Re: [PATCH v2] stable: crypto: sha256 - fix crash at kexec

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Oct 02, 2025 at 04:26:20AM -0700, Breno Leitao wrote:
> Loading a large (~2.1G) files with kexec crashes the host with when
> running:
> 
>   # kexec --load kernel --initrd initrd_with_2G_or_more
> 
>   UBSAN: signed-integer-overflow in ./include/crypto/sha256_base.h:64:19
>   34152083 * 64 cannot be represented in type 'int'
>   ...
>   BUG: unable to handle page fault for address: ff9fffff83b624c0
>   sha256_update (lib/crypto/sha256.c:137)
>   crypto_sha256_update (crypto/sha256_generic.c:40)
>   kexec_calculate_store_digests (kernel/kexec_file.c:769)
>   __se_sys_kexec_file_load (kernel/kexec_file.c:397 kernel/kexec_file.c:332)
>   ...
> 
> (Line numbers based on commit da274362a7bd9 ("Linux 6.12.49")
> 
> This started happening after commit f4da7afe07523f
> ("kexec_file: increase maximum file size to 4G") that landed in v6.0,
> which increased the file size for kexec.
> 
> This is not happening upstream (v6.16+), given that `block` type was
> upgraded from "int" to "size_t" in commit 74a43a2cf5e8 ("crypto:
> lib/sha256 - Move partial block handling out")
> 
> Upgrade the block type similar to the commit above, avoiding hitting the
> overflow.
> 
> This patch is only suitable for the stable tree, and before 6.16, which
> got commit 74a43a2cf5e8 ("crypto: lib/sha256 - Move partial block
> handling out"). This is not required before f4da7afe07523f ("kexec_file:
> increase maximum file size to 4G"). In other words, this fix is required
> between versions v6.0 and v6.16.
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>
> Fixes: f4da7afe07523f ("kexec_file: increase maximum file size to 4G") # Before v6.16
> Reported-by: Michael van der Westhuizen <rmikey@meta.com>
> Reported-by: Tobias Fleig <tfleig@meta.com>
> ---
> Changes in v2:
> - s/size_t/unsigned int/ as suggested by Eric
> - Tag the commit that introduce the problem as Fixes, making backport easier.
> - Link to v1: https://lore.kernel.org/r/20251001-stable_crash-v1-1-3071c0bd795e@debian.org
> ---
>  include/crypto/sha256_base.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/crypto/sha256_base.h b/include/crypto/sha256_base.h
> index e0418818d63c8..e3e610cfe8d30 100644
> --- a/include/crypto/sha256_base.h
> +++ b/include/crypto/sha256_base.h
> @@ -44,7 +44,7 @@ static inline int lib_sha256_base_do_update(struct sha256_state *sctx,
>  	sctx->count += len;
>  
>  	if (unlikely((partial + len) >= SHA256_BLOCK_SIZE)) {
> -		int blocks;
> +		unsigned int blocks;
>  
>  		if (partial) {
>  			int p = SHA256_BLOCK_SIZE - partial;
> 
> ---
> base-commit: da274362a7bd9ab3a6e46d15945029145ebce672
> change-id: 20251001-stable_crash-f2151baf043b

Reviewed-by: Eric Biggers <ebiggers@kernel.org>

- Eric