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")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---

This patch is targeting libcrypto-fixes

 lib/crypto/chacha-block-generic.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/lib/crypto/chacha-block-generic.c b/lib/crypto/chacha-block-generic.c
index 77f68de71066..4a6d627580cb 100644
--- a/lib/crypto/chacha-block-generic.c
+++ b/lib/crypto/chacha-block-generic.c
@@ -85,10 +85,12 @@ void chacha_block_generic(struct chacha_state *state,
 	for (i = 0; i < ARRAY_SIZE(state->x); i++)
 		put_unaligned_le32(permuted_state.x[i] + state->x[i],
 				   &out[i * sizeof(u32)]);
 
 	state->x[12]++;
+
+	chacha_zeroize_state(&permuted_state);
 }
 EXPORT_SYMBOL(chacha_block_generic);
 
 /**
  * hchacha_block_generic - abbreviated ChaCha core, for XChaCha
@@ -108,7 +110,9 @@ void hchacha_block_generic(const struct chacha_state *state,
 
 	chacha_permute(&permuted_state, nrounds);
 
 	memcpy(&out[0], &permuted_state.x[0], 16);
 	memcpy(&out[4], &permuted_state.x[12], 16);
+
+	chacha_zeroize_state(&permuted_state);
 }
 EXPORT_SYMBOL(hchacha_block_generic);

base-commit: 0138af2472dfdef0d56fc4697416eaa0ff2589bd
-- 
2.53.0

On Wed, Mar 25, 2026 at 08:29:20PM -0700, Eric Biggers wrote:
> 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")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@kernel.org>
> ---
> 
> This patch is targeting libcrypto-fixes

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/log/?h=libcrypto-fixes

- Eric

On Thu, 26 Mar 2026, at 04:29, Eric Biggers wrote:
> 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")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@kernel.org>
> ---
>
> This patch is targeting libcrypto-fixes
>
>  lib/crypto/chacha-block-generic.c | 4 ++++
>  1 file changed, 4 insertions(+)
>

Acked-by: Ard Biesheuvel <ardb@kernel.org>