On 2020-05-12 01:25, Greg KH wrote:
> On Tue, May 12, 2020 at 09:22:15AM +0200, Jiri Slaby wrote:
> > On 11. 05. 20, 9:39, Greg KH wrote:
> > > On Mon, May 11, 2020 at 12:23:58AM -0700, rana...@codeaurora.org wrote:
> > >> On 2020-05-09 23:48, Greg KH wrote:
> > >>> On Sat, May 09, 2020 at 06:30:56PM -0700, rana...@codeaurora.org wrote:
> > >>>> On 2020-05-06 02:48, Greg KH wrote:
> > >>>>> On Mon, Apr 27, 2020 at 08:26:01PM -0700, Raghavendra Rao Ananta
wrote:
> > >>>>>> Potentially, hvc_open() can be called in parallel when two tasks
calls
> > >>>>>> open() on /dev/hvcX. In such a scenario, if the
> > >>>>>> hp->ops->notifier_add()
> > >>>>>> callback in the function fails, where it sets the tty->driver_data to
> > >>>>>> NULL, the parallel hvc_open() can see this NULL and cause a memory
> > >>>>>> abort.
> > >>>>>> Hence, serialize hvc_open and check if tty->private_data is NULL
> > >>>>>> before
> > >>>>>> proceeding ahead.
> > >>>>>>
> > >>>>>> The issue can be easily reproduced by launching two tasks
> > >>>>>> simultaneously
> > >>>>>> that does nothing but open() and close() on /dev/hvcX.
> > >>>>>> For example:
> > >>>>>> $ ./simple_open_close /dev/hvc0 & ./simple_open_close /dev/hvc0 &
> > >>>>>>
> > >>>>>> Signed-off-by: Raghavendra Rao Ananta <rana...@codeaurora.org>
> > >>>>>> ---
> > >>>>>> drivers/tty/hvc/hvc_console.c | 16 ++++++++++++++--
> > >>>>>> 1 file changed, 14 insertions(+), 2 deletions(-)
> > >>>>>>
> > >>>>>> diff --git a/drivers/tty/hvc/hvc_console.c
> > >>>>>> b/drivers/tty/hvc/hvc_console.c
> > >>>>>> index 436cc51c92c3..ebe26fe5ac09 100644
> > >>>>>> --- a/drivers/tty/hvc/hvc_console.c
> > >>>>>> +++ b/drivers/tty/hvc/hvc_console.c
> > >>>>>> @@ -75,6 +75,8 @@ static LIST_HEAD(hvc_structs);
> > >>>>>> */
> > >>>>>> static DEFINE_MUTEX(hvc_structs_mutex);
> > >>>>>>
> > >>>>>> +/* Mutex to serialize hvc_open */
> > >>>>>> +static DEFINE_MUTEX(hvc_open_mutex);
> > >>>>>> /*
> > >>>>>> * This value is used to assign a tty->index value to a hvc_struct
> > >>>>>> based
> > >>>>>> * upon order of exposure via hvc_probe(), when we can not match it
> > >>>>>> to
> > >>>>>> @@ -346,16 +348,24 @@ static int hvc_install(struct tty_driver
> > >>>>>> *driver, struct tty_struct *tty)
> > >>>>>> */
> > >>>>>> static int hvc_open(struct tty_struct *tty, struct file * filp)
> > >>>>>> {
> > >>>>>> - struct hvc_struct *hp = tty->driver_data;
> > >>>>>> + struct hvc_struct *hp;
> > >>>>>> unsigned long flags;
> > >>>>>> int rc = 0;
> > >>>>>>
> > >>>>>> + mutex_lock(&hvc_open_mutex);
> > >>>>>> +
> > >>>>>> + hp = tty->driver_data;
> > >>>>>> + if (!hp) {
> > >>>>>> + rc = -EIO;
> > >>>>>> + goto out;
> > >>>>>> + }
> > >>>>>> +
> > >>>>>> spin_lock_irqsave(&hp->port.lock, flags);
> > >>>>>> /* Check and then increment for fast path open. */
> > >>>>>> if (hp->port.count++ > 0) {
> > >>>>>> spin_unlock_irqrestore(&hp->port.lock, flags);
> > >>>>>> hvc_kick();
> > >>>>>> - return 0;
> > >>>>>> + goto out;
> > >>>>>> } /* else count == 0 */
> > >>>>>> spin_unlock_irqrestore(&hp->port.lock, flags);
> > >>>>>
> > >>>>> Wait, why isn't this driver just calling tty_port_open() instead of
> > >>>>> trying to open-code all of this?
> > >>>>>
> > >>>>> Keeping a single mutext for open will not protect it from close, it
will
> > >>>>> just slow things down a bit. There should already be a tty lock held
by
> > >>>>> the tty core for open() to keep it from racing things, right?
> > >>>> The tty lock should have been held, but not likely across
> > >>>> ->install() and
> > >>>> ->open() callbacks, thus resulting in a race between hvc_install() and
> > >>>> hvc_open(),
> > >>>
> > >>> How? The tty lock is held in install, and should not conflict with
> > >>> open(), otherwise, we would be seeing this happen in all tty drivers,
> > >>> right?
> > >>>
> > >> Well, I was expecting the same, but IIRC, I see that the open() was being
> > >> called in parallel for the same device node.
> > >
> > > So open and install are happening at the same time? And the tty_lock()
> > > does not protect the needed fields from being protected properly? If
> > > not, what fields are being touched without the lock?
> > >
> > >> Is it expected that the tty core would allow only one thread to
> > >> access the dev-node, while blocking the other, or is it the client
> > >> driver's responsibility to handle the exclusiveness?
> > >
> > > The tty core should handle this correctly, for things that can mess
> > > stuff up (like install and open at the same time). A driver should not
> > > have to worry about that.
> > >
> > >>>> where hvc_install() sets a data and the hvc_open() clears it.
> > >>>> hvc_open()
> > >>>> doesn't
> > >>>> check if the data was set to NULL and proceeds.
> > >>>
> > >>> What data is being set that hvc_open is checking?
> > >> hvc_install sets tty->private_data to hp, while hvc_open sets it to NULL
(in
> > >> one of the paths).
> > >
> > > I see no use of private_data in drivers/tty/hvc/ so what exactly are you
> > > referring to?
> >
> > He likely means tty->driver_data. And there exactly lays the issue.
> >
> > commit bdb498c20040616e94b05c31a0ceb3e134b7e829
> > Author: Jiri Slaby <jsl...@suse.cz>
> > Date: Tue Aug 7 21:48:04 2012 +0200
> >
> > TTY: hvc_console, add tty install
> >
> > added hvc_install but did not move 'tty->driver_data = NULL;' from
> > hvc_open's fail path to hvc_cleanup.
> >
> > IOW hvc_open now NULLs tty->driver_data even for another task which
> > opened the tty earlier. The same holds for
> > "tty_port_tty_set(&hp->port,
> > NULL);" there. And actually "tty_port_put(&hp->port);" is also
> > incorrect
> > for the 2nd task opening the tty.
> >
> > So, a mutex with tty->driver_data check in open is not definitely the
> > way to go. This mess needs to be sorted out properly. Sure, a good
> > start
> > would be a conversion to tty_port_open. Right after dropping "tty:
> > hvc:
> > Fix data abort due to race in hvc_open" from tty/tty-next :).
>
> I've now reverted this commit so we can start from a "clean" place.
>
> > What I *don't* understand is why hp->ops->notifier_add fails, given
> > the
> > open does not allow multiple opens anyway?
>
> I don't understand that either. Raghavendra, can you show a real trace
> for this issue that shows this?
>
Let me know if this helps:
[ 265.332900] Unable to handle kernel NULL pointer dereference at
virtual
address 00000000000000a8
[ 265.332920] Mem abort info:
[ 265.332934] ESR = 0x96000006
[ 265.332950] EC = 0x25: DABT (current EL), IL = 32 bits
[ 265.332963] SET = 0, FnV = 0
[ 265.332975] EA = 0, S1PTW = 0
[ 265.332985] Data abort info:
[ 265.332997] ISV = 0, ISS = 0x00000006
[ 265.333008] CM = 0, WnR = 0
[ 265.333025] user pgtable: 4k pages, 39-bit VAs,
pgdp=00000001620f3000
[ 265.333038] [00000000000000a8] pgd=00000001620f2003,
pud=00000001620f2003, pmd=0000000000000000
[ 265.333071] Internal error: Oops: 96000006 [#1] PREEMPT SMP
[ 265.333424] CPU: 1 PID: 5653 Comm: stress-ng-dev Tainted: G S
W O
5.4.12-g04866e0 #1
[ 265.333458] pstate: 80400085 (Nzcv daIf +PAN -UAO)
[ 265.333499] pc : _raw_spin_lock_irqsave+0x40/0x7c
[ 265.333517] lr : _raw_spin_lock_irqsave+0x38/0x7c
[ 265.333530] sp : ffffffc02436ba40
[ 265.333542] x29: ffffffc02436ba40 x28: 0000000000020800
[ 265.333562] x27: ffffffdfb4046490 x26: ffffff8101b83400
[ 265.333580] x25: ffffff80e163ad00 x24: ffffffdfb45c7798
[ 265.333598] x23: ffffff8101b83668 x22: ffffffdfb4974000
[ 265.333617] x21: 0000000000000001 x20: 00000000000000a8
[ 265.333634] x19: 0000000000000000 x18: ffffff80e0b0d460
[ 265.333652] x17: 0000000000000000 x16: 0000000001000000
[ 265.333670] x15: 0000000001000000 x14: 00000000f8000000
[ 265.333688] x13: 0000000000000000 x12: 0000000000000001
[ 265.333706] x11: 17f5f16765f64600 x10: 17f5f16765f64600
[ 265.333724] x9 : ffffffdfb3444244 x8 : 0000000000000000
[ 265.333741] x7 : 0000000000000000 x6 : 0000000000000000
[ 265.333759] x5 : 0000000000000000 x4 : 0000000000000002
[ 265.333776] x3 : ffffffc02436b9c0 x2 : ffffffdfb40456e0
[ 265.333794] x1 : ffffffc02436b9c0 x0 : ffffffdfb3444244
[ 265.333812] Call trace:
[ 265.333831] _raw_spin_lock_irqsave+0x40/0x7c
[ 265.333859] 28$61deaf328f140fd7df47c115ec866fa5+0x28/0x174
[ 265.333882] tty_open$86bd494905ebe22944bf63b711173de3+0x3d0/0x584
[ 265.333921]
chrdev_open$4083aaa799bca8e0e1e0c8dc1947aa96+0x1c4/0x248
[ 265.333940] do_dentry_open+0x258/0x3b0
[ 265.333956] vfs_open+0x2c/0x38
[ 265.333975] path_openat+0x898/0xedc
[ 265.333991] do_filp_open+0x78/0x124
[ 265.334006] do_sys_open+0x13c/0x298
[ 265.334022] __arm64_sys_openat+0x28/0x34
[ 265.334044] el0_svc_common+0xb8/0x1b4
[ 265.334059] el0_svc_handler+0x6c/0x88
[ 265.334079] el0_svc+0x8/0xc
[ 265.334110] Code: 52800035 97b9fec7 aa1f03e8 f9800291 (885ffe81)
[ 265.334130] ---[ end trace ac90e3099a98e99f ]---
[ 265.334146] Kernel panic - not syncing: Fatal exception
