Re: [syzbot] [mm?] KMSAN: uninit-value in zswap_rb_insert

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Apr 29, 2024 at 11:02 PM syzbot
<syzbot+9c9d60f1b20b22ce218a@syzkaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    e33c4963bf53 Merge tag 'nfsd-6.9-5' of git://git.kernel.or..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=123d5a0f180000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=776c05250f36d55c
> dashboard link: https://syzkaller.appspot.com/bug?extid=9c9d60f1b20b22ce218a
> 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/a10175188ebb/disk-e33c4963.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/abe743417d16/vmlinux-e33c4963.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/fb10cccc9909/bzImage-e33c4963.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+9c9d60f1b20b22ce218a@syzkaller.appspotmail.com
>
> =====================================================
> BUG: KMSAN: uninit-value in zswap_rb_insert+0x1e9/0x330 mm/zswap.c:842

I am not sure how this could happen. The report is complaining about
the access in the loop condition AFAICT:

while (*link) { /* here */
..
}

*link should start as root->rb_node (which should be NULL as it is
static data), then be updated to point at a zswap_entry->rbnode as
entries get added. *link should keep then be updated to values at
zswap_entry->rbnode->{left/right}.

Although entry->rbnode->{left/right} are not initialized when
allocated by zswap_entry_cache_alloc() as the report mentions, they
are initialized in zswap_rb_insert()->rb_link_node() when they are
added to the tree.

Also, this code is removed with the xarray conversion anyway.

>  zswap_rb_insert+0x1e9/0x330 mm/zswap.c:842
>  zswap_store+0x22f2/0x2ca0 mm/zswap.c:1591
>  swap_writepage+0x126/0x4c0 mm/page_io.c:198
>  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
>  shrink_zones mm/vmscan.c:6152 [inline]
>  do_try_to_free_pages+0x820/0x2570 mm/vmscan.c:6214
>  try_to_free_pages+0xb7b/0x1820 mm/vmscan.c:6449
>  __perform_reclaim mm/page_alloc.c:3774 [inline]
>  __alloc_pages_direct_reclaim mm/page_alloc.c:3796 [inline]
>  __alloc_pages_slowpath+0x1035/0x31a0 mm/page_alloc.c:4202
>  __alloc_pages+0xacf/0xe70 mm/page_alloc.c:4588
>  alloc_pages_mpol+0x299/0x990 mm/mempolicy.c:2264
>  vma_alloc_folio+0x418/0x680 mm/mempolicy.c:2303
>  do_cow_fault mm/memory.c:4918 [inline]
>  do_fault mm/memory.c:5026 [inline]
>  do_pte_missing mm/memory.c:3880 [inline]
>  handle_pte_fault mm/memory.c:5300 [inline]
>  __handle_mm_fault mm/memory.c:5441 [inline]
>  handle_mm_fault+0x4f2f/0xce00 mm/memory.c:5606
>  do_user_addr_fault arch/x86/mm/fault.c:1413 [inline]
>  handle_page_fault arch/x86/mm/fault.c:1505 [inline]
>  exc_page_fault+0x2a0/0x730 arch/x86/mm/fault.c:1563
>  asm_exc_page_fault+0x2b/0x30 arch/x86/include/asm/idtentry.h:623
>  rep_stos_alternative+0x40/0x80 arch/x86/lib/clear_page_64.S:92
>  load_elf_binary+0x212e/0x4d30 fs/binfmt_elf.c:1132
>  search_binary_handler fs/exec.c:1778 [inline]
>  exec_binprm fs/exec.c:1820 [inline]
>  bprm_execve+0xc57/0x21c0 fs/exec.c:1872
>  do_execveat_common+0xceb/0xd70 fs/exec.c:1979
>  do_execve fs/exec.c:2053 [inline]
>  __do_sys_execve fs/exec.c:2129 [inline]
>  __se_sys_execve fs/exec.c:2124 [inline]
>  __x64_sys_execve+0xf4/0x130 fs/exec.c:2124
>  x64_sys_call+0x1612/0x3b50 arch/x86/include/generated/asm/syscalls_64.h:60
>  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
>
> Uninit was created at:
>  __alloc_pages+0x9d6/0xe70 mm/page_alloc.c:4598
>  __alloc_pages_node include/linux/gfp.h:238 [inline]
>  alloc_pages_node include/linux/gfp.h:261 [inline]
>  alloc_slab_page mm/slub.c:2175 [inline]
>  allocate_slab mm/slub.c:2338 [inline]
>  new_slab+0x2de/0x1400 mm/slub.c:2391
>  ___slab_alloc+0x1184/0x33d0 mm/slub.c:3525
>  __slab_alloc mm/slub.c:3610 [inline]
>  __slab_alloc_node mm/slub.c:3663 [inline]
>  slab_alloc_node mm/slub.c:3835 [inline]
>  kmem_cache_alloc_node+0x6ea/0xc50 mm/slub.c:3888
>  zswap_entry_cache_alloc mm/zswap.c:874 [inline]
>  zswap_store+0xb26/0x2ca0 mm/zswap.c:1535
>  swap_writepage+0x126/0x4c0 mm/page_io.c:198
>  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
>  shrink_zones mm/vmscan.c:6152 [inline]
>  do_try_to_free_pages+0x820/0x2570 mm/vmscan.c:6214
>  try_to_free_pages+0xb7b/0x1820 mm/vmscan.c:6449
>  __perform_reclaim mm/page_alloc.c:3774 [inline]
>  __alloc_pages_direct_reclaim mm/page_alloc.c:3796 [inline]
>  __alloc_pages_slowpath+0x1035/0x31a0 mm/page_alloc.c:4202
>  __alloc_pages+0xacf/0xe70 mm/page_alloc.c:4588
>  alloc_pages_mpol+0x299/0x990 mm/mempolicy.c:2264
>  alloc_pages+0x1bf/0x1e0 mm/mempolicy.c:2335
>  vm_area_alloc_pages mm/vmalloc.c:3561 [inline]
>  __vmalloc_area_node mm/vmalloc.c:3637 [inline]
>  __vmalloc_node_range+0x100a/0x28b0 mm/vmalloc.c:3818
>  vmalloc_user+0x90/0xb0 mm/vmalloc.c:3972
>  kcov_ioctl+0x5d/0x660 kernel/kcov.c:704
>  vfs_ioctl fs/ioctl.c:51 [inline]
>  __do_sys_ioctl fs/ioctl.c:904 [inline]
>  __se_sys_ioctl+0x261/0x450 fs/ioctl.c:890
>  __x64_sys_ioctl+0x96/0xe0 fs/ioctl.c:890
>  x64_sys_call+0x1883/0x3b50 arch/x86/include/generated/asm/syscalls_64.h:17
>  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: 0 PID: 5281 Comm: dhcpcd-run-hook Not tainted 6.9.0-rc5-syzkaller-00053-ge33c4963bf53 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
> =====================================================
>
>
> ---
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