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

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 05/01/2023 16:00, Tvrtko Ursulin wrote:
> 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+

Pushed to drm-intel-gt-next - thanks for the fix and reviews.

Regards,

Tvrtko

> 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