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.
base-commit: d295daf505758f9a0e4d05f4ee3bfdfb4192c18f
