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.na...@alphalink.fr>
---
This was patch #2 of the 'ppp: fix locking issues related to ppp_ioctl()'
series. I haven't kept the extra locking of ppp->flags in
ppp_ioctl(PPPIOCGFLAGS), which was added in the original series,
because the ppp_mutex lock ensures we can't enter the PPPIOCSFLAGS case
concurrently.
This is still quite theoretical issue as I've never observed the error
in practice.
drivers/net/ppp/ppp_generic.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index fc8ad00..e8a5936 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -443,9 +443,14 @@ 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)
@@ -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;
--
2.7.0
