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

Tvrtko Ursulin
2023-01-05T16:00:22Z

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+

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