Re: [PATCH bpf v2] bpf: Fix NULL pointer dereference in bpf_sock_from_file

On Fri Aug 21, 2026 at 7:30 PM CEST, Andrii Nakryiko wrote:

On Thu, Aug 20, 2026 at 8:25 AM Syeda Mahnur Asif [off-list ref] wrote:

quoted

bpf_sock_from_file should not dereference a NULL file pointer.
KASAN detects a null-ptr-deref when eBPF tracing
fentry/fexit programs are attached to points such as
__mmap_region and file_map_prot_check kernel functions. This can
result in a NULL file pointer flowing from context to the helper.

A minimal check before dereferencing can fix this.

Fixes: b60da4955f53 ("bpf: Only provide bpf_sock_from_file with CONFIG_NET")
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Syeda Mahnur Asif <redacted>
---
v2:
- Added Reviewed-by from Emil Tsalapatis
- Fixed name in Signed-off-by

 net/core/filter.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 16845987b244..7c3caae4bafc 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -12182,7 +12182,10 @@ const struct bpf_func_proto bpf_skc_to_mptcp_sock_proto = {

 BPF_CALL_1(bpf_sock_from_file, struct file *, file)
 {
-       return (unsigned long)sock_from_file(file);
+       if (file)
+               return (unsigned long)sock_from_file(file);

given this is a legacy ARG_PTR_TO_BTF_ID, it can be not just NULL, but
also a) small value bogus "pointer" resulting from NULL + offset
calculation and/or b) just random garbage casted to struct file
pointer with bpf_core_cast() (maybe this hole we've closed with
explicitly untrusted, not sure).

Either way, this is not a sufficient fix at least.

But I wonder if the proper fix is actually to mark this (and other
similar) argument as explicitly requiring trusted PTR_TO_BTF_ID?

Thoughts?

+1, should bite the bullet and accelerate deprecation of legacy PTR_TO_BTF_ID.
We can mark specific arguments / fields in structs as trusted where there is a
legitimate use case and the kernel guarantees their lifetime appropriately.

quoted

+
+       return (unsigned long)NULL;
 }

 BTF_ID_LIST(bpf_sock_from_file_btf_ids)
--
2.53.0