From 9ecd10b7a0270803fd5f36ab93173e6d5b41b895 Mon Sep 17 00:00:00 2001 From: Eryu Guan Date: Fri, 27 May 2016 14:27:18 -0700 Subject: [PATCH] direct-io: fix direct write stale data exposure from concurrent buffered read Currently direct writes inside i_size on a DIO_SKIP_HOLES filesystem are not allowed to allocate blocks(get_more_blocks() sets 'create' to 0 before calling get_block() callback), if it's a sparse file, direct writes fall back to buffered writes to avoid stale data exposure from concurrent buffered read. But there're two cases that can result in stale data exposure are not correctly detected. 1. The detection for "writing inside i_size" is not sufficient, writes can be treated as "extending writes" wrongly. For example, direct write 1FSB (file system block) to a 1FSB sparse file on ext2/3/4, starting from offset 0, in this case it's writing inside i_size, but 'create' is non-zero, because 'block_in_file' and '(i_size_read(inode) >> blkbits' are both zero. 2. Direct writes starting from or beyong i_size (not inside i_size) also could trigger block allocation and expose stale data. For example, consider a sparse file with i_size of 2k, and a write to offset 2k or 3k into the file, with a filesystem block size of 4k. (Thanks to Jeff Moyer for pointing this case out in his review.) The first problem can be demostrated by running ltp-aiodio test ADSP045 many times. When testing on extN filesystems, I see test failures occasionally, buffered read could read non-zero (stale) data. ADSP045: dio_sparse -a 4k -w 4k -s 2k -n 1 dio_sparse 0 TINFO : Dirtying free blocks dio_sparse 0 TINFO : Starting I/O tests non zero buffer at buf[0] => 0xffffffaa,ffffffaa,ffffffaa,ffffffaa non-zero read at offset 0 dio_sparse 0 TINFO : Killing childrens(s) dio_sparse 1 TFAIL : dio_sparse.c:191: 1 children(s) exited abnormally The second problem can also be reproduced easily by a hacked dio_sparse program, which accepts an option to specify the write offset. What we should really do is to disable block allocation for writes that could result in filling holes inside i_size. Link: http://lkml.kernel.org/r/1463156728-13357-1-git-send-email-guaneryu@gmail.com Reviewed-by: Jan Kara Signed-off-by: Eryu Guan Cc: Al Viro Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/direct-io.c | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-) diff --git a/fs/direct-io.c b/fs/direct-io.c index 3bf3f20f8ecc..f3b4408be590 100644 --- a/fs/direct-io.c +++ b/fs/direct-io.c @@ -628,11 +628,11 @@ static int get_more_blocks(struct dio *dio, struct dio_submit *sdio, map_bh->b_size = fs_count << i_blkbits; /* - * For writes inside i_size on a DIO_SKIP_HOLES filesystem we - * forbid block creations: only overwrites are permitted. - * We will return early to the caller once we see an - * unmapped buffer head returned, and the caller will fall - * back to buffered I/O. + * For writes that could fill holes inside i_size on a + * DIO_SKIP_HOLES filesystem we forbid block creations: only + * overwrites are permitted. We will return early to the caller + * once we see an unmapped buffer head returned, and the caller + * will fall back to buffered I/O. * * Otherwise the decision is left to the get_blocks method, * which may decide to handle it or also return an unmapped @@ -640,8 +640,8 @@ static int get_more_blocks(struct dio *dio, struct dio_submit *sdio, */ create = dio->rw & WRITE; if (dio->flags & DIO_SKIP_HOLES) { - if (sdio->block_in_file < (i_size_read(dio->inode) >> - sdio->blkbits)) + if (fs_startblk <= ((i_size_read(dio->inode) - 1) >> + i_blkbits)) create = 0; } -- 2.20.1