[PATCH 2/2] drm: Prevent drm_copy_field() to attempt copying a NULL pointer

[Javier Martinez Canillas]

Hello Thomas,

Thanks for your feedback.

On 7/4/22 14:30, Thomas Zimmermann wrote:
> Hi
> 
> Am 01.07.22 um 14:07 schrieb Javier Martinez Canillas:
>> The drm_copy_field() helper is used to copy some struct drm_driver fields
>> to userspace through the DRM_IOCTL_VERSION ioctl. But it could be possible
>> that the driver didn't set some of the fields and are instead set to NULL.
>>
>> If that is the case, the drm_copy_field() attempts to copy a NULL pointer:
>>
>> [ +10.395966] Unable to handle kernel access to user memory outside uaccess routines at virtual address 0000000000000000
>> [  +0.010955] Mem abort info:
>> [  +0.002835]   ESR = 0x0000000096000004
>> [  +0.003872]   EC = 0x25: DABT (current EL), IL = 32 bits
>> [  +0.005395]   SET = 0, FnV = 0
>> [  +0.003113]   EA = 0, S1PTW = 0
>> [  +0.003182]   FSC = 0x04: level 0 translation fault
>> [  +0.004964] Data abort info:
>> [  +0.002919]   ISV = 0, ISS = 0x00000004
>> [  +0.003886]   CM = 0, WnR = 0
>> [  +0.003040] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000115dad000
>> [  +0.006536] [0000000000000000] pgd=0000000000000000, p4d=0000000000000000
>> [  +0.006925] Internal error: Oops: 96000004 [#1] SMP
>> ...
>> [  +0.011113] pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>> [  +0.007061] pc : __pi_strlen+0x14/0x150
>> [  +0.003895] lr : drm_copy_field+0x30/0x1a4
>> [  +0.004156] sp : ffff8000094b3a50
>> [  +0.003355] x29: ffff8000094b3a50 x28: ffff8000094b3b70 x27: 0000000000000040
>> [  +0.007242] x26: ffff443743c2ba00 x25: 0000000000000000 x24: 0000000000000040
>> [  +0.007243] x23: ffff443743c2ba00 x22: ffff8000094b3b70 x21: 0000000000000000
>> [  +0.007241] x20: 0000000000000000 x19: ffff8000094b3b90 x18: 0000000000000000
>> [  +0.007241] x17: 0000000000000000 x16: 0000000000000000 x15: 0000aaab14b9af40
>> [  +0.007241] x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
>> [  +0.007239] x11: 0000000000000000 x10: 0000000000000000 x9 : ffffa524ad67d4d8
>> [  +0.007242] x8 : 0101010101010101 x7 : 7f7f7f7f7f7f7f7f x6 : 6c6e6263606e7141
>> [  +0.007239] x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
>> [  +0.007241] x2 : 0000000000000000 x1 : ffff8000094b3b90 x0 : 0000000000000000
>> [  +0.007240] Call trace:
>> [  +0.002475]  __pi_strlen+0x14/0x150
>> [  +0.003537]  drm_version+0x84/0xac
>> [  +0.003448]  drm_ioctl_kernel+0xa8/0x16c
>> [  +0.003975]  drm_ioctl+0x270/0x580
>> [  +0.003448]  __arm64_sys_ioctl+0xb8/0xfc
>> [  +0.003978]  invoke_syscall+0x78/0x100
>> [  +0.003799]  el0_svc_common.constprop.0+0x4c/0xf4
>> [  +0.004767]  do_el0_svc+0x38/0x4c
>> [  +0.003357]  el0_svc+0x34/0x100
>> [  +0.003185]  el0t_64_sync_handler+0x11c/0x150
>> [  +0.004418]  el0t_64_sync+0x190/0x194
>> [  +0.003716] Code: 92402c04 b200c3e8 f13fc09f 5400088c (a9400c02)
>> [  +0.006180] ---[ end trace 0000000000000000 ]---
>>
>> Reported-by: Peter Robinson <pbrobinson at gmail.com>
>> Signed-off-by: Javier Martinez Canillas <javierm at redhat.com>
>> ---
>>
>>   drivers/gpu/drm/drm_ioctl.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
>> index e1b9a03e619c..dacaddc59c82 100644
>> --- a/drivers/gpu/drm/drm_ioctl.c
>> +++ b/drivers/gpu/drm/drm_ioctl.c
>> @@ -474,6 +474,10 @@ static int drm_copy_field(char __user *buf, size_t *buf_len, const char *value)
>>   {
>>   	size_t len;
>>   
>> +	/* don't attempt to copy a NULL pointer */
>> +	if (WARN_ONCE(!value, "BUG: the value to copy was not set!"))
>> +		return -EINVAL;
>> +
> 
> We usually assume that the caller passes the correct arguments. This is 
> different for no reasons. I'd rather not take this patch unless there's 
> a security implication to the ioctl interface (e.g., leaking information 
> because of this NULL ptr).
>

This can lead from an oops (soft panic) to a kernel crash for a buggy driver.

I see from where you are coming from but then I think we should sanitize the
filled struct drm_driver fields in drm_dev_register() and make it fail early.

Would you agree with such a patch? But what I think that we shouldn't allow
is to attempt copying a NULL pointer, if we can easily prevent it.

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat