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

[Breno Leitao <leitao@debian.org>]

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

Best regards,
--  
Breno Leitao <leitao@debian.org>