[PATCH bpf] bpf: Check size of the signature buffer

[PATCH bpf] bpf: Check size of the signature buffer

[KP Singh]

Accept only a SHA256 sized buffer.

Fixes: 349271568303 ("bpf: Implement signature verification for BPF programs")
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: KP Singh <kpsingh@kernel.org>
---
 kernel/bpf/syscall.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 8a129746bd6c..cc5bce20ec86 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2826,6 +2826,9 @@ static int bpf_prog_verify_signature(struct bpf_prog *prog, union bpf_attr *attr
 	void *sig;
 	int err = 0;
 
+	if (attr->signature_size != SHA256_DIGEST_SIZE)
+		return -EINVAL;
+
 	if (system_keyring_id_check(attr->keyring_id) == 0)
 		key = bpf_lookup_system_key(attr->keyring_id);
 	else
-- 
2.43.0

Re: [PATCH bpf] bpf: Check size of the signature buffer

[James Bottomley]

On Sun, 2025-11-02 at 12:37 +0100, KP Singh wrote:
> Accept only a SHA256 sized buffer.
> 
> Fixes: 349271568303 ("bpf: Implement signature verification for BPF
> programs")
> Reported-by: Chris Mason <clm@meta.com>
> Signed-off-by: KP Singh <kpsingh@kernel.org>
> ---
>  kernel/bpf/syscall.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 8a129746bd6c..cc5bce20ec86 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -2826,6 +2826,9 @@ static int bpf_prog_verify_signature(struct
> bpf_prog *prog, union bpf_attr *attr
>  	void *sig;
>  	int err = 0;
>  
> +	if (attr->signature_size != SHA256_DIGEST_SIZE)
> +		return -EINVAL;
> +

That's not going to work unless something really strange is going on: a
pkcs7 signature is way bigger than a simple hash.  I believe Chris was
thinking we should do something like modules do: check to see that the
signature is not bigger than the total size of the passed in file data
size less the program data before doing the vmalloc.

Regards,

James

Re: [PATCH bpf] bpf: Check size of the signature buffer

[KP Singh]

On Mon, Nov 3, 2025 at 1:36 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> On Sun, 2025-11-02 at 12:37 +0100, KP Singh wrote:
> > Accept only a SHA256 sized buffer.
> >
> > Fixes: 349271568303 ("bpf: Implement signature verification for BPF
> > programs")
> > Reported-by: Chris Mason <clm@meta.com>
> > Signed-off-by: KP Singh <kpsingh@kernel.org>
> > ---
> >  kernel/bpf/syscall.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index 8a129746bd6c..cc5bce20ec86 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -2826,6 +2826,9 @@ static int bpf_prog_verify_signature(struct
> > bpf_prog *prog, union bpf_attr *attr
> >       void *sig;
> >       int err = 0;
> >
> > +     if (attr->signature_size != SHA256_DIGEST_SIZE)
> > +             return -EINVAL;
> > +
>
> That's not going to work unless something really strange is going on: a
> pkcs7 signature is way bigger than a simple hash.  I believe Chris was
> thinking we should do something like modules do: check to see that the
> signature is not bigger than the total size of the passed in file data
> size less the program data before doing the vmalloc.

Ah, duh, yes. Apologies.

Will respin.

- KP

>
> Regards,
>
> James
>
>
>