Re: [PATCH] ext4: fix race condition between buffer write and page_mkwrite

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On 2023/5/29 22:44, Jan Kara wrote:
On Mon 29-05-23 16:01:48, Baokun Li wrote:
Syzbot reported a BUG_ON:
==================================================================
EXT4-fs (loop0): mounted filesystem without journal. Quota mode: none.
EXT4-fs error (device loop0): ext4_mb_generate_buddy:1098: group 0, block
      bitmap and bg descriptor inconsistent: 25 vs 150994969 free clusters
------------[ cut here ]------------
kernel BUG at fs/ext4/ext4_jbd2.c:53!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 494 Comm: syz-executor.0 6.1.0-rc7-syzkaller-ga4412fdd49dc #0
RIP: 0010:__ext4_journal_stop+0x1b3/0x1c0
  [...]
Call Trace:
  ext4_write_inline_data_end+0xa39/0xdf0
  ext4_da_write_end+0x1e2/0x950
  generic_perform_write+0x401/0x5f0
  ext4_buffered_write_iter+0x35f/0x640
  ext4_file_write_iter+0x198/0x1cd0
  vfs_write+0x8b5/0xef0
  [...]
==================================================================

The above BUG_ON is triggered by the following race:

            cpu1                    cpu2
________________________|________________________
ksys_write
  vfs_write
   new_sync_write
    ext4_file_write_iter
     ext4_buffered_write_iter
      generic_perform_write
       ext4_da_write_begin
                           do_fault
                            do_page_mkwrite
                             ext4_page_mkwrite
                              ext4_convert_inline_data
                               ext4_convert_inline_data_nolock
                                ext4_destroy_inline_data_nolock
                                 //clear EXT4_STATE_MAY_INLINE_DATA
                                ext4_map_blocks --> return error
        ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA)
        ext4_block_write_begin
                                ext4_restore_inline_data
                                 // set EXT4_STATE_MAY_INLINE_DATA
       ext4_da_write_end
        ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA)
        ext4_write_inline_data_end
         handle=NULL
         ext4_journal_stop(handle)
          __ext4_journal_stop
           ext4_put_nojournal(handle)
            ref_cnt = (unsigned long)handle
            BUG_ON(ref_cnt == 0)  ---> BUG_ON

The root cause of this problem is that the ext4_convert_inline_data() in
ext4_page_mkwrite() does not grab i_rwsem, so it may race with
ext4_buffered_write_iter() and cause the write_begin() and write_end()
functions to be inconsistent and trigger BUG_ON.

To solve the above issue, we cannot add inode_lock directly to
ext4_page_mkwrite(), because this function is a hot path and frequent calls
to inode_lock will cause performance degradation for multi-threaded reads
and writes. Hence, we move ext4_convert_inline_data() to ext4_file_mmap(),
and only when inline_data is enabled and mmap a file in shared write mode,
we hold the lock to convert, which can reduce the impact on performance.

Reported-by: Jun Nie <jun.nie@xxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/63903521.5040307@xxxxxxxxxx/t/
Reported-by: syzbot+a158d886ca08a3fecca4@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?id=899b37f20ce4072bcdfecfe1647b39602e956e36
Fixes: 7b4cc9787fe3 ("ext4: evict inline data when writing to memory map")
CC: stable@xxxxxxxxxxxxxxx # 4.12+
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
Thanks for the patch! The problem with i_rwsem in ext4_page_mkwrite() is
not so much about performance as about lock ordering. In
ext4_page_mkwrite() we are called with mmap_sem held and so we cannot
acquire i_rwsem because it ranks about it.

Thank you for your review!

I'm sorry I didn't make myself clear here.

Yes, we can't get i_rwsem after holding mmap_sem at any time, otherwise
ABBA deadlock may occur. The "add inode_lock directly" in my patch
description actually looks like this:
```
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index b98d2d58b900..c9318dc2a613 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -6025,12 +6025,14 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
        sb_start_pagefault(inode->i_sb);
        file_update_time(vma->vm_file);

-       filemap_invalidate_lock_shared(mapping);
-
+       inode_lock(inode);
        err = ext4_convert_inline_data(inode);
+       inode_unlock(inode);
        if (err)
                goto out_ret;

+       filemap_invalidate_lock_shared(mapping);
+
        /*
         * On data journalling we skip straight to the transaction handle:
         * there's no delalloc; page truncated will be checked later; the
```
Originally, ext4_page_mkwrite could be freely concurrent with write,
but after adding inode_lock as in the above scenario, the two would
compete for inode_lock, thus having a performance impact on the
scenario where ext4_page_mkwrite and write are called at the same
time. The ext4_file_map will definitely be called before calling
ext4_page_mkwrite, and ext4_file_map will be called much less often,
so the logic for locking and performing convert is moved here to
reduce the performance impact.


---
  fs/ext4/file.c  | 24 +++++++++++++++++++++++-
  fs/ext4/inode.c |  4 ----
  2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index d101b3b0c7da..7a04376c33f2 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -795,7 +795,8 @@ static const struct vm_operations_struct ext4_file_vm_ops = {
  static int ext4_file_mmap(struct file *file, struct vm_area_struct *vma)
  {
  	struct inode *inode = file->f_mapping->host;
-	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
+	struct super_block *sb = inode->i_sb;
+	struct ext4_sb_info *sbi = EXT4_SB(sb);
  	struct dax_device *dax_dev = sbi->s_daxdev;
if (unlikely(ext4_forced_shutdown(sbi)))
@@ -808,6 +809,27 @@ static int ext4_file_mmap(struct file *file, struct vm_area_struct *vma)
  	if (!daxdev_mapping_supported(vma, dax_dev))
  		return -EOPNOTSUPP;
+ /*
+	 * Writing via mmap has no logic to handle inline data, so we
+	 * need to call ext4_convert_inline_data() to convert the inode
+	 * to normal format before doing so, otherwise a BUG_ON will be
+	 * triggered in ext4_writepages() due to the
+	 * EXT4_STATE_MAY_INLINE_DATA flag. Moreover, we need to grab
+	 * i_rwsem during conversion, since clearing and setting the
+	 * inline data flag may race with ext4_buffered_write_iter()
+	 * to trigger a BUG_ON.
+	 */
+	if (ext4_has_feature_inline_data(sb) &&
+	    vma->vm_flags & VM_SHARED && vma->vm_flags & VM_WRITE) {
Sadly this does not work because we can mmap(2) the file read-only and then
use mprotect(2) to make file writeable.

I'm very sorry I didn't take this situation into account.

Thank you for pointing out the problem!

But we can test for VM_MAYWRITE
which gets set when mapping can be made writeable (basically anytime when
the file descriptor itself is writeable).

Otherwise the patch looks good.

								Honza

Indeed!

I will send a patch V2 with the changes suggested by you.

Thanks again!

--
With Best Regards,
Baokun Li
.



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