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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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@chromium.org>
>>>
>>> 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@chromium.org>
>>
>> Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
>>
>> 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@vger.kernel.org> # 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