[PATCH 3.13 071/187] debugfs: Fix corrupted loop in debugfs_remove_recursive

Kamal Mostafa
2014-09-15T22:59:22Z

From: Steven Rostedt <rostedt@goodmis.org>

commit 485d44022a152c0254dd63445fdb81c4194cbf0e upstream.

[ I'm currently running my tests on it now, and so far, after a few
hours it has yet to blow up. I'll run it for 24 hours which it never
succeeded in the past. ]

The tracing code has a way to make directories within the debugfs file
system as well as deleting them using mkdir/rmdir in the instance
directory. This is very limited in functionality, such as there is
no renames, and the parent directory "instance" can not be modified.
The tracing code creates the instance directory from the debugfs code
and then replaces the dentry->d_inode->i_op with its own to allow
for mkdir/rmdir to work.

When these are called, the d_entry and inode locks need to be released
to call the instance creation and deletion code. That code has its own
accounting and locking to serialize everything to prevent multiple
users from causing harm. As the parent "instance" directory can not
be modified this simplifies things.

I created a stress test that creates several threads that randomly
creates and deletes directories thousands of times a second. The code
stood up to this test and I submitted it a while ago.

Recently I added a new test that adds readers to the mix. While the
instance directories were being added and deleted, readers would read
from these directories and even enable tracing within them. This test
was able to trigger a bug:

general protection fault: 0000 [#1] PREEMPT SMP
Modules linked in: ...
CPU: 3 PID: 17789 Comm: rmdir Tainted: G W 3.15.0-rc2-test+ #41
Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./To be filled by O.E.M., BIOS SDBLI944.86P 05/08/2007
task: ffff88003786ca60 ti: ffff880077018000 task.ti: ffff880077018000
RIP: 0010:[<ffffffff811ed5eb>] [<ffffffff811ed5eb>] debugfs_remove_recursive+0x1bd/0x367
RSP: 0018:ffff880077019df8 EFLAGS: 00010246
RAX: 0000000000000002 RBX: ffff88006f0fe490 RCX: 0000000000000000
RDX: dead000000100058 RSI: 0000000000000246 RDI: ffff88003786d454
RBP: ffff88006f0fe640 R08: 0000000000000628 R09: 0000000000000000
R10: 0000000000000628 R11: ffff8800795110a0 R12: ffff88006f0fe640
R13: ffff88006f0fe640 R14: ffffffff81817d0b R15: ffffffff818188b7
FS: 00007ff13ae24700(0000) GS:ffff88007d580000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 0000003054ec7be0 CR3: 0000000076d51000 CR4: 00000000000007e0
Stack:
ffff88007a41ebe0 dead000000100058 00000000fffffffe ffff88006f0fe640
0000000000000000 ffff88006f0fe678 ffff88007a41ebe0 ffff88003793a000
00000000fffffffe ffffffff810bde82 ffff88006f0fe640 ffff88007a41eb28
Call Trace:
[<ffffffff810bde82>] ? instance_rmdir+0x15b/0x1de
[<ffffffff81132e2d>] ? vfs_rmdir+0x80/0xd3
[<ffffffff81132f51>] ? do_rmdir+0xd1/0x139
[<ffffffff8124ad9e>] ? trace_hardirqs_on_thunk+0x3a/0x3c
[<ffffffff814fea62>] ? system_call_fastpath+0x16/0x1b
Code: fe ff ff 48 8d 75 30 48 89 df e8 c9 fd ff ff 85 c0 75 13 48 c7 c6 b8 cc d2 81 48 c7 c7 b0 cc d2 81 e8 8c 7a f5 ff 48 8b 54 24 08 <48> 8b 82 a8 00 00 00 48 89 d3 48 2d a8 00 00 00 48 89 44 24 08
RIP [<ffffffff811ed5eb>] debugfs_remove_recursive+0x1bd/0x367
RSP <ffff880077019df8>

It took a while, but every time it triggered, it was always in the
same place:

list_for_each_entry_safe(child, next, &parent->d_subdirs, d_u.d_child) {

Where the child->d_u.d_child seemed to be corrupted. I added lots of
trace_printk()s to see what was wrong, and sure enough, it was always
the child's d_u.d_child field. I looked around to see what touches
it and noticed that in __dentry_kill() which calls dentry_free():

static void dentry_free(struct dentry *dentry)
{
/* if dentry was never visible to RCU, immediate free is OK */
if (!(dentry->d_flags & DCACHE_RCUACCESS))
__d_free(&dentry->d_u.d_rcu);
else
call_rcu(&dentry->d_u.d_rcu, __d_free);
}

I also noticed that __dentry_kill() unlinks the child->d_u.child
under the parent->d_lock spin_lock.

Looking back at the loop in debugfs_remove_recursive() it never takes the
parent->d_lock to do the list walk. Adding more tracing, I was able to
prove this was the issue:

ftrace-t-15385 1.... 246662024us : dentry_kill <ffffffff81138b91>: free ffff88006d573600
rmdir-15409 2.... 246662024us : debugfs_remove_recursive <ffffffff811ec7e5>: child=ffff88006d573600 next=dead000000100058

The dentry_kill freed ffff88006d573600 just as the remove recursive was walking
it.

In order to fix this, the list walk needs to be modified a bit to take
the parent->d_lock. The safe version is no longer necessary, as every
time we remove a child, the parent->d_lock must be released and the
list walk must start over. Each time a child is removed, even though it
may still be on the list, it should be skipped by the first check
in the loop:

if (!debugfs_positive(child))
continue;

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
---
fs/debugfs/inode.c | 33 ++++++++++++++++++++++++++-------
1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index 9c0444c..1576195 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -533,7 +533,7 @@ EXPORT_SYMBOL_GPL(debugfs_remove);
*/
void debugfs_remove_recursive(struct dentry *dentry)
{
- struct dentry *child, *next, *parent;
+ struct dentry *child, *parent;

if (IS_ERR_OR_NULL(dentry))
return;
@@ -545,30 +545,49 @@ void debugfs_remove_recursive(struct dentry *dentry)
parent = dentry;
down:
mutex_lock(&parent->d_inode->i_mutex);
- list_for_each_entry_safe(child, next, &parent->d_subdirs, d_u.d_child) {
+ loop:
+ /*
+ * The parent->d_subdirs is protected by the d_lock. Outside that
+ * lock, the child can be unlinked and set to be freed which can
+ * use the d_u.d_child as the rcu head and corrupt this list.
+ */
+ spin_lock(&parent->d_lock);
+ list_for_each_entry(child, &parent->d_subdirs, d_u.d_child) {
if (!debugfs_positive(child))
continue;

/* perhaps simple_empty(child) makes more sense */
if (!list_empty(&child->d_subdirs)) {
+ spin_unlock(&parent->d_lock);
mutex_unlock(&parent->d_inode->i_mutex);
parent = child;
goto down;
}
- up:
+
+ spin_unlock(&parent->d_lock);
+
if (!__debugfs_remove(child, parent))
simple_release_fs(&debugfs_mount, &debugfs_mount_count);
+
+ /*
+ * The parent->d_lock protects agaist child from unlinking
+ * from d_subdirs. When releasing the parent->d_lock we can
+ * no longer trust that the next pointer is valid.
+ * Restart the loop. We'll skip this one with the
+ * debugfs_positive() check.
+ */
+ goto loop;
}
+ spin_unlock(&parent->d_lock);

mutex_unlock(&parent->d_inode->i_mutex);
child = parent;
parent = parent->d_parent;
mutex_lock(&parent->d_inode->i_mutex);

- if (child != dentry) {
- next = list_next_entry(child, d_u.d_child);
- goto up;
- }
+ if (child != dentry)
+ /* go up */
+ goto loop;

if (!__debugfs_remove(child, parent))
simple_release_fs(&debugfs_mount, &debugfs_mount_count);
--
1.9.1