Re: [PATCH net] ppp: limit MRU to 64K

On Mon, Nov 13, 2023 at 4:17 AM Willem de Bruijn willemdebruijn.kernel@gmail.com wrote:

From: Willem de Bruijn willemb@google.com

ppp_sync_ioctl allows setting device MRU, but does not sanity check this input.

Limit to a sane upper bound of 64KB.

No implementation I could find generates larger than 64KB frames. RFC 2823 mentions an upper bound of PPP over SDL of 64KB based on the 16-bit length field. Other protocols will be smaller, such as PPPoE (9KB jumbo frame) and PPPoA (18190 maximum CPCS-SDU size, RFC 2364). PPTP and L2TP encapsulate in IP.

Syzbot managed to trigger alloc warning in __alloc_pages:

    if (WARN_ON_ONCE_GFP(order > MAX_ORDER, gfp))

WARNING: CPU: 1 PID: 37 at mm/page_alloc.c:4544 __alloc_pages+0x3ab/0x4a0 mm/page_alloc.c:4544

__alloc_skb+0x12b/0x330 net/core/skbuff.c:651
__netdev_alloc_skb+0x72/0x3f0 net/core/skbuff.c:715
netdev_alloc_skb include/linux/skbuff.h:3225 [inline]
dev_alloc_skb include/linux/skbuff.h:3238 [inline]
ppp_sync_input drivers/net/ppp/ppp_synctty.c:669 [inline]
ppp_sync_receive+0xff/0x680 drivers/net/ppp/ppp_synctty.c:334
tty_ldisc_receive_buf+0x14c/0x180 drivers/tty/tty_buffer.c:390
tty_port_default_receive_buf+0x70/0xb0 drivers/tty/tty_port.c:37
receive_buf drivers/tty/tty_buffer.c:444 [inline]
flush_to_ldisc+0x261/0x780 drivers/tty/tty_buffer.c:494
process_one_work+0x884/0x15c0 kernel/workqueue.c:2630

With call

ioctl$PPPIOCSMRU1(r1, 0x40047452, &(0x7f0000000100)=0x5e6417a8)

Similar code exists in other drivers that implement ppp_channel_ops ioctl PPPIOCSMRU. Those might also be in scope. Notably excluded from this are pppol2tp_ioctl and pppoe_ioctl.

This code goes back to the start of git history.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Reported-by: syzbot+6177e1f90d92583bcc58@syzkaller.appspotmail.com Signed-off-by: Willem de Bruijn willemb@google.com

---

drivers/net/ppp/ppp_synctty.c | 4 ++++ 1 file changed, 4 insertions(+)

diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c index ea261a628786..52d05ce4a281 100644 --- a/drivers/net/ppp/ppp_synctty.c +++ b/drivers/net/ppp/ppp_synctty.c @@ -453,6 +453,10 @@ ppp_sync_ioctl(struct ppp_channel *chan, unsigned int cmd, unsigned long arg) case PPPIOCSMRU: if (get_user(val, (int __user *) argp)) break;

•           if (val > U16_MAX) {
  

•                   err = -EINVAL;
  

•                   break;
  

•           }
            if (val < PPP_MRU)
                    val = PPP_MRU;
            ap->mru = val;
  

-- 2.43.0.rc0.421.g78406f8d94-goog

Reviewed-by: Eric Dumazet edumazet@google.com

It is unclear why ppp_sync_input() does not allocate an skb based on @count argument though...

diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c index ebcdffdf4f0e0193635d2b479e8a9f7a32703509..79cb7916ff03c9b311109372e3d3e434e3669fa6 100644 --- a/drivers/net/ppp/ppp_synctty.c +++ b/drivers/net/ppp/ppp_synctty.c @@ -659,14 +659,14 @@ ppp_sync_input(struct syncppp *ap, const u8 *buf, const u8 *flags, int count) struct sk_buff *skb; unsigned char *p;

- if (count == 0) + if (count <= 0) return;

if (ap->flags & SC_LOG_INPKT) ppp_print_buffer ("receive buffer", buf, count);

/* stuff the chars in the skb */ - skb = dev_alloc_skb(ap->mru + PPP_HDRLEN + 2); + skb = dev_alloc_skb(count + PPP_HDRLEN + 2); if (!skb) { printk(KERN_ERR "PPPsync: no memory (input pkt)\n"); goto err;