On Tue, Jun 11, 2024 at 2:14 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > On Mon, 10 Jun 2024, Yosry Ahmed wrote: > > On Sat, Jun 8, 2024 at 2:09 PM syzbot > > <syzbot+9c1fe13fcb51574b249b@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > Hello, > > > > > > syzbot found the following issue on: > > > > > > HEAD commit: 614da38e2f7a Merge tag 'hid-for-linus-2024051401' of git:/.. > > > git tree: upstream > > > console output: https://syzkaller.appspot.com/x/log.txt?x=113f3fd6980000 > > > kernel config: https://syzkaller.appspot.com/x/.config?x=f5d2cbf33633f507 > > > dashboard link: https://syzkaller.appspot.com/bug?extid=9c1fe13fcb51574b249b > > > compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40 > > > > > > Unfortunately, I don't have any reproducer for this issue yet. > > > > > > Downloadable assets: > > > disk image: https://storage.googleapis.com/syzbot-assets/89eafb874b71/disk-614da38e.raw.xz > > > vmlinux: https://storage.googleapis.com/syzbot-assets/356000512ad9/vmlinux-614da38e.xz > > > kernel image: https://storage.googleapis.com/syzbot-assets/839c73939115/bzImage-614da38e.xz > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > > > Reported-by: syzbot+9c1fe13fcb51574b249b@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > This doesn't look like a zswap bug, and perhaps not a bug at all but I > > am not sure. Zswap is reading the data in a folio to check if it is > > filled with a repeated pattern, and it is encountering uninitialized > > data. I am not sure if this is expected behavior or not. > > I agree, it doesn't look like a zswap bug: zswap is just following orders. > > > > > Could it just be that the size of the data written by syzbot is not > > divisible by PAGE_SIZE, so part of the final page is left > > uninitialized? Do we keep the unwritten parts of a shmem page > > uninitialized by any chance? > > Shmem can keep uninitialized data pages around, but should be zeroing > what's still uninitialized before it can reach the outside world - > in this case being written to swap. I don't know of a shmem bug here. Thanks for taking a look at this and stating that shmem will not pass uninitialized memory to swap/zswap. > > > > > Hugh, do you mind taking a quick look at whether this is a real bug? > > > > If this is expected behavior, perhaps there is some annotation we can > > use in zswap that it is fine to encounter uninitialized data when > > reading the folio. > > I've not been faced with a KMSAN report before, so I might well be > misunderstanding its language: but this looks like an ext4 "bug" to me. > > I think the story that the three KMSAN stacks are telling is this: > syzbot has an ext4 filesystem on a loop device on a tmpfs file (I do > exactly that too); ext4 is doing some ext4_xattr_inode_write() business, > in the course of which it writes back a not-fully-initialized block to > the loop device; shmem faithfully copies that data into its pagecache, > then later when under memory pressure that page gets "written" out to > zswap: where zswap_is_page_same_filled() takes an interest in the data, > and KMSAN objects because some of it was not originally initialized. > > If that's a correct interpretation, then it's probably not a big deal: > it's probably the uninitialized end of a buffer that's written out, > not any part of the "disk" which ext4 would ever show to a user; but > I do agree with KMSAN that ext4 would do better to clear that area, > rather than accidentally storing someone else's super-secret info. Interesting, I never saw an ext4 filesystem on a loop device on a tmpfs file :) My understanding of the KMSAN stacks seems to match yours. Thanks for looping in ext4 folks, hopefully they will shed more light on this from their end. > > Hugh > > > > > > > > > ===================================================== > > > BUG: KMSAN: uninit-value in zswap_is_page_same_filled mm/zswap.c:1481 [inline] > > > BUG: KMSAN: uninit-value in zswap_store+0x1008/0x2ca0 mm/zswap.c:1553 > > > zswap_is_page_same_filled mm/zswap.c:1481 [inline] > > > zswap_store+0x1008/0x2ca0 mm/zswap.c:1553 > > > swap_writepage+0x126/0x4c0 mm/page_io.c:198 > > > shmem_writepage+0x1826/0x1f70 mm/shmem.c:1518 > > > pageout mm/vmscan.c:660 [inline] > > > shrink_folio_list+0x4a55/0x7910 mm/vmscan.c:1323 > > > evict_folios+0x9d7f/0xcc20 mm/vmscan.c:4537 > > > try_to_shrink_lruvec+0x160e/0x1a50 mm/vmscan.c:4733 > > > shrink_one+0x66f/0xd40 mm/vmscan.c:4772 > > > shrink_many mm/vmscan.c:4835 [inline] > > > lru_gen_shrink_node mm/vmscan.c:4935 [inline] > > > shrink_node+0x4856/0x55f0 mm/vmscan.c:5894 > > > kswapd_shrink_node mm/vmscan.c:6704 [inline] > > > balance_pgdat mm/vmscan.c:6895 [inline] > > > kswapd+0x1eba/0x4460 mm/vmscan.c:7164 > > > kthread+0x3e2/0x540 kernel/kthread.c:389 > > > ret_from_fork+0x6d/0x90 arch/x86/kernel/process.c:147 > > > ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244 > > > > > > Uninit was stored to memory at: > > > memcpy_from_iter lib/iov_iter.c:73 [inline] > > > iterate_bvec include/linux/iov_iter.h:122 [inline] > > > iterate_and_advance2 include/linux/iov_iter.h:249 [inline] > > > iterate_and_advance include/linux/iov_iter.h:271 [inline] > > > __copy_from_iter lib/iov_iter.c:249 [inline] > > > copy_page_from_iter_atomic+0x12b7/0x2ae0 lib/iov_iter.c:481 > > > generic_perform_write+0x4c1/0xc60 mm/filemap.c:3982 > > > shmem_file_write_iter+0x2bd/0x2f0 mm/shmem.c:2920 > > > do_iter_readv_writev+0x7e6/0x960 > > > vfs_iter_write+0x459/0xd00 fs/read_write.c:895 > > > lo_write_bvec drivers/block/loop.c:246 [inline] > > > lo_write_simple drivers/block/loop.c:267 [inline] > > > do_req_filebacked drivers/block/loop.c:491 [inline] > > > loop_handle_cmd drivers/block/loop.c:1907 [inline] > > > loop_process_work+0x1502/0x3440 drivers/block/loop.c:1942 > > > loop_rootcg_workfn+0x2b/0x40 drivers/block/loop.c:1973 > > > process_one_work kernel/workqueue.c:3267 [inline] > > > process_scheduled_works+0xa81/0x1bd0 kernel/workqueue.c:3348 > > > worker_thread+0xea5/0x1560 kernel/workqueue.c:3429 > > > kthread+0x3e2/0x540 kernel/kthread.c:389 > > > ret_from_fork+0x6d/0x90 arch/x86/kernel/process.c:147 > > > ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244 > > > > > > Uninit was created at: > > > __alloc_pages+0x9d6/0xe70 mm/page_alloc.c:4598 > > > alloc_pages_mpol+0x299/0x990 mm/mempolicy.c:2264 > > > alloc_pages mm/mempolicy.c:2335 [inline] > > > folio_alloc+0x1d0/0x230 mm/mempolicy.c:2342 > > > filemap_alloc_folio+0xa6/0x440 mm/filemap.c:984 > > > __filemap_get_folio+0xa10/0x14b0 mm/filemap.c:1926 > > > grow_dev_folio fs/buffer.c:1042 [inline] > > > grow_buffers fs/buffer.c:1108 [inline] > > > __getblk_slow fs/buffer.c:1134 [inline] > > > bdev_getblk+0x39b/0xc80 fs/buffer.c:1429 > > > __getblk include/linux/buffer_head.h:355 [inline] > > > sb_getblk include/linux/buffer_head.h:361 [inline] > > > ext4_getblk+0x3da/0xe00 fs/ext4/inode.c:843 > > > ext4_xattr_inode_write fs/ext4/xattr.c:1421 [inline] > > > ext4_xattr_inode_lookup_create fs/ext4/xattr.c:1596 [inline] > > > ext4_xattr_set_entry+0x574d/0x6880 fs/ext4/xattr.c:1718 > > > ext4_xattr_block_set+0xb94/0x4fb0 fs/ext4/xattr.c:2037 > > > ext4_xattr_move_to_block fs/ext4/xattr.c:2654 [inline] > > > ext4_xattr_make_inode_space fs/ext4/xattr.c:2729 [inline] > > > ext4_expand_extra_isize_ea+0x20bd/0x3560 fs/ext4/xattr.c:2821 > > > __ext4_expand_extra_isize+0x5dc/0x680 fs/ext4/inode.c:5789 > > > ext4_try_to_expand_extra_isize fs/ext4/inode.c:5832 [inline] > > > __ext4_mark_inode_dirty+0x70d/0xa10 fs/ext4/inode.c:5910 > > > ext4_delete_inline_entry+0x650/0x7d0 fs/ext4/inline.c:1753 > > > ext4_delete_entry+0x13f/0x7d0 fs/ext4/namei.c:2719 > > > __ext4_unlink+0x9b8/0x11b0 fs/ext4/namei.c:3273 > > > ext4_unlink+0x226/0x630 fs/ext4/namei.c:3321 > > > vfs_unlink+0x676/0xa30 fs/namei.c:4343 > > > do_unlinkat+0x823/0xe10 fs/namei.c:4407 > > > __do_sys_unlinkat fs/namei.c:4450 [inline] > > > __se_sys_unlinkat fs/namei.c:4443 [inline] > > > __x64_sys_unlinkat+0x17c/0x230 fs/namei.c:4443 > > > x64_sys_call+0x846/0x3b50 arch/x86/include/generated/asm/syscalls_64.h:264 > > > do_syscall_x64 arch/x86/entry/common.c:52 [inline] > > > do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83 > > > entry_SYSCALL_64_after_hwframe+0x77/0x7f > > > > > > CPU: 1 PID: 88 Comm: kswapd1 Not tainted 6.9.0-syzkaller-02707-g614da38e2f7a #0 > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/02/2024 > > > ===================================================== > > > > > > > > > --- > > > This report is generated by a bot. It may contain errors. > > > See https://goo.gl/tpsmEJ for more information about syzbot. > > > syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxx. > > > > > > syzbot will keep track of this issue. See: > > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot. > > > > > > If the report is already addressed, let syzbot know by replying with: > > > #syz fix: exact-commit-title > > > > > > If you want to overwrite report's subsystems, reply with: > > > #syz set subsystems: new-subsystem > > > (See the list of subsystem names on the web dashboard) > > > > > > If the report is a duplicate of another one, reply with: > > > #syz dup: exact-subject-of-another-report > > > > > > If you want to undo deduplication, reply with: > > > #syz undup > >
