Messages in this thread | | | Date | Fri, 6 Jul 2018 12:39:28 -0600 | From | Tycho Andersen <> | Subject | Re: [PATCH v3] uart: fix race between uart_put_char() and uart_shutdown() |
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On Fri, Jul 06, 2018 at 07:49:09PM +0300, Andy Shevchenko wrote: > On Fri, Jul 6, 2018 at 7:24 PM, Tycho Andersen <tycho@tycho.ws> wrote: > > > Looking in uart_port_startup(), it seems that circ->buf (state->xmit.buf) > > protected by the "per-port mutex", which based on uart_port_check() is > > state->port.mutex. Indeed, the lock acquired in uart_put_char() is > > uport->lock, i.e. not the same lock. > > > > Anyway, since the lock is not acquired, if uart_shutdown() is called, the > > last chunk of that function may release state->xmit.buf before its assigned > > to null, and cause the race above. > > > > To fix it, let's lock uport->lock when allocating/deallocating > > state->xmit.buf in addition to the per-port mutex. > > Thanks for fixing this! > > Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com> > > Some nitpicks though. > > > + unsigned long page, flags = 0; > > I would rather put on separate lines and btw assignment is not needed. > It all goes through macros.
Sure, I can split it up, but without the initialization I get,
CC drivers/tty/serial/serial_core.o In file included from ./include/linux/seqlock.h:36:0, from ./include/linux/time.h:6, from ./include/linux/stat.h:19, from ./include/linux/module.h:10, from drivers/tty/serial/serial_core.c:10: drivers/tty/serial/serial_core.c: In function ‘uart_startup.part.20’: ./include/linux/spinlock.h:260:3: warning: ‘flags’ may be used uninitialized in this function -Wmaybe-uninitialized] _raw_spin_unlock_irqrestore(lock, flags); \ ^~~~~~~~~~~~~~~~~~~~~~~~~~~ drivers/tty/serial/serial_core.c:184:22: note: ‘flags’ was declared here unsigned long page, flags; ^~~~~
> > - if (!state->xmit.buf) { > > - /* This is protected by the per port mutex */ > > - page = get_zeroed_page(GFP_KERNEL); > > - if (!page) > > - return -ENOMEM; > > + page = get_zeroed_page(GFP_KERNEL); > > + if (!page) > > + return -ENOMEM; > > + if (!state->xmit.buf) { > > state->xmit.buf = (unsigned char *) page; > > uart_circ_clear(&state->xmit); > > + } else { > > + free_page(page); > > } > > I see original code, but since you are adding else, does it make sense > to switch to positive condition?
Sure, I'll switch it.
> > + unsigned long flags = 0; > > Ditto about assignment.
And in this case too,
drivers/tty/serial/serial_core.c:184:22: note: ‘flags’ was declared here unsigned long page, flags; ^~~~~ In file included from ./include/linux/seqlock.h:36:0, from ./include/linux/time.h:6, from ./include/linux/stat.h:19, from ./include/linux/module.h:10, from drivers/tty/serial/serial_core.c:10: drivers/tty/serial/serial_core.c: In function ‘uart_shutdown’: ./include/linux/spinlock.h:260:3: warning: ‘flags’ may be used uninitialized in this function -Wmaybe-uninitialized] _raw_spin_unlock_irqrestore(lock, flags); \ ^~~~~~~~~~~~~~~~~~~~~~~~~~~ drivers/tty/serial/serial_core.c:269:16: note: ‘flags’ was declared here unsigned long flags; ^~~~~
Tycho
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