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

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 05/01/2023 15:52, Andi Shyti wrote:
> Hi Rob,
>
> On Tue, Jan 03, 2023 at 03:49:46PM -0800, 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.
> >
> > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
>
> Reviewed-by: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx>
>
> I also agree with Tvrtko that we should add Stable: and Fixes:.

Yeah I'll add them all when merging. Just waiting for full CI results. It will be like this:

Fixes: eb4dedae920a ("drm/i915/gem: Delay tracking the GEM context until it is registered")
Fixes: a4c1cdd34e2c ("drm/i915/gem: Delay context creation (v3)")
Fixes: 49bd54b390c2 ("drm/i915: Track all user contexts per client")
Cc: <stable@xxxxxxxxxxxxxxx> # v5.10+

Regards,

Tvrtko
 
> One little thing, "user after free" is clearer that UAF :)
>
> Thanks,
> Andi
>
> > ---
> >   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);
> >   }
> >   
> >   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;
> >   }
> >   
> >   struct i915_gem_context *
> > --
> > 2.38.1