[PATCH 5.8 62/70] drm/i915/gem: Serialise debugfs i915_gem_objects with ctx->mutex

[Greg Kroah-Hartman]

From: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>

commit 4fe9af8e881d946bf60790eeb37a7c4f96e28382 upstream.

Since the debugfs may peek into the GEM contexts as the corresponding
client/fd is being closed, we may try and follow a dangling pointer.
However, the context closure itself is serialised with the ctx->mutex,
so if we hold that mutex as we inspect the state coupled in the context,
we know the pointers within the context are stable and will remain valid
as we inspect their tables.

Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: CQ Tang <cq.tang@xxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/20200723172119.17649-3-chris@xxxxxxxxxxxxxxxxxx
(cherry picked from commit 102f5aa491f262c818e607fc4fee08a724a76c69)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/gpu/drm/i915/i915_debugfs.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -323,6 +323,7 @@ static void print_context_stats(struct s
 		}
 		i915_gem_context_unlock_engines(ctx);
 
+		mutex_lock(&ctx->mutex);
 		if (!IS_ERR_OR_NULL(ctx->file_priv)) {
 			struct file_stats stats = {
 				.vm = rcu_access_pointer(ctx->vm),
@@ -343,6 +344,7 @@ static void print_context_stats(struct s
 
 			print_file_stats(m, name, stats);
 		}
+		mutex_unlock(&ctx->mutex);
 
 		spin_lock(&i915->gem.contexts.lock);
 		list_safe_reset_next(ctx, cn, link);