[PATCH v2 3/4] ext2: fix race between setxattr and write back

[Ye Bin]

There's a issue as follows:
Block Allocation Reservation Windows Map (ext2_try_to_allocate_with_rsv):
reservation window 0x000000006f105382 start: 0, end: 0
reservation window 0x000000008fd1a555 start: 1044, end: 1059
Window map complete.
kernel BUG at fs/ext2/balloc.c:1158!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN
RIP: 0010:ext2_try_to_allocate_with_rsv.isra.0+0x15c4/0x1800
Call Trace:
 <TASK>
 ext2_new_blocks+0x935/0x1690
 ext2_new_block+0x73/0xa0
 ext2_xattr_set2+0x74f/0x1730
 ext2_xattr_set+0x12b6/0x2260
 ext2_xattr_user_set+0x9c/0x110
 __vfs_setxattr+0x139/0x1d0
 __vfs_setxattr_noperm+0xfc/0x370
 __vfs_setxattr_locked+0x205/0x2c0
 vfs_setxattr+0x19d/0x3b0
 do_setxattr+0xff/0x220
 setxattr+0x123/0x150
 path_setxattr+0x193/0x1e0
 __x64_sys_setxattr+0xc8/0x170
 do_syscall_64+0x35/0x80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

Above issue may happens as follows:
        setxattr                             write back
ext2_xattr_set
  ext2_xattr_set2
    ext2_new_block
      ext2_new_blocks
        ext2_try_to_allocate_with_rsv
          alloc_new_reservation
          --> group=0 [0, 1023] rsv [1016, 1023]
                                        do_writepages
                                          mpage_writepages
                                            write_cache_pages
                                              __mpage_writepage
                                                ext2_get_block
                                                  ext2_get_blocks
                                                   ext2_alloc_branch
                                                    ext2_new_blocks
                                                     ext2_try_to_allocate_with_rsv
                                                       alloc_new_reservation
                                     -->group=1 [1024, 2047] rsv [1044, 1059]
          if ((my_rsv->rsv_start > group_last_block) ||
              (my_rsv->rsv_end < group_first_block)
              rsv_window_dump
              BUG();
Now ext2 mkwrite delay allocate new blocks. So there maybe allocate blocks when
do write back. However, there is no concurrent protection between
ext2_xattr_set() and do_writepages().
To solve about issue don't use reservation window when allocate block for xattr.

Signed-off-by: Ye Bin <yebin10@xxxxxxxxxx>
---
 fs/ext2/balloc.c | 15 +++++++++------
 fs/ext2/xattr.c  |  4 ++--
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/fs/ext2/balloc.c b/fs/ext2/balloc.c
index 99a19f2ecd6f..4ff1d831bb80 100644
--- a/fs/ext2/balloc.c
+++ b/fs/ext2/balloc.c
@@ -1247,13 +1247,16 @@ ext2_fsblk_t ext2_new_blocks(struct inode *inode, ext2_fsblk_t goal,
 	 * it's a regular file, and
 	 * the desired window size is greater than 0 (One could use ioctl
 	 * command EXT2_IOC_SETRSVSZ to set the window size to 0 to turn off
-	 * reservation on that particular file)
+	 * reservation on that particular file), and unspecified Do not use
+	 * reservation window.
 	 */
-	block_i = EXT2_I(inode)->i_block_alloc_info;
-	if (block_i) {
-		windowsz = block_i->rsv_window_node.rsv_goal_size;
-		if (windowsz > 0)
-			my_rsv = &block_i->rsv_window_node;
+	if (!(flag & EXT2_ALLOC_NORESERVE)) {
+		block_i = EXT2_I(inode)->i_block_alloc_info;
+		if (block_i) {
+			windowsz = block_i->rsv_window_node.rsv_goal_size;
+			if (windowsz > 0)
+				my_rsv = &block_i->rsv_window_node;
+		}
 	}
 
 	if (!ext2_has_free_blocks(sbi)) {
diff --git a/fs/ext2/xattr.c b/fs/ext2/xattr.c
index e076386a26f4..21725b8d04c8 100644
--- a/fs/ext2/xattr.c
+++ b/fs/ext2/xattr.c
@@ -743,8 +743,8 @@ ext2_xattr_set2(struct inode *inode, struct buffer_head *old_bh,
 			ext2_fsblk_t goal = ext2_group_first_block_no(sb,
 						EXT2_I(inode)->i_block_group);
 			unsigned long count = 1;
-			int block = ext2_new_blocks(inode, goal, &count,
-						    &error, 0);
+			int block = ext2_new_blocks(inode, goal, &count, &error,
+						    EXT2_ALLOC_NORESERVE);
 			if (error)
 				goto cleanup;
 			ea_idebug(inode, "creating block %d", block);
-- 
2.31.1