linux-ext4 - [PATCH 1/2] ext2: fix race between setxattr and write back

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Date:   Fri, 11 Aug 2023 11:38:56 +0800
From:   Ye Bin <yebin10@...wei.com>
To:     <jack@...e.cz>, <linux-ext4@...r.kernel.org>
CC:     <linux-kernel@...r.kernel.org>, Ye Bin <yebin10@...wei.com>
Subject: [PATCH 1/2] ext2: fix race between setxattr and write back

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 hold '&ei->truncate_mutex' lock when new block for xattr.

Signed-off-by: Ye Bin <yebin10@...wei.com>
---
 fs/ext2/balloc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/ext2/balloc.c b/fs/ext2/balloc.c
index c8049c90323d..039f655febfd 100644
--- a/fs/ext2/balloc.c
+++ b/fs/ext2/balloc.c
@@ -1432,8 +1432,14 @@ ext2_fsblk_t ext2_new_blocks(struct inode *inode, ext2_fsblk_t goal,
 ext2_fsblk_t ext2_new_block(struct inode *inode, unsigned long goal, int *errp)
 {
 	unsigned long count = 1;
+	struct ext2_inode_info *ei = EXT2_I(inode);
+	ext2_fsblk_t ret;
+
+	mutex_lock(&ei->truncate_mutex);
+	ret = ext2_new_blocks(inode, goal, &count, errp);
+	mutex_unlock(&ei->truncate_mutex);
 
-	return ext2_new_blocks(inode, goal, &count, errp);
+	return ret;
 }
 
 #ifdef EXT2FS_DEBUG
-- 
2.31.1