Re: [PATCH] ext4: fix race condition between buffer write and page_mkwrite
From: Baokun Li
Date: Mon May 29 2023 - 22:20:24 EST
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
.