Re: [PATCH] fsverity: Fix bpf_get_fsverity_digest() dynptr assumptions

[Kumar Kartikeya Dwivedi]

On Fri Jul 31, 2026 at 2:39 AM CEST, Eric Biggers wrote:
> The BPF verifier and the dynptr abstraction ensure that the memory space
> referenced by a dynptr remains valid.  They do not, however, provide any
> guarantee that the contents of the memory are stable.  kfuncs are
> expected to remain memory-safe even if concurrent modifications occur.
>
> bpf_get_fsverity_digest() didn't follow that: it could crash if
> arg->digest_size was concurrently modified.
>
> Fix that by using the known-good value hash_alg->digest_size instead.
>

This makes sense.

> Also correctly handle sizes over INT_MAX, which previously caused an
> integer overflow and crash.  __bpf_dynptr_size() returns a u64.
>

I guess this occurs when using file dynptrs? Otherwise the size is capped to
DYNPTR_SIZE_MASK.

> Fixes: 67814c00de31 ("bpf, fsverity: Add kfunc bpf_get_fsverity_digest")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> ---

Acked-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>

> [...]