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

Tvrtko Ursulin
2023-01-04T09:34:48Z

On 03/01/2023 23:49, 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.

Any backtraces from oopses or notes on how it was found to record in the commit message?

> Signed-off-by: Rob Clark <robdclark@chromium.org>

Fixes: a4c1cdd34e2c ("drm/i915/gem: Delay context creation (v3)")
References: 3aa9945a528e ("drm/i915: Separate GEM context construction and registration to userspace")
Cc: <stable@vger.kernel.org> # 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.

> }
>
> 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@intel.com>

Regards,

Tvrtko

> }
>
> struct i915_gem_context *