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

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

[Rob Clark]

From: Rob Clark <robdclark@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@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).

 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;
-- 
2.38.1

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

[Tvrtko Ursulin]

+ some more people based on e1a7ab4fca0c

On 19/01/2023 17:32, Rob Clark wrote:
> From: Rob Clark <robdclark@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@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?

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;

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

[Matthew Auld]

On Mon, 23 Jan 2023 at 16:57, Tvrtko Ursulin
<tvrtko.ursulin@linux.intel.com> wrote:
>
>
> + some more people based on e1a7ab4fca0c
>
> On 19/01/2023 17:32, Rob Clark wrote:
> > From: Rob Clark <robdclark@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@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@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@vger.kernel.org> # v5.16+

>
> 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;

Re: [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@linux.intel.com> wrote:
>>
>>
>> + some more people based on e1a7ab4fca0c
>>
>> On 19/01/2023 17:32, Rob Clark wrote:
>>> From: Rob Clark <robdclark@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@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@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@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;