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Subject[PATCH] f2fs: fix to avoid potential deadlock
Date
There is a potential deadlock reported by syzbot as below:

F2FS-fs (loop2): invalid crc value
F2FS-fs (loop2): Found nat_bits in checkpoint
F2FS-fs (loop2): Mounted with checkpoint version = 48b305e4
======================================================
WARNING: possible circular locking dependency detected
6.1.0-rc8-syzkaller-33330-ga5541c0811a0 #0 Not tainted
------------------------------------------------------
syz-executor.2/32123 is trying to acquire lock:
ffff0000c0e1a608 (&mm->mmap_lock){++++}-{3:3}, at: __might_fault+0x54/0xb4 mm/memory.c:5644

but task is already holding lock:
ffff0001317c6088 (&sbi->sb_lock){++++}-{3:3}, at: f2fs_down_write fs/f2fs/f2fs.h:2205 [inline]
ffff0001317c6088 (&sbi->sb_lock){++++}-{3:3}, at: f2fs_ioc_get_encryption_pwsalt fs/f2fs/file.c:2334 [inline]
ffff0001317c6088 (&sbi->sb_lock){++++}-{3:3}, at: __f2fs_ioctl+0x1370/0x3318 fs/f2fs/file.c:4151

which lock already depends on the new lock.

Chain exists of:
&mm->mmap_lock --> &nm_i->nat_tree_lock --> &sbi->sb_lock

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&sbi->sb_lock);
lock(&nm_i->nat_tree_lock);
lock(&sbi->sb_lock);
lock(&mm->mmap_lock);

Let's try to avoid above deadlock condition by moving __might_fault()
out of sbi->sb_lock coverage.

Fixes: 95fa90c9e5a7 ("f2fs: support recording errors into superblock")
Link: https://lore.kernel.org/linux-f2fs-devel/000000000000cd5fe305ef617fe2@google.com/T/#u
Reported-by: syzbot+4793f6096d174c90b4f7@syzkaller.appspotmail.com
Signed-off-by: Chao Yu <chao@kernel.org>
---
fs/f2fs/file.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index cad4bdd6f097..4bc98dbe8292 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -2336,6 +2336,7 @@ static int f2fs_ioc_get_encryption_pwsalt(struct file *filp, unsigned long arg)
{
struct inode *inode = file_inode(filp);
struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
+ u8 encrypt_pw_salt[16];
int err;

if (!f2fs_sb_has_encrypt(sbi))
@@ -2360,12 +2361,14 @@ static int f2fs_ioc_get_encryption_pwsalt(struct file *filp, unsigned long arg)
goto out_err;
}
got_it:
- if (copy_to_user((__u8 __user *)arg, sbi->raw_super->encrypt_pw_salt,
- 16))
- err = -EFAULT;
+ memcpy(encrypt_pw_salt, sbi->raw_super->encrypt_pw_salt, 16);
out_err:
f2fs_up_write(&sbi->sb_lock);
mnt_drop_write_file(filp);
+
+ if (!err && copy_to_user((__u8 __user *)arg, encrypt_pw_salt, 16))
+ err = -EFAULT;
+
return err;
}

--
2.36.1
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