Re: [PATCH] BPF: Fix potential bad pointer dereference in bpf_sys_bpf

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On 7/28/22 7:16 PM, Yonghong Song wrote:


On 7/28/22 11:01 AM, Jinghao Jia wrote:

On 7/28/22 10:52 AM, Yonghong Song wrote:


On 7/27/22 6:29 AM, Jinghao Jia wrote:
The bpf_sys_bpf() helper function allows an eBPF program to load another
eBPF program from within the kernel. In this case the argument union
bpf_attr pointer (as well as the insns and license pointers inside) is a
kernel address instead of a userspace address (which is the case of a
usual bpf() syscall). To make the memory copying process in the syscall
work in both cases, bpfptr_t [1] was introduced to wrap around the
pointer and distinguish its origin. Specifically, when copying memory
contents from a bpfptr_t, a copy_from_user() is performed in case of a
userspace address and a memcpy() is performed for a kernel address [2].

This can lead to problems because the in-kernel pointer is never checked
for validity. If an eBPF syscall program tries to call bpf_sys_bpf()
with a bad insns pointer, say 0xdeadbeef (which is supposed to point to
the start of the instruction array) in the bpf_attr union, memcpy() is
always happy to dereference the bad pointer to cause a un-handle-able
page fault and in turn an oops. However, this is not supposed to happen
because at that point the eBPF program is already verified and should
not cause a memory error. The same issue in userspace is handled
gracefully by copy_from_user(), which would return -EFAULT in such a
case.

Replace memcpy() with the safer copy_from_kernel_nofault() and
strncpy_from_kernel_nofault().

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/bpfptr.h [2]: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/sockptr.h#n44

Signed-off-by: Jinghao Jia <jinghao@xxxxxxxxxxxxx>
---
  include/linux/sockptr.h | 11 +++--------
  1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/include/linux/sockptr.h b/include/linux/sockptr.h
index d45902fb4cad..3b8a41c82516 100644
--- a/include/linux/sockptr.h
+++ b/include/linux/sockptr.h
@@ -46,8 +46,7 @@ static inline int copy_from_sockptr_offset(void *dst, sockptr_t src,
  {
      if (!sockptr_is_kernel(src))
          return copy_from_user(dst, src.user + offset, size);
-    memcpy(dst, src.kernel + offset, size);
-    return 0;
+    return copy_from_kernel_nofault(dst, src.kernel + offset, size);
  }

The subject and commit message mentioned it is bpf_sys_bpf() helper
might have issues. But the patch itself tries to modify copy_from_sockptr_offset() and strncpy_from_sockptr(), why?


Hi Yonghong,

Sorry for the confusion. The problem happens when bpf_sys_bpf() helper is called with a bad kernel address but the dereference takes place in the copy_from_sockptr_offset() and strncpy_from_sockptr() functions.

Let's assume we are doing a BPF_PROG_LOAD operation using bpf_sys_bpf() and our insns pointer inside the union bpf_attr argument is set to NULL (could be any other bad address). The helper calls __sys_bpf() which would then call bpf_prog_load() to load the program. bpf_prog_load() is responsible for copying the eBPF instructions to the newly allocated memory for the program, which uses the bpfptr_t API [1]. Internally, all bpfptr_t operations are backed by the corresponding sockptr_t operations. In other words, the code that performs the copy (and therefore the deref of the pointer) is inside copy_from_sockptr_offset() and strncpy_from_sockptr().

Thanks for explanation. It would be great if you can put the above
details in the commit message (esp. call stack) which leads to
the kernel panic(?).


[1]: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/kernel/bpf/syscall.c#n2566

Best,
Jinghao
    static inline int copy_from_sockptr(void *dst, sockptr_t src, size_t size) @@ -93,12 +92,8 @@ static inline void *memdup_sockptr_nul(sockptr_t src, size_t len)     static inline long strncpy_from_sockptr(char *dst, sockptr_t src, size_t count)
  {
-    if (sockptr_is_kernel(src)) {
-        size_t len = min(strnlen(src.kernel, count - 1) + 1, count);
-
-        memcpy(dst, src.kernel, len);
-        return len;
-    }
+    if (sockptr_is_kernel(src))
+        return strncpy_from_kernel_nofault(dst, src.kernel, count);
      return strncpy_from_user(dst, src.user, count);
  }

I think we should not modify copy_from_sockptr() and strncpy_from_sockptr(). These two functions are used by networking
as well and nofault version should not be called for calls in
networking stack.

So I suggest you directly modify copy_from_bpfptr() and strncpy_from_bpfptr() since these two functions indeed might
have invalid kernel address and may cause fault.


Thanks a lot for the feedback! I do agree that the changes on sockptr functions might be problematic. I will roll out a V2 based on your comments.

--Jinghao

base-commit: d295daf505758f9a0e4d05f4ee3bfdfb4192c18f



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