On Sat, Dec 30, 2017 at 09:37:44AM +0100, Steffen Klassert wrote:
> On Sat, Dec 23, 2017 at 02:29:42PM -0600, Eric Biggers wrote:
> > [+Cc Steffen Klassert <steffen.klassert@xxxxxxxxxxx>]
> >
> >
> > I was able to reproduce this by trying to use 'pcrypt' recursively. I am not
> > 100% sure it is the exact same bug, but it probably is. Here is a C reproducer:
> >
> > #include <linux/if_alg.h>
> > #include <sys/socket.h>
> > #include <unistd.h>
> >
> > int main()
> > {
> > struct sockaddr_alg addr = {
> > .salg_type = "aead",
> > .salg_name = "pcrypt(pcrypt(rfc4106-gcm-aesni))",
> > };
> > int algfd, reqfd;
> > char buf[32] = { 0 };
> >
> > algfd = socket(AF_ALG, SOCK_SEQPACKET, 0);
> > bind(algfd, (void *)&addr, sizeof(addr));
> > setsockopt(algfd, SOL_ALG, ALG_SET_KEY, buf, 20);
> >
> > reqfd = accept(algfd, 0, 0);
> > write(reqfd, buf, 32);
> > read(reqfd, buf, 16);
> > }
> >
> > It seems the problem is that all 'pcrypt' instances use the same
> > 'padata_instance', which completes works in the order they are submitted. But
> > with nested use, the outer work is submitted before the inner work, so the inner
> > work isn't allowed to complete until the outer work does, which deadlocks
> > because actually the inner work needs to complete first.
> >
> > What a mess. Maybe there should be a separate 'padata_instance' per pcrypt
> > instance? Or maybe there should be a way for an algorithm to declare that it
> > can only appear in the stack one time?
>
> Having two nested pcrypt templates in one algorithm instance
> does not make so much sense in the first place. I thought
> that the crypto layer would refuse to build an instance
> with two nested templates of the same type.
>
> At least for pcrypt, refusing such instantiations would
> be the correct behaviour. Are there any other templates
> where a nested use would make sense?
Maybe. But either way, I don't see a straightforward way to prevent it
currently. In particular, the links from an instance to its inner algorithms
are stored in the crypto_instance_ctx() which has a template-specific format, so
it isn't currently possible to recursively search an instance to check whether a
particular template is present. We could perhaps add such links in a standard
format, though...
Eric
