[PATCH v2] drm/i915: Fix the race between the GEM close and debugfs

[Nikunj A. Dadhania]

As we close GEM object and set file_priv to -EBADF which is protected
by ctx->mutex, populating the GEM debugfs info is not protected
and results in the crash shown below.

Make sure to protect the access to file_priv using ctx->mutex to avoid
race.

BUG: unable to handle page fault for address: ffffffffffffffff
RIP: 0010:i915_gem_object_info+0x26b/0x3eb
Code: 89 44 24 48 48 89 44 24 40 48 89 44 24 38 48 89 44 24 30 48 89 44 24 28 48 89 44 24 20 49 8b 46 f0 48 89 44 24 20 49 8b 46 a0 <48> 8b 58 08 b9 0a 00 00 00 48 b8 aa aa aa aa aa aa aa aa 48 8d bc
RSP: 0018:ffffac81c14cfc30 EFLAGS: 00010246
RAX: fffffffffffffff7 RBX: ffff95094429c218 RCX: ffff95096756c740
RDX: 0000000000000000 RSI: ffffffff919b93ee RDI: ffff95094429c218
RBP: ffffac81c14cfd58 R08: ffff9509746fab80 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: ffff9509753f8e80
R13: ffffac81c14cfc98 R14: ffff95094429c268 R15: ffffac81c14cfc88
FS:  00007a1bdcd52900(0000) GS:ffff950977e00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffff CR3: 000000026b4e0000 CR4: 0000000000340ef0
Call Trace:
 seq_read+0x162/0x3ca
 full_proxy_read+0x5b/0x8d
 __vfs_read+0x45/0x1b9
 vfs_read+0xc9/0x15e
 ksys_read+0x7e/0xde
 do_syscall_64+0x54/0x7e
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7a1bdd34cf03

Signed-off-by: Nikunj A. Dadhania <nikunj.dadhania at linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: stable at vger.kernel.org
---
 drivers/gpu/drm/i915/i915_debugfs.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
index 784219962193..ea469168cd44 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -326,6 +326,7 @@ static void print_context_stats(struct seq_file *m,
 		}
 		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),
@@ -346,6 +347,7 @@ static void print_context_stats(struct seq_file *m,
 
 			print_file_stats(m, name, stats);
 		}
+		mutex_unlock(&ctx->mutex);
 
 		spin_lock(&i915->gem.contexts.lock);
 		list_safe_reset_next(ctx, cn, link);
-- 
2.17.1