Re: [PATCH] crypto: af_alg - avoid undefined behavior accessing salg_name

[Eric Biggers]

On Mon, Oct 26, 2020 at 10:23:35PM +0100, 'Jann Horn' via syzkaller-bugs wrote:
> On Mon, Oct 26, 2020 at 9:08 PM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
> > Commit 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm
> > names") made the kernel start accepting arbitrarily long algorithm names
> > in sockaddr_alg.
> 
> That's not true; it's still limited by the size of struct
> sockaddr_storage (128 bytes total for the entire address).

Interesting, so the actual limit is 104 bytes.  It seems like the intent of that
commit was to make it unlimited, though...

> If you make it longer, __copy_msghdr_from_user() will silently truncate the
> size.

That's used for sys_sendmsg(), which AFAICT isn't relevant here.  sockaddr_alg
is used with sys_bind(), which fails with EINVAL if the address is longer than
sizeof(struct sockaddr_storage).

However, since sys_sendmsg() is truncating overly-long addresses, it's probably
the case that sizeof(struct sockaddr_storage) can never be increased in the
future...

> 
> > This is broken because the kernel can access indices >= 64 in salg_name,
> > which is undefined behavior -- even though the memory that is accessed
> > is still located within the sockaddr structure.  It would only be
> > defined behavior if the array were properly marked as arbitrary-length
> > (either by making it a flexible array, which is the recommended way
> > these days, or by making it an array of length 0 or 1).
> >
> > We can't simply change salg_name into a flexible array, since that would
> > break source compatibility with userspace programs that embed
> > sockaddr_alg into another struct, or (more commonly) declare a
> > sockaddr_alg like 'struct sockaddr_alg sa = { .salg_name = "foo" };'.
> >
> > One solution would be to change salg_name into a flexible array only
> > when '#ifdef __KERNEL__'.  However, that would keep userspace without an
> > easy way to actually use the longer algorithm names.
> >
> > Instead, add a new structure 'sockaddr_alg_new' that has the flexible
> > array field, and expose it to both userspace and the kernel.
> > Make the kernel use it correctly in alg_bind().
> [...]
> > @@ -147,7 +147,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
> >         const u32 allowed = CRYPTO_ALG_KERN_DRIVER_ONLY;
> >         struct sock *sk = sock->sk;
> >         struct alg_sock *ask = alg_sk(sk);
> > -       struct sockaddr_alg *sa = (void *)uaddr;
> > +       struct sockaddr_alg_new *sa = (void *)uaddr;
> >         const struct af_alg_type *type;
> >         void *private;
> >         int err;
> > @@ -155,7 +155,11 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
> >         if (sock->state == SS_CONNECTED)
> >                 return -EINVAL;
> >
> > -       if (addr_len < sizeof(*sa))
> > +       BUILD_BUG_ON(offsetof(struct sockaddr_alg_new, salg_name) !=
> > +                    offsetof(struct sockaddr_alg, salg_name));
> > +       BUILD_BUG_ON(offsetof(struct sockaddr_alg, salg_name) != sizeof(*sa));
> > +
> > +       if (addr_len < sizeof(*sa) + 1)
> >                 return -EINVAL;
> >
> >         /* If caller uses non-allowed flag, return error. */
> > @@ -163,7 +167,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
> >                 return -EINVAL;
> >
> >         sa->salg_type[sizeof(sa->salg_type) - 1] = 0;
> > -       sa->salg_name[sizeof(sa->salg_name) + addr_len - sizeof(*sa) - 1] = 0;
> > +       sa->salg_name[addr_len - sizeof(*sa) - 1] = 0;
> 
> This looks like an out-of-bounds write in the case `addr_len ==
> sizeof(struct sockaddr_storage)`.

I think you mean addr_len == sizeof(*sa)?  That's what the
'if (addr_len < sizeof(*sa) + 1) return -EINVAL' above is for.

- Eric