Re: [PATCH] drm/i915: Fix potential context UAFs

[Rob Clark]

On Wed, Jan 4, 2023 at 1:34 AM Tvrtko Ursulin
<tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
>
>
> On 03/01/2023 23:49, Rob Clark wrote:
> > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> >
> > gem_context_register() makes the context visible to userspace, and which
> > point a separate thread can trigger the I915_GEM_CONTEXT_DESTROY ioctl.
> > So we need to ensure that nothing uses the ctx ptr after this.  And we
> > need to ensure that adding the ctx to the xarray is the *last* thing
> > that gem_context_register() does with the ctx pointer.
>
> Any backtraces from oopses or notes on how it was found to record in the commit message?

It was a UAF bug that was reported to us

https://bugs.chromium.org/p/chromium/issues/detail?id=1401594 (but I
guess security bugs are not going to be visible)

>
> > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
>
> Fixes: a4c1cdd34e2c ("drm/i915/gem: Delay context creation (v3)")
> References: 3aa9945a528e ("drm/i915: Separate GEM context construction and registration to userspace")
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.15+
>
> > ---
> >   drivers/gpu/drm/i915/gem/i915_gem_context.c | 24 +++++++++++++++------
> >   1 file changed, 18 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > index 7f2831efc798..6250de9b9196 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > @@ -1688,6 +1688,10 @@ void i915_gem_init__contexts(struct drm_i915_private *i915)
> >       init_contexts(&i915->gem.contexts);
> >   }
> >
> > +/*
> > + * Note that this implicitly consumes the ctx reference, by placing
> > + * the ctx in the context_xa.
> > + */
> >   static void gem_context_register(struct i915_gem_context *ctx,
> >                                struct drm_i915_file_private *fpriv,
> >                                u32 id)
> > @@ -1703,10 +1707,6 @@ static void gem_context_register(struct i915_gem_context *ctx,
> >       snprintf(ctx->name, sizeof(ctx->name), "%s[%d]",
> >                current->comm, pid_nr(ctx->pid));
> >
> > -     /* And finally expose ourselves to userspace via the idr */
> > -     old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
> > -     WARN_ON(old);
> > -
> >       spin_lock(&ctx->client->ctx_lock);
> >       list_add_tail_rcu(&ctx->client_link, &ctx->client->ctx_list);
> >       spin_unlock(&ctx->client->ctx_lock);
> > @@ -1714,6 +1714,10 @@ static void gem_context_register(struct i915_gem_context *ctx,
> >       spin_lock(&i915->gem.contexts.lock);
> >       list_add_tail(&ctx->link, &i915->gem.contexts.list);
> >       spin_unlock(&i915->gem.contexts.lock);
> > +
> > +     /* And finally expose ourselves to userspace via the idr */
> > +     old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
> > +     WARN_ON(old);
>
> Have you seen that this hunk is needed or just moving it for a good measure? To be clear, it is probably best to move it even if the current placement cannot cause any problems, I am just double-checking if you had any concrete observations here while mulling over easier stable backports if we would omit it.
>

This was actually the originally reported issue, the
finalize_create_context_locked() part was something I found when the
original report prompted me to audit gem_context_register() call
paths.


> >   }
> >
> >   int i915_gem_context_open(struct drm_i915_private *i915,
> > @@ -2199,14 +2203,22 @@ finalize_create_context_locked(struct drm_i915_file_private *file_priv,
> >       if (IS_ERR(ctx))
> >               return ctx;
> >
> > +     /*
> > +      * One for the xarray and one for the caller.  We need to grab
> > +      * the reference *prior* to making the ctx visble to userspace
> > +      * in gem_context_register(), as at any point after that
> > +      * userspace can try to race us with another thread destroying
> > +      * the context under our feet.
> > +      */
> > +     i915_gem_context_get(ctx);
> > +
> >       gem_context_register(ctx, file_priv, id);
> >
> >       old = xa_erase(&file_priv->proto_context_xa, id);
> >       GEM_BUG_ON(old != pc);
> >       proto_context_close(file_priv->dev_priv, pc);
> >
> > -     /* One for the xarray and one for the caller */
> > -     return i915_gem_context_get(ctx);
> > +     return ctx;
>
> Otherwise userspace can look up a context which hasn't had it's reference count increased yep. I can add the Fixes: and Stable: tags while merging if no complaints.
>
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

Thanks

BR,
-R

>
> Regards,
>
> Tvrtko
>
> >   }
> >
> >   struct i915_gem_context *