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  1. 06 Mar, 2022 3 commits
    • Hao Luo's avatar
      bpf: Reject programs that try to load __percpu memory. · 5844101a
      Hao Luo authored 
      With the introduction of the btf_type_tag "percpu", we can add a
MEM_PERCPU to identify those pointers that point to percpu memory.
The ability of differetiating percpu pointers from regular memory
pointers have two benefits:

 1. It forbids unexpected use of percpu pointers, such as direct loads.
    In kernel, there are special functions used for accessing percpu
    memory. Directly loading percpu memory is meaningless. We already
    have BPF helpers like bpf_per_cpu_ptr() and bpf_this_cpu_ptr() that
    wrap the kernel percpu functions. So we can now convert percpu
    pointers into regular pointers in a safe way.

 2. Previously, bpf_per_cpu_ptr() and bpf_this_cpu_ptr() only work on
    PTR_TO_PERCPU_BTF_ID, a special reg_type which describes static
    percpu variables in kernel (we rely on pahole to encode them into
    vmlinux BTF). Now, since we can identify __percpu tagged pointers,
    we can also identify dynamically allocated percpu memory as well.
    It means we can use bpf_xxx_cpu_ptr() on dynamic percpu memory.
    This would be very convenient when accessing fields like
    "cgroup->rstat_cpu".
Signed-off-by: default avatarHao Luo <haoluo@google.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarYonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220304191657.981240-4-haoluo@google.com
      5844101a
    • Hao Luo's avatar
      compiler_types: Define __percpu as __attribute__((btf_type_tag("percpu"))) · 9216c916
      Hao Luo authored 
      This is similar to commit 7472d5a6 ("compiler_types: define __user as
__attribute__((btf_type_tag("user")))"), where a type tag "user" was
introduced to identify the pointers that point to user memory. With that
change, the newest compile toolchain can encode __user information into
vmlinux BTF, which can be used by the BPF verifier to enforce safe
program behaviors.

Similarly, we have __percpu attribute, which is mainly used to indicate
memory is allocated in percpu region. The __percpu pointers in kernel
are supposed to be used together with functions like per_cpu_ptr() and
this_cpu_ptr(), which perform necessary calculation on the pointer's
base address. Without the btf_type_tag introduced in this patch,
__percpu pointers will be treated as regular memory pointers in vmlinux
BTF and BPF programs are allowed to directly dereference them, generating
incorrect behaviors. Now with "percpu" btf_type_tag, the BPF verifier is
able to differentiate __percpu pointers from regular pointers and forbids
unexpected behaviors like direct load.

The following is an example similar to the one given in commit
7472d5a6:

  [$ ~] cat test.c
  #define __percpu __attribute__((btf_type_tag("percpu")))
  int foo(int __percpu *arg) {
  	return *arg;
  }
  [$ ~] clang -O2 -g -c test.c
  [$ ~] pahole -JV test.o
  ...
  File test.o:
  [1] INT int size=4 nr_bits=32 encoding=SIGNED
  [2] TYPE_TAG percpu type_id=1
  [3] PTR (anon) type_id=2
  [4] FUNC_PROTO (anon) return=1 args=(3 arg)
  [5] FUNC foo type_id=4
  [$ ~]

for the function argument "int __percpu *arg", its type is described as
	PTR -> TYPE_TAG(percpu) -> INT
The kernel can use this information for bpf verification or other
use cases.

Like commit 7472d5a6, this feature requires clang (>= clang14) and
pahole (>= 1.23).
Signed-off-by: default avatarHao Luo <haoluo@google.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarYonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220304191657.981240-3-haoluo@google.com
      9216c916
    • Hao Luo's avatar
      bpf: Fix checking PTR_TO_BTF_ID in check_mem_access · bff61f6f
      Hao Luo authored 
      With the introduction of MEM_USER in

 commit c6f1bfe8 ("bpf: reject program if a __user tagged memory accessed in kernel way")

PTR_TO_BTF_ID can be combined with a MEM_USER tag. Therefore, most
likely, when we compare reg_type against PTR_TO_BTF_ID, we want to use
the reg's base_type. Previously the check in check_mem_access() wants
to say: if the reg is BTF_ID but not NULL, the execution flow falls
into the 'then' branch. But now a reg of (BTF_ID | MEM_USER), which
should go into the 'then' branch, goes into the 'else'.

The end results before and after this patch are the same: regs tagged
with MEM_USER get rejected, but not in a way we intended. So fix the
condition, the error message now is correct.

Before (log from commit 696c3901):

  $ ./test_progs -v -n 22/3
  ...
  libbpf: prog 'test_user1': BPF program load failed: Permission denied
  libbpf: prog 'test_user1': -- BEGIN PROG LOAD LOG --
  R1 type=ctx expected=fp
  0: R1=ctx(id=0,off=0,imm=0) R10=fp0
  ; int BPF_PROG(test_user1, struct bpf_testmod_btf_type_tag_1 *arg)
  0: (79) r1 = *(u64 *)(r1 +0)
  func 'bpf_testmod_test_btf_type_tag_user_1' arg0 has btf_id 136561 type STRUCT 'bpf_testmod_btf_type_tag_1'
  1: R1_w=user_ptr_bpf_testmod_btf_type_tag_1(id=0,off=0,imm=0)
  ; g = arg->a;
  1: (61) r1 = *(u32 *)(r1 +0)
  R1 invalid mem access 'user_ptr_'

Now:

  libbpf: prog 'test_user1': BPF program load failed: Permission denied
  libbpf: prog 'test_user1': -- BEGIN PROG LOAD LOG --
  R1 type=ctx expected=fp
  0: R1=ctx(id=0,off=0,imm=0) R10=fp0
  ; int BPF_PROG(test_user1, struct bpf_testmod_btf_type_tag_1 *arg)
  0: (79) r1 = *(u64 *)(r1 +0)
  func 'bpf_testmod_test_btf_type_tag_user_1' arg0 has btf_id 104036 type STRUCT 'bpf_testmod_btf_type_tag_1'
  1: R1_w=user_ptr_bpf_testmod_btf_type_tag_1(id=0,ref_obj_id=0,off=0,imm=0)
  ; g = arg->a;
  1: (61) r1 = *(u32 *)(r1 +0)
  R1 is ptr_bpf_testmod_btf_type_tag_1 access user memory: off=0

Note the error message for the reason of rejection.

Fixes: c6f1bfe8 ("bpf: reject program if a __user tagged memory accessed in kernel way")
Signed-off-by: default avatarHao Luo <haoluo@google.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarYonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220304191657.981240-2-haoluo@google.com
      bff61f6f
  3. 05 Mar, 2022 37 commits
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