[PATCH] Fix sync. in blkdev_write_iter() acessing i_flags

[Alexander Lochmann]

inode.i_flags might be altered without proper
synchronisation when the inode belongs to devtmpfs.
blkdev_write_iter() starts writing via __generic_file_write_iter()
which sets S_NOSEC bit without any synchronisation.
The following stacktrace shows how to get there:
13: entry_SYSENTER_32:460
12: do_fast_syscall_32:410
11: _static_cpu_has:146
10: do_syscall_32_irqs_on:322
09: SyS_pwrite64:636
08: SYSC_pwrite64:650
07: fdput:38
06: vfs_write:560
05: __vfs_write:512
04: new_sync_write:500
03: blkdev_write_iter:1977
02: __generic_file_write_iter:2897
01: file_remove_privs:1818
00: inode_has_no_xattr:3163
If S_NOSEC is *not* set, i_rwsem is acquired around
__generic_file_write_iter().

Found by LockDoc (Alexander Lochmann, Horst Schirmeier and Olaf
Spinczyk)

Signed-off-by: Alexander Lochmann <alexander.lochmann@xxxxxxxxxxxxxx>
Signed-off-by: Horst Schirmeier <horst.schirmeier@xxxxxxxxxxxxxx>
---
 fs/block_dev.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index a80b4f0ee7c4..b4ece62e3c05 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1894,8 +1894,10 @@ ssize_t blkdev_write_iter(struct kiocb *iocb,
struct iov_iter *from)
 {
 	struct file *file = iocb->ki_filp;
 	struct inode *bd_inode = bdev_file_inode(file);
+	struct inode *inode = file_inode(file);
 	loff_t size = i_size_read(bd_inode);
 	struct blk_plug plug;
+	int locked = 0;
 	ssize_t ret;

 	if (bdev_read_only(I_BDEV(bd_inode)))
@@ -1913,9 +1915,19 @@ ssize_t blkdev_write_iter(struct kiocb *iocb,
struct iov_iter *from)
 	iov_iter_truncate(from, size - iocb->ki_pos);

 	blk_start_plug(&plug);
+	/*
+	 * Ensure excl. access to i_flags in __generic_file_write_iter().
+	 * Otherwise, it would race with chmod adding SUID bit.
+	 */
+	if (!IS_NOSEC(inode)) {
+		inode_lock(inode);
+		locked = 1;
+	}
 	ret = __generic_file_write_iter(iocb, from);
 	if (ret > 0)
 		ret = generic_write_sync(iocb, ret);
+	if (locked)
+		inode_unlock(inode);
 	blk_finish_plug(&plug);
 	return ret;
 }
-- 
2.19.2

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