[PATCH] fs/ntfs3: Keep write operations atomic

[Lizhi Xu <lizhi.xu@windriver.com>]

syzbot reported a NULL pointer dereference in __generic_file_write_iter. [1]

Before the write operation is completed, the user executes ioctl[2] to clear
the compress flag of the file, which causes the is_compressed() judgment to
return 0, further causing the program to enter the wrong process and call the
wrong ops ntfs_aops_cmpr, which triggers the null pointer dereference of
write_begin.

Use inode lock to synchronize ioctl and write to avoid this case.

[1]
Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
Mem abort info:
  ESR = 0x0000000086000006
  EC = 0x21: IABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
  FSC = 0x06: level 2 translation fault
user pgtable: 4k pages, 48-bit VAs, pgdp=000000011896d000
[0000000000000000] pgd=0800000118b44403, p4d=0800000118b44403, pud=0800000117517403, pmd=0000000000000000
Internal error: Oops: 0000000086000006 [#1] PREEMPT SMP
Modules linked in:
CPU: 0 UID: 0 PID: 6427 Comm: syz-executor347 Not tainted 6.13.0-rc3-syzkaller-g573067a5a685 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : 0x0
lr : generic_perform_write+0x29c/0x868 mm/filemap.c:4055
sp : ffff80009d4978a0
x29: ffff80009d4979c0 x28: dfff800000000000 x27: ffff80009d497bc8
x26: 0000000000000000 x25: ffff80009d497960 x24: ffff80008ba71c68
x23: 0000000000000000 x22: ffff0000c655dac0 x21: 0000000000001000
x20: 000000000000000c x19: 1ffff00013a92f2c x18: ffff0000e183aa1c
x17: 0004060000000014 x16: ffff800083275834 x15: 0000000000000001
x14: 0000000000000000 x13: 0000000000000001 x12: ffff0000c655dac0
x11: 0000000000ff0100 x10: 0000000000ff0100 x9 : 0000000000000000
x8 : 0000000000000000 x7 : 0000000000000000 x6 : 0000000000000000
x5 : ffff80009d497980 x4 : ffff80009d497960 x3 : 0000000000001000
x2 : 0000000000000000 x1 : ffff0000e183a928 x0 : ffff0000d60b0fc0
Call trace:
 0x0 (P)
 __generic_file_write_iter+0xfc/0x204 mm/filemap.c:4156
 ntfs_file_write_iter+0x54c/0x630 fs/ntfs3/file.c:1267
 new_sync_write fs/read_write.c:586 [inline]
 vfs_write+0x920/0xcf4 fs/read_write.c:679
 ksys_write+0x15c/0x26c fs/read_write.c:731
 __do_sys_write fs/read_write.c:742 [inline]
 __se_sys_write fs/read_write.c:739 [inline]
 __arm64_sys_write+0x7c/0x90 fs/read_write.c:739
 __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
 invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:49
 el0_svc_common+0x130/0x23c arch/arm64/kernel/syscall.c:132
 do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
 el0_svc+0x54/0x168 arch/arm64/kernel/entry-common.c:744
 el0t_64_sync_handler+0x84/0x108 arch/arm64/kernel/entry-common.c:762

[2]
ioctl$FS_IOC_SETFLAGS(r0, 0x40086602, &(0x7f00000000c0)=0x20)

Reported-by: syzbot+5d0bdc98770e6c55a0fd@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5d0bdc98770e6c55a0fd
Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
---
 fs/ntfs3/file.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/ntfs3/file.c b/fs/ntfs3/file.c
index 3f96a11804c9..ec6103c97321 100644
--- a/fs/ntfs3/file.c
+++ b/fs/ntfs3/file.c
@@ -1228,6 +1228,12 @@ static ssize_t ntfs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	ssize_t ret;
 	int err;
 
+	if (!inode_trylock(inode)) {
+		if (iocb->ki_flags & IOCB_NOWAIT)
+			return -EAGAIN;
+		inode_lock(inode);
+	}
+
 	err = check_write_restriction(inode);
 	if (err)
 		return err;
@@ -1237,12 +1243,6 @@ static ssize_t ntfs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		return -EOPNOTSUPP;
 	}
 
-	if (!inode_trylock(inode)) {
-		if (iocb->ki_flags & IOCB_NOWAIT)
-			return -EAGAIN;
-		inode_lock(inode);
-	}
-
 	ret = generic_write_checks(iocb, from);
 	if (ret <= 0)
 		goto out;
-- 
2.43.0