[PATCH] drm/i915: Fix potential context UAFs

Rob Clark
2023-01-03T23:50:13Z

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>
---
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