[PATCH v2] lib/crypto: chacha20poly1305: zeroize state in __chacha20poly1305_decrypt

[Mohamad Raizudeen <raizudeen.kerneldev@gmail.com>]

The `__chacha20poly1305_decrypt` function does not zeroize the chacha
state, unlike its encrypt counterpart. The regular
`chacha20poly1305_decrypt` function handles this by manually calling
chacha_zeroize_state(). However, `xchacha20poly1305_decrypt` returns
the result directly without clearing the state, leaving the derived
chacha20 subkey on the stack.

Fix this by moving the chacha_zeroize_state() call into
__chacha20poly1305_decrypt() itself, so the helper cleans up after
itself just like __chacha20poly1305_encrypt does. This ensures all
callers are secure without needing manual cleanup.

Fixes: ed20078b7e333 ("crypto: chacha20poly1305 - import construction and selftest from Zinc")
Cc: stable@vger.kernel.org
Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Mohamad Raizudeen <raizudeen.kerneldev@gmail.com>
---
Changes in v2:
- Move chacha_zeroize_state() into __chacha20poly1305_decrypt() as
  suggested by Ard and requested by Eric.
- Update subject prefix to "lib/crypto: chacha20poly1305:"

Link to v1: https://lore.kernel.org/all/20261003060826.7792-1-raizudeen.kerneldev@gmail.com/T/

 lib/crypto/chacha20poly1305.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/lib/crypto/chacha20poly1305.c b/lib/crypto/chacha20poly1305.c
index ea42a28f4ff7..80904321458b 100644
--- a/lib/crypto/chacha20poly1305.c
+++ b/lib/crypto/chacha20poly1305.c
@@ -137,8 +137,10 @@ __chacha20poly1305_decrypt(u8 *dst, const u8 *src, const size_t src_len,
 		__le64 lens[2];
 	} b;
 
-	if (unlikely(src_len < POLY1305_DIGEST_SIZE))
+	if (unlikely(src_len < POLY1305_DIGEST_SIZE)) {
+		chacha_zeroize_state(chacha_state);
 		return false;
+	}
 
 	chacha20_crypt(chacha_state, b.block0, pad0, sizeof(b.block0));
 	poly1305_init(&poly1305_state, b.block0);
@@ -164,6 +166,7 @@ __chacha20poly1305_decrypt(u8 *dst, const u8 *src, const size_t src_len,
 
 	memzero_explicit(&b, sizeof(b));
 
+	chacha_zeroize_state(chacha_state);
 	return !ret;
 }
 
@@ -186,7 +189,6 @@ bool chacha20poly1305_decrypt(u8 *dst, const u8 *src, const size_t src_len,
 	ret = __chacha20poly1305_decrypt(dst, src, src_len, ad, ad_len,
 					 &chacha_state);
 
-	chacha_zeroize_state(&chacha_state);
 	memzero_explicit(iv, sizeof(iv));
 	memzero_explicit(k, sizeof(k));
 	return ret;
-- 
2.53.0