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

[Eric Biggers <ebiggers@kernel.org>]

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@vger.kernel.org
> > Signed-off-by: Mohamad Raizudeen <raizudeen.kerneldev@gmail.com>
> > ---
> >  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.

- Eric