Commit edffc217 authored Feb 26, 2016 by Guillaume Nault Committed by David S. Miller Mar 01, 2016

ppp: lock ppp->flags in ppp_read() and ppp_poll()

ppp_read() and ppp_poll() can be called concurrently with ppp_ioctl().
In this case, ppp_ioctl() might call ppp_ccp_closed(), which may update
ppp->flags while ppp_read() or ppp_poll() is reading it.
The update done by ppp_ccp_closed() isn't atomic due to the bit mask
operation ('ppp->flags &= ~(SC_CCP_OPEN | SC_CCP_UP)'), so concurrent
readers might get transient values.
Reading incorrect ppp->flags may disturb the 'ppp->flags & SC_LOOP_TRAFFIC'
test in ppp_read() and ppp_poll(), which in turn can lead to improper
decision on whether the PPP unit file is ready for reading or not.

Since ppp_ccp_closed() is protected by the Rx and Tx locks (with
ppp_lock()), taking the Rx lock is enough for ppp_read() and ppp_poll()
to guarantee that ppp_ccp_closed() won't update ppp->flags
concurrently.

The same reasoning applies to ppp->n_channels. The 'n_channels' field
can also be written to concurrently by ppp_ioctl() (through
ppp_connect_channel() or ppp_disconnect_channel()). These writes aren't
atomic (simple increment/decrement), but are protected by both the Rx
and Tx locks (like in the ppp->flags case). So holding the Rx lock
before reading ppp->n_channels also prevents concurrent writes.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent a7f4b988

drivers/net/ppp/ppp_generic.c

+9 −1

Original line number	Diff line number	Diff line
		@@ -443,10 +443,15 @@ static ssize_t ppp_read(struct file file, char __user buf,
		* network traffic (demand mode).
		*/
		struct ppp *ppp = PF_TO_PPP(pf);

		ppp_recv_lock(ppp);
		if (ppp->n_channels == 0 &&
		(ppp->flags & SC_LOOP_TRAFFIC) == 0)
		(ppp->flags & SC_LOOP_TRAFFIC) == 0) {
		ppp_recv_unlock(ppp);
		break;
		}
		ppp_recv_unlock(ppp);
		}
		ret = -EAGAIN;
		if (file->f_flags & O_NONBLOCK)
		break;
		@@ -532,9 +537,12 @@ static unsigned int ppp_poll(struct file file, poll_table wait)
		else if (pf->kind == INTERFACE) {
		/* see comment in ppp_read */
		struct ppp *ppp = PF_TO_PPP(pf);

		ppp_recv_lock(ppp);
		if (ppp->n_channels == 0 &&
		(ppp->flags & SC_LOOP_TRAFFIC) == 0)
		mask \|= POLLIN \| POLLRDNORM;
		ppp_recv_unlock(ppp);
		}

		return mask;