[PATCH] f2fs: fix to avoid mmap vs set_compress_option case

[Chao Yu <chao@kernel.org>]

Compression option in inode should not be changed after they have
been used, however, it may happen in below race case:

Thread A				Thread B
- f2fs_ioc_set_compress_option
 - check f2fs_is_mmap_file()
 - check get_dirty_pages()
 - check F2FS_HAS_BLOCKS()
					- f2fs_file_mmap
					 - set_inode_flag(FI_MMAP_FILE)
					- fault
					 - do_page_mkwrite
					  - f2fs_vm_page_mkwrite
					  - f2fs_get_block_locked
					 - fault_dirty_shared_page
					  - set_page_dirty
 - update i_compress_algorithm
 - update i_log_cluster_size
 - update i_cluster_size

Avoid such race condition by covering f2fs_file_mmap() w/ inode lock.

Fixes: e1e8debec656 ("f2fs: add F2FS_IOC_SET_COMPRESS_OPTION ioctl")
Signed-off-by: Chao Yu <chao@kernel.org>
---
 fs/f2fs/file.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 7b097ab2f5e4..685ded62fc28 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -519,17 +519,25 @@ static loff_t f2fs_llseek(struct file *file, loff_t offset, int whence)
 static int f2fs_file_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	struct inode *inode = file_inode(file);
+	int ret = 0;
 
 	if (unlikely(f2fs_cp_error(F2FS_I_SB(inode))))
 		return -EIO;
 
-	if (!f2fs_is_compress_backend_ready(inode))
-		return -EOPNOTSUPP;
+	inode_lock(inode);
+
+	if (!f2fs_is_compress_backend_ready(inode)) {
+		ret = -EOPNOTSUPP;
+		goto out_unlock;
+	}
 
 	file_accessed(file);
 	vma->vm_ops = &f2fs_file_vm_ops;
 	set_inode_flag(inode, FI_MMAP_FILE);
-	return 0;
+
+out_unlock:
+	inode_unlock(inode);
+	return ret;
 }
 
 static int f2fs_file_open(struct inode *inode, struct file *filp)
-- 
2.40.1