linux-kernel - [PATCH v8 2/5] drm: avoid blocking in drm_clients

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20210712043508.11584-3-desmondcheongzx@gmail.com>
Date:   Mon, 12 Jul 2021 12:35:05 +0800
From:   Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
To:     maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...ux.ie, daniel@...ll.ch,
        sumit.semwal@...aro.org, christian.koenig@....com
Cc:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org, skhan@...uxfoundation.org,
        gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        emil.l.velikov@...il.com
Subject: [PATCH v8 2/5] drm: avoid blocking in drm_clients_info's rcu section

Inside drm_clients_info, the rcu_read_lock is held to lock
pid_task()->comm. However, within this protected section, a call to
drm_is_current_master is made, which involves a mutex lock in a future
patch. However, this is illegal because the mutex lock might block
while in the RCU read-side critical section.

Since drm_is_current_master isn't protected by rcu_read_lock, we avoid
this by moving it out of the RCU critical section.

The following report came from intel-gfx ci's
igt@...ugfs_test@...d_all_entries testcase:

=============================
[ BUG: Invalid wait context ]
5.13.0-CI-Patchwork_20515+ #1 Tainted: G        W
-----------------------------
debugfs_test/1101 is trying to lock:
ffff888132d901a8 (&dev->master_mutex){+.+.}-{3:3}, at:
drm_is_current_master+0x1e/0x50
other info that might help us debug this:
context-{4:4}
3 locks held by debugfs_test/1101:
 #0: ffff88810fdffc90 (&p->lock){+.+.}-{3:3}, at:
 seq_read_iter+0x53/0x3b0
 #1: ffff888132d90240 (&dev->filelist_mutex){+.+.}-{3:3}, at:
 drm_clients_info+0x63/0x2a0
 #2: ffffffff82734220 (rcu_read_lock){....}-{1:2}, at:
 drm_clients_info+0x1b1/0x2a0
stack backtrace:
CPU: 8 PID: 1101 Comm: debugfs_test Tainted: G        W
5.13.0-CI-Patchwork_20515+ #1
Hardware name: Intel Corporation CometLake Client Platform/CometLake S
UDIMM (ERB/CRB), BIOS CMLSFWR1.R00.1263.D00.1906260926 06/26/2019
Call Trace:
 dump_stack+0x7f/0xad
 __lock_acquire.cold.78+0x2af/0x2ca
 lock_acquire+0xd3/0x300
 ? drm_is_current_master+0x1e/0x50
 ? __mutex_lock+0x76/0x970
 ? lockdep_hardirqs_on+0xbf/0x130
 __mutex_lock+0xab/0x970
 ? drm_is_current_master+0x1e/0x50
 ? drm_is_current_master+0x1e/0x50
 ? drm_is_current_master+0x1e/0x50
 drm_is_current_master+0x1e/0x50
 drm_clients_info+0x107/0x2a0
 seq_read_iter+0x178/0x3b0
 seq_read+0x104/0x150
 full_proxy_read+0x4e/0x80
 vfs_read+0xa5/0x1b0
 ksys_read+0x5a/0xd0
 do_syscall_64+0x39/0xb0
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
---
 drivers/gpu/drm/drm_debugfs.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_debugfs.c b/drivers/gpu/drm/drm_debugfs.c
index 3d7182001004..b0a826489488 100644
--- a/drivers/gpu/drm/drm_debugfs.c
+++ b/drivers/gpu/drm/drm_debugfs.c
@@ -91,6 +91,7 @@ static int drm_clients_info(struct seq_file *m, void *data)
 	mutex_lock(&dev->filelist_mutex);
 	list_for_each_entry_reverse(priv, &dev->filelist, lhead) {
 		struct task_struct *task;
+		bool is_current_master = drm_is_current_master(priv);
 
 		rcu_read_lock(); /* locks pid_task()->comm */
 		task = pid_task(priv->pid, PIDTYPE_PID);
@@ -99,7 +100,7 @@ static int drm_clients_info(struct seq_file *m, void *data)
 			   task ? task->comm : "<unknown>",
 			   pid_vnr(priv->pid),
 			   priv->minor->index,
-			   drm_is_current_master(priv) ? 'y' : 'n',
+			   is_current_master ? 'y' : 'n',
 			   priv->authenticated ? 'y' : 'n',
 			   from_kuid_munged(seq_user_ns(m), uid),
 			   priv->magic);
-- 
2.25.1