[PATCH 6.12, 6.6, 6.1, 5.15, 5.10] lib/crypto: chacha: Zeroize permuted_state before it leaves scope

[Eric Biggers]

commit e5046823f8fa3677341b541a25af2fcb99a5b1e0 upstream.

Since the ChaCha permutation is invertible, the local variable
'permuted_state' is sufficient to compute the original 'state', and thus
the key, even after the permutation has been done.

While the kernel is quite inconsistent about zeroizing secrets on the
stack (and some prominent userspace crypto libraries don't bother at all
since it's not guaranteed to work anyway), the kernel does try to do it
as a best practice, especially in cases involving the RNG.

Thus, explicitly zeroize 'permuted_state' before it goes out of scope.

Fixes: c08d0e647305 ("crypto: chacha20 - Add a generic ChaCha20 stream cipher implementation")
Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20260326032920.39408-1-ebiggers@xxxxxxxxxx
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
 lib/crypto/chacha.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/lib/crypto/chacha.c b/lib/crypto/chacha.c
index 3cdda3b5ee060..1271a7de1ba79 100644
--- a/lib/crypto/chacha.c
+++ b/lib/crypto/chacha.c
@@ -84,10 +84,12 @@ void chacha_block_generic(u32 *state, u8 *stream, int nrounds)
 
 	for (i = 0; i < ARRAY_SIZE(x); i++)
 		put_unaligned_le32(x[i] + state[i], &stream[i * sizeof(u32)]);
 
 	state[12]++;
+
+	memzero_explicit(x, sizeof(x));
 }
 EXPORT_SYMBOL(chacha_block_generic);
 
 /**
  * hchacha_block_generic - abbreviated ChaCha core, for XChaCha
@@ -108,7 +110,9 @@ void hchacha_block_generic(const u32 *state, u32 *stream, int nrounds)
 
 	chacha_permute(x, nrounds);
 
 	memcpy(&stream[0], &x[0], 16);
 	memcpy(&stream[4], &x[12], 16);
+
+	memzero_explicit(x, sizeof(x));
 }
 EXPORT_SYMBOL(hchacha_block_generic);

base-commit: e7a3953084a7050ca349010deb22546834c2e196
-- 
2.53.0