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Guillaume Nault authored
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>
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