On Tue, Jun 18, 2019 at 2:41 AM Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
>
> On Tue, 18 Jun 2019 at 11:39, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> > On Tue, Jun 18, 2019 at 2:32 AM Ard Biesheuvel
> > <ard.biesheuvel@xxxxxxxxxx> wrote:
> > >
> > > Some changes to the TCP fastopen code to make it more robust
> > > against future changes in the choice of key/cookie size, etc.
> > >
> > > - Instead of keeping the SipHash key in an untyped u8[] buffer
> > > and casting it to the right type upon use, use the correct
> > > siphash_key_t type directly. This ensures that the key will
> > > appear at the correct alignment if we ever change the way
> > > these data structures are allocated. (Currently, they are
> > > only allocated via kmalloc so they always appear at the
> > > correct alignment)
> > >
> > > - Use DIV_ROUND_UP when sizing the u64[] array to hold the
> > > cookie, so it is always of sufficient size, even when
> > > TCP_FASTOPEN_COOKIE_MAX is no longer a multiple of 8.
> > >
> > > - Add a key length check to tcp_fastopen_reset_cipher(). No
> > > callers exist currently that fail this check (they all pass
> > > compile constant values that equal TCP_FASTOPEN_KEY_LENGTH),
> > > but future changes might create problems, e.g., by leaving part
> > > of the key uninitialized, or overflowing the key buffers.
> > >
> > > Note that none of these are functional changes wrt the current
> > > state of the code.
> > >
> > ...
> >
> > > - memcpy(ctx->key[0], primary_key, len);
> > > + if (unlikely(len != TCP_FASTOPEN_KEY_LENGTH)) {
> > > + pr_err("TCP: TFO key length %u invalid\n", len);
> > > + err = -EINVAL;
> > > + goto out;
> > > + }
> >
> >
> > Why a pr_err() is there ?
> >
> > Can unpriv users flood the syslog ?
>
> They can if they could do so before: there was a call to
> crypto_cipher_setkey() in the original pre-SipHash code which would
> also result in a pr_err() on an invalid key length. That call got
> removed along with the AES cipher handling, and this basically
> reinstates it, as suggested by EricB.
This tcp_fastopen_reset_cipher() function is internal to TCP stack, all callers
always pass the correct length.
We could add checks all over the place, and end up having a TCP stack
full of defensive
checks and 10,000 additional lines of code :/
I would prefer not reinstating this.
