The patch titled
Subject: mm/filemap: avoid buffered read/write race to read inconsistent data
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-filemap-avoid-buffered-read-write-race-to-read-inconsistent-data.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-filemap-avoid-buffered-read-write-race-to-read-inconsistent-data.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days
------------------------------------------------------
From: Baokun Li <libaokun1@xxxxxxxxxx>
Subject: mm/filemap: avoid buffered read/write race to read inconsistent data
Date: Wed, 13 Dec 2023 14:23:24 +0800
The following concurrency may cause the data read to be inconsistent with
the data on disk:
cpu1 cpu2
------------------------------|------------------------------
// Buffered write 2048 from 0
ext4_buffered_write_iter
generic_perform_write
copy_page_from_iter_atomic
ext4_da_write_end
ext4_da_do_write_end
block_write_end
__block_commit_write
folio_mark_uptodate
// Buffered read 4096 from 0 smp_wmb()
ext4_file_read_iter set_bit(PG_uptodate, folio_flags)
generic_file_read_iter i_size_write // 2048
filemap_read unlock_page(page)
filemap_get_pages
filemap_get_read_batch
folio_test_uptodate(folio)
ret = test_bit(PG_uptodate, folio_flags)
if (ret)
smp_rmb();
// Ensure that the data in page 0-2048 is up-to-date.
// New buffered write 2048 from 2048
ext4_buffered_write_iter
generic_perform_write
copy_page_from_iter_atomic
ext4_da_write_end
ext4_da_do_write_end
block_write_end
__block_commit_write
folio_mark_uptodate
smp_wmb()
set_bit(PG_uptodate, folio_flags)
i_size_write // 4096
unlock_page(page)
isize = i_size_read(inode) // 4096
// Read the latest isize 4096, but without smp_rmb(), there may be
// Load-Load disorder resulting in the data in the 2048-4096 range
// in the page is not up-to-date.
copy_page_to_iter
// copyout 4096
In the concurrency above, we read the updated i_size, but there is no read
barrier to ensure that the data in the page is the same as the i_size at
this point, so we may copy the unsynchronized page out. Hence adding the
missing read memory barrier to fix this.
This is a Load-Load reordering issue, which only occurs on some weak
mem-ordering architectures (e.g. ARM64, ALPHA), but not on strong
mem-ordering architectures (e.g. X86). And theoretically the problem
doesn't only happen on ext4, filesystems that call filemap_read() but
don't hold inode lock (e.g. btrfs, f2fs, ubifs ...) will have this
problem, while filesystems with inode lock (e.g. xfs, nfs) won't have
this problem.
Link: https://lkml.kernel.org/r/20231213062324.739009-1-libaokun1@xxxxxxxxxx
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: Andreas Dilger <adilger.kernel@xxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
Cc: Theodore Ts'o <tytso@xxxxxxx>
Cc: yangerkun <yangerkun@xxxxxxxxxx>
Cc: Yu Kuai <yukuai3@xxxxxxxxxx>
Cc: Zhang Yi <yi.zhang@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/filemap.c | 9 +++++++++
1 file changed, 9 insertions(+)
--- a/mm/filemap.c~mm-filemap-avoid-buffered-read-write-race-to-read-inconsistent-data
+++ a/mm/filemap.c
@@ -2608,6 +2608,15 @@ ssize_t filemap_read(struct kiocb *iocb,
end_offset = min_t(loff_t, isize, iocb->ki_pos + iter->count);
/*
+ * Pairs with a barrier in
+ * block_write_end()->mark_buffer_dirty() or other page
+ * dirtying routines like iomap_write_end() to ensure
+ * changes to page contents are visible before we see
+ * increased inode size.
+ */
+ smp_rmb();
+
+ /*
* Once we start copying data, we don't want to be touching any
* cachelines that might be contended:
*/
_
Patches currently in -mm which might be from libaokun1@xxxxxxxxxx are
mm-filemap-avoid-buffered-read-write-race-to-read-inconsistent-data.patch
