arm64: avoid executing padding bytes during kexec / hibernation

Currently we rely on the HIBERNATE_TEXT section starting with the entry
point to swsusp_arch_suspend_exit, and the KEXEC_TEXT section starting
with the entry point to arm64_relocate_new_kernel. In both cases we copy
the entire section into a dynamically-allocated page, and then later
branch to the start of this page.

SYM_FUNC_START() will align the function entry points to
CONFIG_FUNCTION_ALIGNMENT, and when the linker later processes the
assembled code it will place padding bytes before the function entry
point if the location counter was not already sufficiently aligned. The
linker happens to use the value zero for these padding bytes.

This padding may end up being applied whenever CONFIG_FUNCTION_ALIGNMENT
is greater than 4, which can be the case with
CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B=y or
CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS=y.

When such padding is applied, attempting to kexec or resume from
hibernate will result ina crash: the kernel will branch to the padding
bytes as the start of the dynamically-allocated page, and as those bytes
are zero they will decode as UDF #0, which reliably triggers an
UNDEFINED exception. For example:

| # ./kexec --reuse-cmdline -f Image
| [   46.965800] kexec_core: Starting new kernel
| [   47.143641] psci: CPU1 killed (polled 0 ms)
| [   47.233653] psci: CPU2 killed (polled 0 ms)
| [   47.323465] psci: CPU3 killed (polled 0 ms)
| [   47.324776] Bye!
| [   47.327072] Internal error: Oops - Undefined instruction: 0000000002000000 [#1] SMP
| [   47.328510] Modules linked in:
| [   47.329086] CPU: 0 PID: 259 Comm: kexec Not tainted 6.2.0-rc5+ #3
| [   47.330223] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
| [   47.331497] pstate: 604003c5 (nZCv DAIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
| [   47.332782] pc : 0x43a95000
| [   47.333338] lr : machine_kexec+0x190/0x1e0
| [   47.334169] sp : ffff80000d293b70
| [   47.334845] x29: ffff80000d293b70 x28: ffff000002cc0000 x27: 0000000000000000
| [   47.336292] x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000
| [   47.337744] x23: ffff80000a837858 x22: 0000000048ec9000 x21: 0000000000000010
| [   47.339192] x20: 00000000adc83000 x19: ffff000000827000 x18: 0000000000000006
| [   47.340638] x17: ffff800075a61000 x16: ffff800008000000 x15: ffff80000d293658
| [   47.342085] x14: 0000000000000000 x13: ffff80000d2937f7 x12: ffff80000a7ff6e0
| [   47.343530] x11: 00000000ffffdfff x10: ffff80000a8ef8e0 x9 : ffff80000813ef00
| [   47.344976] x8 : 000000000002ffe8 x7 : c0000000ffffdfff x6 : 00000000000affa8
| [   47.346431] x5 : 0000000000001fff x4 : 0000000000000001 x3 : ffff80000a0a3008
| [   47.347877] x2 : ffff80000a8220f8 x1 : 0000000043a95000 x0 : ffff000000827000
| [   47.349334] Call trace:
| [   47.349834]  0x43a95000
| [   47.350338]  kernel_kexec+0x88/0x100
| [   47.351070]  __do_sys_reboot+0x108/0x268
| [   47.351873]  __arm64_sys_reboot+0x2c/0x40
| [   47.352689]  invoke_syscall+0x78/0x108
| [   47.353458]  el0_svc_common.constprop.0+0x4c/0x100
| [   47.354426]  do_el0_svc+0x34/0x50
| [   47.355102]  el0_svc+0x34/0x108
| [   47.355747]  el0t_64_sync_handler+0xf4/0x120
| [   47.356617]  el0t_64_sync+0x194/0x198
| [   47.357374] Code: bad PC value
| [   47.357999] ---[ end trace 0000000000000000 ]---
| [   47.358937] Kernel panic - not syncing: Oops - Undefined instruction: Fatal exception
| [   47.360515] Kernel Offset: disabled
| [   47.361230] CPU features: 0x002000,00050108,c8004203
| [   47.362232] Memory Limit: none

Note: Unfortunately the code dump reports "bad PC value" as it attempts
to dump some instructions prior to the UDF (i.e. before the start of the
page), and terminates early upon a fault, obscuring the problem.

This patch fixes this issue by aligning the section starter markes to
CONFIG_FUNCTION_ALIGNMENT using the ALIGN_FUNCTION() helper, which
ensures that the linker never needs to place padding bytes within the
section. Assertions are added to verify each section begins with the
function we expect, making our implicit requirement explicit.

In future it might be nice to rework the kexec and hibernation code to
decouple the section start from the entry point, but that involves much
more significant changes that come with a higher risk of error, so I've
tried to keep this fix as simple as possible for now.

Fixes: 47a15aa5 ("arm64: Extend support for CONFIG_FUNCTION_ALIGNMENT")
Reported-by: CKI Project <cki-project@redhat.com>
Link: https://lore.kernel.org/linux-arm-kernel/29992.123012504212600261@us-mta-139.us.mimecast.lan/Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/vmlinux.lds.S

@@ -93,6 +93,7 @@ jiffies = jiffies_64;
 
 #ifdef CONFIG_HIBERNATION
 #define HIBERNATE_TEXT					\
+	ALIGN_FUNCTION();				\
 	__hibernate_exit_text_start = .;		\
 	*(.hibernate_exit.text)				\
 	__hibernate_exit_text_end = .;
@@ -102,6 +103,7 @@ jiffies = jiffies_64;
 
 #ifdef CONFIG_KEXEC_CORE
 #define KEXEC_TEXT					\
+	ALIGN_FUNCTION();				\
 	__relocate_new_kernel_start = .;		\
 	*(.kexec_relocate.text)				\
 	__relocate_new_kernel_end = .;
@@ -355,6 +357,8 @@ ASSERT(__idmap_text_end - (__idmap_text_start & ~(SZ_4K - 1)) <= SZ_4K,
 #ifdef CONFIG_HIBERNATION
 ASSERT(__hibernate_exit_text_end - __hibernate_exit_text_start <= SZ_4K,
        "Hibernate exit text is bigger than 4 KiB")
+ASSERT(__hibernate_exit_text_start == swsusp_arch_suspend_exit,
+       "Hibernate exit text does not start with swsusp_arch_suspend_exit")
 #endif
 #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
 ASSERT((__entry_tramp_text_end - __entry_tramp_text_start) <= 3*PAGE_SIZE,
@@ -381,4 +385,6 @@ ASSERT(swapper_pg_dir - tramp_pg_dir == TRAMP_SWAPPER_OFFSET,
 ASSERT(__relocate_new_kernel_end - __relocate_new_kernel_start <= SZ_4K,
        "kexec relocation code is bigger than 4 KiB")
 ASSERT(KEXEC_CONTROL_PAGE_SIZE >= SZ_4K, "KEXEC_CONTROL_PAGE_SIZE is broken")
+ASSERT(__relocate_new_kernel_start == arm64_relocate_new_kernel,
+       "kexec control page does not start with arm64_relocate_new_kernel")
 #endif