6.6-stable review patch. If anyone has any objections, please let me know.
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From: Baokun Li libaokun1@huawei.com
commit e2c27b803bb664748e090d99042ac128b3f88d92 upstream.
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@huawei.com Signed-off-by: Baokun Li libaokun1@huawei.com Reviewed-by: Jan Kara jack@suse.cz Cc: Andreas Dilger adilger.kernel@dilger.ca Cc: Christoph Hellwig hch@infradead.org Cc: Dave Chinner david@fromorbit.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Ritesh Harjani (IBM) ritesh.list@gmail.com Cc: Theodore Ts'o tytso@mit.edu Cc: yangerkun yangerkun@huawei.com Cc: Yu Kuai yukuai3@huawei.com Cc: Zhang Yi yi.zhang@huawei.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/filemap.c | 9 +++++++++ 1 file changed, 9 insertions(+)
--- a/mm/filemap.c +++ b/mm/filemap.c @@ -2667,6 +2667,15 @@ 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: */