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

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

[Syeda Mahnur Asif]

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 <s.mahnur.a@gmail.com>
---
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);
+
+	return (unsigned long)NULL;
 }
 
 BTF_ID_LIST(bpf_sock_from_file_btf_ids)
-- 
2.53.0

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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

Pre-existing issues:
- [High] Incomplete validation allows ERR_PTR to cause kernel panic in bpf_sock_from_file.
- [High] Missing NULL pointer checks in BPF helpers taking ARG_PTR_TO_BTF_ID lead to kernel panics.
--

--- Patch [1]: [PATCH bpf v2] bpf: Fix NULL pointer dereference in bpf_sock_from_file ---
commit a54db842fd2086671fb2b904beab51f691e8cc8b
Author: Syeda Mahnur Asif <s.mahnur.a@gmail.com>

bpf: Fix NULL pointer dereference in bpf_sock_from_file

Adds a NULL check to bpf_sock_from_file to prevent kernel panics when
fentry/fexit programs pass a NULL file pointer.

> diff --git a/net/core/filter.c b/net/core/filter.c
> index 16845987b2449..7c3caae4bafc3 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);

[Severity: High]
This isn't a bug introduced by this patch, but does this check adequately
handle error pointers?

If an eBPF fexit program attaches to a kernel function returning a
struct file pointer that fails (e.g., returning ERR_PTR(-ENOENT)), the BPF
verifier treats this as a valid PTR_TO_BTF_ID.

Since an error pointer is not NULL, this condition will pass, and
sock_from_file() will attempt to dereference it, leading to a crash.

Could we use IS_ERR_OR_NULL(file) instead to catch these cases as well?

Also, while not introduced by this patch, are there similar missing NULL
pointer checks in other BPF helpers taking ARG_PTR_TO_BTF_ID?

When fentry/fexit programs pass context arguments, the verifier lacks BTF
nullability annotations for most parameters. This means helpers can receive
and blindly dereference NULL pointers.

For instance, bpf_task_pt_regs() directly dereferences the task without
checking:

kernel/trace/bpf_trace.c:bpf_task_pt_regs() {
	return (unsigned long) task_pt_regs(task);
}

Similarly, __bpf_get_task_stack() calls try_get_task_stack() which will
dereference the task:

kernel/bpf/stackmap.c:__bpf_get_task_stack() {
	if (!try_get_task_stack(task))
		return -EFAULT;
}

And bpf_xdp_get_buff_len() accesses the xdp buffer directly:

net/core/filter.c:bpf_xdp_get_buff_len() {
	return xdp_get_buff_len(xdp);
}

Do these helpers need similar runtime validation to prevent panics in the
same execution context?

> +
> +	return (unsigned long)NULL;
>  }
>  
>  BTF_ID_LIST(bpf_sock_from_file_btf_ids)

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260820152532.621639-1-s.mahnur.a@gmail.com?part=1

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

[Andrii Nakryiko]

On Thu, Aug 20, 2026 at 8:25 AM Syeda Mahnur Asif <s.mahnur.a@gmail.com> wrote:
>
> 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 <s.mahnur.a@gmail.com>
> ---
> 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?

> +
> +       return (unsigned long)NULL;
>  }
>
>  BTF_ID_LIST(bpf_sock_from_file_btf_ids)
> --
> 2.53.0
>

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

[Kumar Kartikeya Dwivedi]

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 <s.mahnur.a@gmail.com> wrote:
>>
>> 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 <s.mahnur.a@gmail.com>
>> ---
>> 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.

>> +
>> +       return (unsigned long)NULL;
>>  }
>>
>>  BTF_ID_LIST(bpf_sock_from_file_btf_ids)
>> --
>> 2.53.0
>>