIG_EFI_STUB can prevent kernels from booting via EFI in certain circumstances. This check was added by 8f24f8c2 ("efi/libstub: Annotate firmware routines as __efiapi") to ensure that __attribute__((ms_abi)) was available, as -mabi=ms is not actually used in any cflags. According to the GCC documentation, this attribute has been supported since GCC 4.4.7. The kernel currently requires GCC 5.1 so this check is not necessary; even when that change landed in 5.6, the kernel required GCC 4.9 so it was unnecessary then as well. Clang supports __attribute__((ms_abi)) for all versions that are supported for building the kernel so no additional check is needed. Remove the 'depends on' line altogether to allow CONFIG_EFI_STUB to be selected when CONFIG_EFI is enabled, regardless of compiler. Fixes: 8f24f8c2 ("efi/libstub: Annotate firmware routines as __efiapi") Signed-off-by:
Nathan Chancellor <nathan@kernel.org> Signed-off-by:
Nick Desaulniers <ndesaulniers@google.com> Acked-by:
Ard Biesheuvel <ardb@kernel.org> Cc: stable@vger.kernel.org Link: https://github.com/llvm/llvm-project/commit/d1ad006a8f64bdc17f618deffa9e7c91d82c444d
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Peter Zijlstra (Intel) authored
Debug aid, allows running retbleed=force,stuff on non-affected uarchs Signed-off-by: -
Thomas Gleixner authored
The fully secure mitigation for RSB underflow on Intel SKL CPUs is IBRS, which inflicts up to 30% penalty for pathological syscall heavy work loads. Software based call depth tracking and RSB refill is not perfect, but reduces the attack surface massively. The penalty for the pathological case is about 8% which is still annoying but definitely more palatable than IBRS. Add a retbleed=stuff command line option to enable the call depth tracking and software refill of the RSB. This gives admins a choice. IBeeRS are safe and cause headaches, call depth tracking is considered to be s(t)ufficiently safe. Signed-off-by:
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Peter Zijlstra authored
Since ftrace has trampolines, don't use thunks for the __fentry__ site but instead require that every function called from there includes accounting. This very much includes all the direct-call functions. Additionally, ftrace uses ROP tricks in two places: - return_to_handler(), and - ftrace_regs_caller() when pt_regs->orig_ax is set by a direct-call. return_to_handler() already uses a retpoline to replace an indirect-jump to defeat IBT, since this is a jump-type retpoline, make sure there is no accounting done and ALTERNATIVE the RET into a ret. ftrace_regs_caller() does much the same and gets the same treatment. Signed-off-by:
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Peter Zijlstra authored
ftrace_regs_caller() uses a PUSH;RET pattern to tail-call into a direct-call function, this unbalances the RSB, fix that. Signed-off-by:
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Peter Zijlstra authored
Remove the weird jumps to RET and simply use RET. This then promotes ftrace_stub() to a real function; which becomes important for kcfi. Signed-off-by:
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Thomas Gleixner authored
Ensure that calls in BPF jitted programs are emitting call depth accounting when enabled to keep the call/return balanced. The return thunk jump is already injected due to the earlier retbleed mitigations. Signed-off-by:
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Peter Zijlstra authored
Callthunks addresses on the stack would confuse the ORC unwinder. Handle them correctly and tell ORC to proceed further down the stack. Signed-off-by:
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Peter Zijlstra authored
Since the pre-symbol function padding is an integral part of the symbol make kallsyms report it as part of the symbol by reporting it as sym-x instead of prev_sym+y. Signed-off-by:
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Peter Zijlstra authored
When indirect calls are switched to direct calls then it has to be ensured that the call target is not the function, but the call thunk when call depth tracking is enabled. But static calls are available before call thunks have been set up. Ensure a second run through the static call patching code after call thunks have been created. When call thunks are not enabled this has no side effects. Signed-off-by:
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Thomas Gleixner authored
Add a debuigfs mechanism to validate the accounting, e.g. vs. call/ret balance and to gather statistics about the stuffing to call ratio. Signed-off-by:
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Thomas Gleixner authored
Add the actual SKL call thunk for call depth accounting. Signed-off-by:
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Peter Zijlstra authored
Ensure that retpolines do the proper call accounting so that the return accounting works correctly. Specifically; retpolines are used to replace both 'jmp *%reg' and 'call *%reg', however these two cases do not have the same accounting requirements. Therefore split things up and provide two different retpoline arrays for SKL. The 'jmp *%reg' case needs no accounting, the __x86_indirect_jump_thunk_array[] covers this. The retpoline is changed to not use the return thunk; it's a simple call;ret construct. [ strictly speaking it should do: andq $(~0x1f), PER_CPU_VAR(__x86_call_depth) but we can argue this can be covered by the fuzz we already have in the accounting depth (12) vs the RSB depth (16) ] The 'call *%reg' case does need accounting, the __x86_indirect_call_thunk_array[] covers this. Again, this retpoline avoids the use of the return-thunk, in this case to avoid double accounting. Signed-off-by:
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Thomas Gleixner authored
To address the Intel SKL RSB underflow issue in software it's required to do call depth tracking. Provide a return thunk for call depth tracking on Intel SKL CPUs. The tracking does not use a counter. It uses uses arithmetic shift right on call entry and logical shift left on return. The depth tracking variable is initialized to 0x8000.... when the call depth is zero. The arithmetic shift right sign extends the MSB and saturates after the 12th call. The shift count is 5 so the tracking covers 12 nested calls. On return the variable is shifted left logically so it becomes zero again. CALL RET 0: 0x8000000000000000 0x0000000000000000 1: 0xfc00000000000000 0xf000000000000000 ... 11: 0xfffffffffffffff8 0xfffffffffffffc00 12: 0xffffffffffffffff 0xffffffffffffffe0 After a return buffer fill the depth is credited 12 calls before the next stuffing has to take place. There is a inaccuracy for situations like this: 10 calls 5 returns 3 calls 4 returns 3 calls .... The shift count might cause this to be off by one in either direction, but there is still a cushion vs. the RSB depth. The algorithm does not claim to be perfect, but it should obfuscate the problem enough to make exploitation extremly difficult. The theory behind this is: RSB is a stack with depth 16 which is filled on every call. On the return path speculation "pops" entries to speculate down the call chain. Once the speculative RSB is empty it switches to other predictors, e.g. the Branch History Buffer, which can be mistrained by user space and misguide the speculation path to a gadget. Call depth tracking is designed to break this speculation path by stuffing speculation trap calls into the RSB which are never getting a corresponding return executed. This stalls the prediction path until it gets resteered, The assumption is that stuffing at the 12th return is sufficient to break the speculation before it hits the underflow and the fallback to the other predictors. Testing confirms that it works. Johannes, one of the retbleed researchers. tried to attack this approach but failed. There is obviously no scientific proof that this will withstand future research progress, but all we can do right now is to speculate about it. The SAR/SHL usage was suggested by Andi Kleen. Signed-off-by: -
Peter Zijlstra authored
Fairly straight forward adaptation/extention of ALTERNATIVE_2. Required for call depth tracking. Signed-off-by:
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