Re: [PATCH v2] arm64/crypto: poly1305 fix a read out-of-bound

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Jul 20, 2022 at 05:57:30PM +0800, Guozihua (Scott) wrote:
> On 2022/7/20 17:41, Will Deacon wrote:
> > On Tue, Jul 12, 2022 at 03:50:31PM +0800, GUO Zihua wrote:
> > > A kasan error was reported during fuzzing:
> > 
> > [...]
> > 
> > > This patch fixes the issue by calling poly1305_init_arm64() instead of
> > > poly1305_init_arch(). This is also the implementation for the same
> > > algorithm on arm platform.
> > > 
> > > Fixes: f569ca164751 ("crypto: arm64/poly1305 - incorporate OpenSSL/CRYPTOGAMS NEON implementation")
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: GUO Zihua <guozihua@huawei.com>
> > > ---
> > >   arch/arm64/crypto/poly1305-glue.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > I'm not a crypto guy by any stretch of the imagination, but Ard is out
> > at the moment and this looks like an important fix so I had a crack at
> > reviewing it.
> > 
> > > diff --git a/arch/arm64/crypto/poly1305-glue.c b/arch/arm64/crypto/poly1305-glue.c
> > > index 9c3d86e397bf..1fae18ba11ed 100644
> > > --- a/arch/arm64/crypto/poly1305-glue.c
> > > +++ b/arch/arm64/crypto/poly1305-glue.c
> > > @@ -52,7 +52,7 @@ static void neon_poly1305_blocks(struct poly1305_desc_ctx *dctx, const u8 *src,
> > >   {
> > >   	if (unlikely(!dctx->sset)) {
> > >   		if (!dctx->rset) {
> > > -			poly1305_init_arch(dctx, src);
> > > +			poly1305_init_arm64(&dctx->h, src);
> > >   			src += POLY1305_BLOCK_SIZE;
> > >   			len -= POLY1305_BLOCK_SIZE;
> > >   			dctx->rset = 1;
> > 
> > With this change, we no longer initialise dctx->buflen to 0 as part of the
> > initialisation. Looking at neon_poly1305_do_update(), I'm a bit worried
> > that we could land in the 'if (likely(len >= POLY1305_BLOCK_SIZE))' block,
> > end up with len == 0 and fail to set dctx->buflen. Is this a problem, or is
> > my ignorance showing?
> > 
> > Will
> > .
> 
> Thanks Will.
> 
> I noticed this as well, but I leaved it out so that the behavior is the same
> as the implementation for arm. The buflen here seems to be used for
> maintaining any excessive data after the last block, and is zeroed during
> init. I am not sure why it should be zeroed again during key initialization.
> Maybe the thought was that the very first block of the data is always used
> for initializing rset and that is also considered to be the "initialization"
> process for the algorithm, thus the zeroing of buflen. I could be completely
> wrong though.
> 

buflen is initialized by neon_poly1305_init(), so there's no issue here.

- Eric

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