Re: [PATCH] mac80211: aes-cmac: remove VLA usage

[Gustavo A. R. Silva]

On 03/21/2018 08:48 AM, Johannes Berg wrote:
> On Wed, 2018-03-21 at 08:42 -0500, Gustavo A. R. Silva wrote:
> > In preparation to enabling -Wvla, remove VLAs and replace them
> > with dynamic memory allocation instead.
> >
> > The use of stack Variable Length Arrays needs to be avoided, as they
> > can be a vector for stack exhaustion, which can be both a runtime bug
> > or a security flaw. Also, in general, as code evolves it is easy to
> > lose track of how big a VLA can get. Thus, we can end up having runtime
> > failures that are hard to debug.
> >
> > Also, fixed as part of the directive to remove all VLAs from
> > the kernel: https://lkml.org/lkml/2018/3/7/621
> >
> > Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
> > ---
> >   net/mac80211/aes_cmac.c | 36 ++++++++++++++++++++++++------------
> >   1 file changed, 24 insertions(+), 12 deletions(-)
> >
> > diff --git a/net/mac80211/aes_cmac.c b/net/mac80211/aes_cmac.c
> > index 2fb6558..c9444bf 100644
> > --- a/net/mac80211/aes_cmac.c
> > +++ b/net/mac80211/aes_cmac.c
> > @@ -27,30 +27,42 @@ static const u8 zero[CMAC_TLEN_256];
> >   void ieee80211_aes_cmac(struct crypto_shash *tfm, const u8 *aad,
> >   			const u8 *data, size_t data_len, u8 *mic)
> >   {
> > -	SHASH_DESC_ON_STACK(desc, tfm);
> > +	struct shash_desc *shash;
> >   	u8 out[AES_BLOCK_SIZE];
> >   
> > -	desc->tfm = tfm;
> > +	shash = kmalloc(sizeof(*shash) + crypto_shash_descsize(tfm),
> > +			GFP_KERNEL);
> > +	if (!shash)
> > +		return;
>
> Honestly, this seems like a really bad idea - you're now hitting
> kmalloc for every TX/RX frame here.
>
> SHA_DESC_ON_STACK() should just be fixed to not need a VLA, but take
> some sort of maximum, I guess?

SHA_DESC_ON_STACK is currently being used in multiple places. But, yeah, 
I think we can define multiple macros of the same kind and adjust to the 
characteristics of each the component.

How big do you think tfm can get?

Thanks
--
Gustavo