Re: [PATCH] crypto: chacha20poly1305 - Fix missing state zeroization in xchacha decrypt

[Ard Biesheuvel]

On Sat, 3 Oct 2026, at 10:57, Eric Biggers wrote:
> On Sat, Oct 03, 2026 at 09:34:21AM +0200, Ard Biesheuvel wrote:
>> 
>> 
>> On Sat, 3 Oct 2026, at 08:08, Mohamad Raizudeen wrote:
>> > 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.
>> >
>> > This leaves the derived chacha20 subkey on the stack after the function
>> > returns. Fix this by storing the return value, calling
>> > chacha_zeroize_state() and then returning the result, matching the
>> > logic in `chacha20poly1305_decrypt`.
>> >
>> > Fixes: ed20078b7e333 ("crypto: chacha20poly1305 - import construction 
>> > and selftest from Zinc")
>> > Cc: stable@xxxxxxxxxxxxxxx
>> > Signed-off-by: Mohamad Raizudeen <raizudeen.kerneldev@xxxxxxxxx>
>> > ---
>> >  lib/crypto/chacha20poly1305.c | 5 ++++-
>> >  1 file changed, 4 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/lib/crypto/chacha20poly1305.c 
>> > b/lib/crypto/chacha20poly1305.c
>> > index ea42a28f4ff7..03b14d272520 100644
>> > --- a/lib/crypto/chacha20poly1305.c
>> > +++ b/lib/crypto/chacha20poly1305.c
>> > @@ -199,10 +199,13 @@ bool xchacha20poly1305_decrypt(u8 *dst, const u8 
>> > *src, const size_t src_len,
>> >  			       const u8 key[at_least CHACHA20POLY1305_KEY_SIZE])
>> >  {
>> >  	struct chacha_state chacha_state;
>> > +	bool ret;
>> > 
>> >  	xchacha_init(&chacha_state, key, nonce);
>> > -	return __chacha20poly1305_decrypt(dst, src, src_len, ad, ad_len,
>> > +	ret = __chacha20poly1305_decrypt(dst, src, src_len, ad, ad_len,
>> >  					  &chacha_state);
>> > +	chacha_zeroize_state(&chacha_state);
>> > +	return ret;
>> >  }
>> >  EXPORT_SYMBOL(xchacha20poly1305_decrypt);
>> > 
>> 
>> Wouldn't it be better to move the existing call from chacha20poly1305_decrypt()
>> to __chacha20poly1305_decrypt()?
>
> Since __chacha20poly1305_decrypt() has two return statements that would
> need to be considered, I think this patch (which makes each
> *chacha_init() clearly paired with chacha_zeroize_state()) is slightly
> cleaner.
>

Fair enough.