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Subject[PATCH] fs: Fix ovl_i_mutex_dir_key/p->lock/cred cred_guard_mutex deadlock
From
These 3 locks are acquired simultaneously in different order causing
deadlock:

https://syzkaller.appspot.com/bug?id=00f119b8bb35a3acbcfafb9d36a2752b364e8d66

======================================================
WARNING: possible circular locking dependency detected
4.19.0-rc5+ #253 Not tainted
------------------------------------------------------
syz-executor1/545 is trying to acquire lock:
00000000b04209e4 (&ovl_i_mutex_dir_key[depth]){++++}, at: inode_lock_shared include/linux/fs.h:748 [inline]
00000000b04209e4 (&ovl_i_mutex_dir_key[depth]){++++}, at: do_last fs/namei.c:3323 [inline]
00000000b04209e4 (&ovl_i_mutex_dir_key[depth]){++++}, at: path_openat+0x250d/0x5160 fs/namei.c:3534

but task is already holding lock:
0000000044500cca (&sig->cred_guard_mutex){+.+.}, at: prepare_bprm_creds+0x53/0x120 fs/exec.c:1404

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #3 (&sig->cred_guard_mutex){+.+.}:
__mutex_lock_common kernel/locking/mutex.c:925 [inline]
__mutex_lock+0x166/0x1700 kernel/locking/mutex.c:1072
mutex_lock_killable_nested+0x16/0x20 kernel/locking/mutex.c:1102
lock_trace+0x4c/0xe0 fs/proc/base.c:384
proc_pid_stack+0x196/0x3b0 fs/proc/base.c:420
proc_single_show+0x101/0x190 fs/proc/base.c:723
seq_read+0x4af/0x1150 fs/seq_file.c:229
do_loop_readv_writev fs/read_write.c:700 [inline]
do_iter_read+0x4a3/0x650 fs/read_write.c:924
vfs_readv+0x175/0x1c0 fs/read_write.c:986
do_preadv+0x1cc/0x280 fs/read_write.c:1070
__do_sys_preadv fs/read_write.c:1120 [inline]
__se_sys_preadv fs/read_write.c:1115 [inline]
__x64_sys_preadv+0x9a/0xf0 fs/read_write.c:1115
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #2 (&p->lock){+.+.}:
__mutex_lock_common kernel/locking/mutex.c:925 [inline]
__mutex_lock+0x166/0x1700 kernel/locking/mutex.c:1072
mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1087
seq_read+0x71/0x1150 fs/seq_file.c:161
do_loop_readv_writev fs/read_write.c:700 [inline]
do_iter_read+0x4a3/0x650 fs/read_write.c:924
vfs_readv+0x175/0x1c0 fs/read_write.c:986
kernel_readv fs/splice.c:362 [inline]
default_file_splice_read+0x53c/0xb20 fs/splice.c:417
do_splice_to+0x12e/0x190 fs/splice.c:881
splice_direct_to_actor+0x270/0x8f0 fs/splice.c:953
do_splice_direct+0x2d4/0x420 fs/splice.c:1062
do_sendfile+0x62a/0xe20 fs/read_write.c:1440
__do_sys_sendfile64 fs/read_write.c:1495 [inline]
__se_sys_sendfile64 fs/read_write.c:1487 [inline]
__x64_sys_sendfile64+0x15d/0x250 fs/read_write.c:1487
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #1 (sb_writers#5){.+.+}:
percpu_down_read_preempt_disable include/linux/percpu-rwsem.h:36 [inline]
percpu_down_read include/linux/percpu-rwsem.h:59 [inline]
__sb_start_write+0x214/0x370 fs/super.c:1387
sb_start_write include/linux/fs.h:1566 [inline]
mnt_want_write+0x3f/0xc0 fs/namespace.c:360
ovl_want_write+0x76/0xa0 fs/overlayfs/util.c:24
ovl_create_object+0x142/0x3a0 fs/overlayfs/dir.c:596
ovl_create+0x2b/0x30 fs/overlayfs/dir.c:627
lookup_open+0x1319/0x1b90 fs/namei.c:3234
do_last fs/namei.c:3324 [inline]
path_openat+0x15e7/0x5160 fs/namei.c:3534
do_filp_open+0x255/0x380 fs/namei.c:3564
do_sys_open+0x568/0x700 fs/open.c:1063
ksys_open include/linux/syscalls.h:1276 [inline]
__do_sys_creat fs/open.c:1121 [inline]
__se_sys_creat fs/open.c:1119 [inline]
__x64_sys_creat+0x61/0x80 fs/open.c:1119
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #0 (&ovl_i_mutex_dir_key[depth]){++++}:
lock_acquire+0x1ed/0x520 kernel/locking/lockdep.c:3900
down_read+0xb0/0x1d0 kernel/locking/rwsem.c:24
inode_lock_shared include/linux/fs.h:748 [inline]
do_last fs/namei.c:3323 [inline]
path_openat+0x250d/0x5160 fs/namei.c:3534
do_filp_open+0x255/0x380 fs/namei.c:3564
do_open_execat+0x221/0x8e0 fs/exec.c:853
__do_execve_file.isra.33+0x173f/0x2540 fs/exec.c:1755
do_execveat_common fs/exec.c:1866 [inline]
do_execve fs/exec.c:1883 [inline]
__do_sys_execve fs/exec.c:1964 [inline]
__se_sys_execve fs/exec.c:1959 [inline]
__x64_sys_execve+0x8f/0xc0 fs/exec.c:1959
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

other info that might help us debug this:

Chain exists of:
&ovl_i_mutex_dir_key[depth] --> &p->lock --> &sig->cred_guard_mutex

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&sig->cred_guard_mutex);
lock(&p->lock);
lock(&sig->cred_guard_mutex);
lock(&ovl_i_mutex_dir_key[depth]);

*** DEADLOCK ***

Solution: I establish this locking order for these locks:

1. ovl_i_mutex_dir_key
2. p->lock
3. sig->cred_guard_mutex

In this change i fix the locking order of exec.c, which is the only
instance that voilates this order.

Signed-off-by: Mina Almasry <almasrymina@google.com>
---
fs/exec.c | 20 ++++++++------------
1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 2e0033348d8e..423d90bc75cc 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1742,6 +1742,12 @@ static int __do_execve_file(int fd, struct filename *filename,
if (retval)
goto out_ret;

+ if (!file)
+ file = do_open_execat(fd, filename, flags);
+ retval = PTR_ERR(file);
+ if (IS_ERR(file))
+ goto out_free;
+
retval = -ENOMEM;
bprm = kzalloc(sizeof(*bprm), GFP_KERNEL);
if (!bprm)
@@ -1754,12 +1760,6 @@ static int __do_execve_file(int fd, struct filename *filename,
check_unsafe_exec(bprm);
current->in_execve = 1;

- if (!file)
- file = do_open_execat(fd, filename, flags);
- retval = PTR_ERR(file);
- if (IS_ERR(file))
- goto out_unmark;
-
sched_exec();

bprm->file = file;
@@ -1775,7 +1775,7 @@ static int __do_execve_file(int fd, struct filename *filename,
fd, filename->name);
if (!pathbuf) {
retval = -ENOMEM;
- goto out_unmark;
+ goto out_free;
}
/*
* Record that a name derived from an O_CLOEXEC fd will be
@@ -1790,7 +1790,7 @@ static int __do_execve_file(int fd, struct filename *filename,

retval = bprm_mm_init(bprm);
if (retval)
- goto out_unmark;
+ goto out_free;

retval = prepare_arg_pages(bprm, argv, envp);
if (retval < 0)
@@ -1840,10 +1840,6 @@ static int __do_execve_file(int fd, struct filename *filename,
mmput(bprm->mm);
}

-out_unmark:
- current->fs->in_exec = 0;
- current->in_execve = 0;
-
out_free:
free_bprm(bprm);
kfree(pathbuf);
--
2.21.0.392.gf8f6787159e-goog
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