[PATCH] crypto: shash - Fix buffer overrun in import function

[Herbert Xu <herbert@gondor.apana.org.au>]

On Mon, May 26, 2025 at 01:39:36AM -0700, syzbot wrote:
> 
> syzbot found the following issue on:
> 
> HEAD commit:    176e917e010c Add linux-next specific files for 20250523
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=175c1ad4580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=e7902c752bef748
> dashboard link: https://syzkaller.appspot.com/bug?extid=4851c19615d35f0e4d68
> compiler:       Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14f92170580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17bd88e8580000

This patch should fix the bug:

---8<---
Only set the partial block length to zero if the algorithm is
block-only.  Otherwise the descriptor context could be empty,
e.g., for digest_null.

Reported-by: syzbot+4851c19615d35f0e4d68@syzkaller.appspotmail.com
Fixes: 7650f826f7b2 ("crypto: shash - Handle partial blocks in API")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/crypto/shash.c b/crypto/shash.c
index 37537d7995c7..4721f5f134f4 100644
--- a/crypto/shash.c
+++ b/crypto/shash.c
@@ -257,12 +257,13 @@ static int __crypto_shash_import(struct shash_desc *desc, const void *in,
 	if (crypto_shash_get_flags(tfm) & CRYPTO_TFM_NEED_KEY)
 		return -ENOKEY;
 
-	plen = crypto_shash_blocksize(tfm) + 1;
-	descsize = crypto_shash_descsize(tfm);
 	ss = crypto_shash_statesize(tfm);
-	buf[descsize - 1] = 0;
-	if (crypto_shash_block_only(tfm))
+	if (crypto_shash_block_only(tfm)) {
+		plen = crypto_shash_blocksize(tfm) + 1;
 		ss -= plen;
+		descsize = crypto_shash_descsize(tfm);
+		buf[descsize - 1] = 0;
+	}
 	if (!import) {
 		memcpy(buf, in, ss);
 		return 0;
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt