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

[Jinghao Jia <jinghao@linux.ibm.com>]

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@linux.ibm.com>
---
 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);
 }
 
 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);
 }
 

base-commit: d295daf505758f9a0e4d05f4ee3bfdfb4192c18f
-- 
2.35.1