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@xxxxxxxxxxxxxxx> wrote:
>
>
> + some more people based on e1a7ab4fca0c
>
> On 19/01/2023 17:32, Rob Clark wrote:
> > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxx>
> > ---
> > 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@xxxxxxxxx>

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@xxxxxxxxxxxxxxx> # 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;