Subject: Re: [PATCH] tty: hvc: Fix data abort due to race in hvc_open

From: [email-masked]
Date: Thu, 14 May 2020 16:22:10 -0700

On 2020-05-13 00:04, Greg KH wrote:
> On Tue, May 12, 2020 at 02:39:50PM -0700, [email-masked] wrote:
>> 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, [email-masked] wrote:
>> > > >> On 2020-05-09 23:48, Greg KH wrote:
>> > > >>> On Sat, May 09, 2020 at 06:30:56PM -0700, [email-masked] 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 <[email-masked]>
>> > > >>>>>> ---
>> > > >>>>>>  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 <[email-masked]>
>> > > 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
> 
> Hm, do you have a strace showing the close happening at the same time?
> What about install()?

Yes, I do see the close happening, at which point the issue is not seen.
It's only seen if the second task came in before this close was called. 
For this task, as the file was already opened, it doesn't go through 
hvc_install.

(I figured adding some logs in the drivers would be helpful than straces 
to also include hvc_install)

These are the traces you get when the issue happens:
[  154.212291] hvc_install called for pid: 666
[  154.216705] hvc_open called for pid: 666
[  154.233657] hvc_open: request_irq failed with rc -22.
[  154.238934] hvc_open called for pid: 678
[  154.243012] Unable to handle kernel NULL pointer dereference at 
virtual address 00000000000000c4
# hvc_install isn't called for pid: 678 as the file wasn't closed yet.

And these are the traces we get when things are normal:
[   76.318861] hvc_install called for pid: 537
[   76.323240] hvc_open called for pid: 537
[   76.340177] hvc_open: request_irq failed with rc -22.
[   76.345384] hvc_close called for pid: 537
[   76.349555] hvc_install called for pid: 538
[   76.353921] hvc_open called for pid: 538
# hvc_open for the second task (pid: 538) seems to be fine here as the 
file was closed prior to the second task tried to open the file.

> 
> And what line in hvc_open() does that offset correspond to?
It's the point where it tries to acquire the spinlock for the first 
time: spin_lock_irqsave(&hp->port.lock, flags);

> thanks,
> greg k-h

Thank you.
Raghavendra