[PATCH v2 2/4] f2fs: require write permission for F2FS_IOC_RELEASE_COMPRESS_BLOCKS

[Hungyu Lin <dennylin0707@gmail.com>]

F2FS_IOC_RELEASE_COMPRESS_BLOCKS does not check who the caller is, so
any user who can open a compressed file, even read-only, can release
its compressed blocks. After that, writes to the file fail with -EPERM
until the blocks are reserved again or the file is truncated to zero.

Require write permission on the inode. inode_permission() does not
depend on how the file was opened, so a caller with write permission
can still release the blocks through a read-only file descriptor, as
f2fs_io and Android do.

Fixes: ef8d563f184e ("f2fs: introduce F2FS_IOC_RELEASE_COMPRESS_BLOCKS")
Suggested-by: Chao Yu <chao@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
 fs/f2fs/file.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index cfd24a5fa271..93f892b11915 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -4238,6 +4238,9 @@ static int f2fs_release_compress_blocks(struct file *filp, unsigned long arg)
 	if (f2fs_readonly(sbi->sb))
 		return -EROFS;
 
+	if (inode_permission(file_mnt_idmap(filp), inode, MAY_WRITE))
+		return -EACCES;
+
 	ret = mnt_want_write_file(filp);
 	if (ret)
 		return ret;
-- 
2.43.0