[Intel-gfx] [PATCH] drm/i915: Avoid potential vm use-after-free

[Tvrtko Ursulin]

On 27/01/2023 13:10, Matthew Auld wrote:
> On Mon, 23 Jan 2023 at 16:57, Tvrtko Ursulin
> <tvrtko.ursulin at linux.intel.com> wrote:
>>
>>
>> + some more people based on e1a7ab4fca0c
>>
>> On 19/01/2023 17:32, Rob Clark wrote:
>>> From: Rob Clark <robdclark at chromium.org>
>>>
>>> Adding the vm to the vm_xa table makes it visible to userspace, which
>>> could try to race with us to close the vm.  So we need to take our extra
>>> reference before putting it in the table.
>>>
>>> Signed-off-by: Rob Clark <robdclark at chromium.org>
>>> ---
>>> Note, you could list commit e1a7ab4fca0c ("drm/i915: Remove the vm open
>>> count") as the "fixed" commit, but really the issue seems to go back
>>> much further (with the fix needing some backporting in the process).
>>
>> It would probably be rather essential to identify the correct Fixes: tag.
>>
>> Since Thomas, Matt and Niranjana you were directly involved in the patch
>> which changed significantly how this works, perhaps there is something
>> still somewhat easily retrievable from your memory lanes to help with this?
> 
> Sorry for the delay. Fix looks good to me,
> Reviewed-by: Matthew Auld <matthew.auld at intel.com>
> 
> Looking at the git history, the fixes tag I think needs to be:
> 
> Fixes: 9ec8795e7d91 ("drm/i915: Drop __rcu from gem_context->vm")
> Cc: <stable at vger.kernel.org> # v5.16+

As discussed offline this looks correct to me too. Thanks for looking 
into it!

Since the CI was green I have now merged the patch. Thanks for the fix Rob!

Regards,

Tvrtko

P.S. Backport to kernels which do not contain e1a7ab4fca0c ("drm/i915: 
Remove the vm open count"), so 5.16 to 5.18, will require a slightly 
different patch as Matt has also mentioned offline.

> 
>>
>> Regards,
>>
>> Tvrtko
>>
>>>
>>>    drivers/gpu/drm/i915/gem/i915_gem_context.c | 14 +++++++++++---
>>>    1 file changed, 11 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>> index 6250de9b9196..e4b78ab4773b 100644
>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>> @@ -1861,11 +1861,19 @@ static int get_ppgtt(struct drm_i915_file_private *file_priv,
>>>        vm = ctx->vm;
>>>        GEM_BUG_ON(!vm);
>>>
>>> +     /*
>>> +      * Get a reference for the allocated handle.  Once the handle is
>>> +      * visible in the vm_xa table, userspace could try to close it
>>> +      * from under our feet, so we need to hold the extra reference
>>> +      * first.
>>> +      */
>>> +     i915_vm_get(vm);
>>> +
>>>        err = xa_alloc(&file_priv->vm_xa, &id, vm, xa_limit_32b, GFP_KERNEL);
>>> -     if (err)
>>> +     if (err) {
>>> +             i915_vm_put(vm);
>>>                return err;
>>> -
>>> -     i915_vm_get(vm);
>>> +     }
>>>
>>>        GEM_BUG_ON(id == 0); /* reserved for invalid/unassigned ppgtt */
>>>        args->value = id;