[PATCH 3.16 093/192] CIFS: fix circular locking dependency

Ben Hutchings
2017-10-09T12:55:02Z

From: Rabin Vincent <rabinv@axis.com>

commit 966681c9f029afd5decee069b7658bb58ad0a863 upstream.

When a CIFS filesystem is mounted with the forcemand option and the
following command is run on it, lockdep warns about a circular locking
dependency between CifsInodeInfo::lock_sem and the inode lock.

while echo foo > hello; do :; done & while touch -c hello; do :; done

cifs_writev() takes the locks in the wrong order, but note that we can't
only flip the order around because it releases the inode lock before the
call to generic_write_sync() while it holds the lock_sem across that
call.

But, AFAICS, there is no need to hold the CifsInodeInfo::lock_sem across
the generic_write_sync() call either, so we can release both the locks
before generic_write_sync(), and change the order.

======================================================
WARNING: possible circular locking dependency detected
4.12.0-rc7+ #9 Not tainted
------------------------------------------------------
touch/487 is trying to acquire lock:
(&cifsi->lock_sem){++++..}, at: cifsFileInfo_put+0x88f/0x16a0

but task is already holding lock:
(&sb->s_type->i_mutex_key#11){+.+.+.}, at: utimes_common+0x3ad/0x870

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&sb->s_type->i_mutex_key#11){+.+.+.}:
__lock_acquire+0x1f74/0x38f0
lock_acquire+0x1cc/0x600
down_write+0x74/0x110
cifs_strict_writev+0x3cb/0x8c0
__vfs_write+0x4c1/0x930
vfs_write+0x14c/0x2d0
SyS_write+0xf7/0x240
entry_SYSCALL_64_fastpath+0x1f/0xbe

-> #0 (&cifsi->lock_sem){++++..}:
check_prevs_add+0xfa0/0x1d10
__lock_acquire+0x1f74/0x38f0
lock_acquire+0x1cc/0x600
down_write+0x74/0x110
cifsFileInfo_put+0x88f/0x16a0
cifs_setattr+0x992/0x1680
notify_change+0x61a/0xa80
utimes_common+0x3d4/0x870
do_utimes+0x1c1/0x220
SyS_utimensat+0x84/0x1a0
entry_SYSCALL_64_fastpath+0x1f/0xbe

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&sb->s_type->i_mutex_key#11);
lock(&cifsi->lock_sem);
lock(&sb->s_type->i_mutex_key#11);
lock(&cifsi->lock_sem);

*** DEADLOCK ***

2 locks held by touch/487:
#0: (sb_writers#10){.+.+.+}, at: mnt_want_write+0x41/0xb0
#1: (&sb->s_type->i_mutex_key#11){+.+.+.}, at: utimes_common+0x3ad/0x870

stack backtrace:
CPU: 0 PID: 487 Comm: touch Not tainted 4.12.0-rc7+ #9
Call Trace:
dump_stack+0xdb/0x185
print_circular_bug+0x45b/0x790
__lock_acquire+0x1f74/0x38f0
lock_acquire+0x1cc/0x600
down_write+0x74/0x110
cifsFileInfo_put+0x88f/0x16a0
cifs_setattr+0x992/0x1680
notify_change+0x61a/0xa80
utimes_common+0x3d4/0x870
do_utimes+0x1c1/0x220
SyS_utimensat+0x84/0x1a0
entry_SYSCALL_64_fastpath+0x1f/0xbe

Fixes: 19dfc1f5f2ef03a52 ("cifs: fix the race in cifs_writev()")
Signed-off-by: Rabin Vincent <rabinv@axis.com>
Signed-off-by: Steve French <smfrench@gmail.com>
Acked-by: Pavel Shilovsky <pshilov@microsoft.com>
[bwh: Backported to 3.16:
- Keep using mutex_{,un}lock()
- Update both branches of if (!cifs_find_lock_conflict(...))]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -2593,18 +2593,19 @@ cifs_writev(struct kiocb *iocb, struct i
ssize_t rc = -EACCES;
loff_t lock_pos = iocb->ki_pos;

+ mutex_lock(&inode->i_mutex);
/*
* We need to hold the sem to be sure nobody modifies lock list
* with a brlock that prevents writing.
*/
down_read(&cinode->lock_sem);
- mutex_lock(&inode->i_mutex);
if (file->f_flags & O_APPEND)
lock_pos = i_size_read(inode);
if (!cifs_find_lock_conflict(cfile, lock_pos, iov_iter_count(from),
server->vals->exclusive_lock_type, NULL,
CIFS_WRITE_OP)) {
rc = __generic_file_write_iter(iocb, from);
+ up_read(&cinode->lock_sem);
mutex_unlock(&inode->i_mutex);

if (rc > 0) {
@@ -2615,9 +2616,9 @@ cifs_writev(struct kiocb *iocb, struct i
rc = err;
}
} else {
+ up_read(&cinode->lock_sem);
mutex_unlock(&inode->i_mutex);
}
- up_read(&cinode->lock_sem);
return rc;
}