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

From: rananta
Date: Tue May 12 2020 - 17:40:03 EST

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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, rananta@xxxxxxxxxxxxxx wrote:
>> On 2020-05-09 23:48, Greg KH wrote:
>>> On Sat, May 09, 2020 at 06:30:56PM -0700, rananta@xxxxxxxxxxxxxx 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 <rananta@xxxxxxxxxxxxxx>
>>>>>> ---
>>>>>> 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 <jslaby@xxxxxxx>
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

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In reply to: Greg KH: "Re: [PATCH] tty: hvc: Fix data abort due to race in hvc_open"
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