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

From: [email-masked]
Date: Tue, 12 May 2020 14:39:50 -0700

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]  hvc_open$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
> thanks,
> greg k-h

Thank you.
Raghavendra