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

[John Hubbard <jhubbard@nvidia.com>]

On 2/13/26 4:26 AM, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    05f7e89ab973 Linux 6.19
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=16c324aa580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=9682a42d8ec8b05c
> dashboard link: https://syzkaller.appspot.com/bug?extid=2aee6839a252e612ce34
> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/b9c0cdc82aa9/disk-05f7e89a.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/e43670d95943/vmlinux-05f7e89a.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/2c8401722932/bzImage-05f7e89a.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+2aee6839a252e612ce34@syzkaller.appspotmail.com

I wasn't able to boot a KMSAN kernel locally to test at all, but I did
notice some page allocation paths that call post_alloc_hook() but skip
kmsan_alloc_page(), leaving stale KMSAN shadow on allocated pages:

  alloc_pages_bulk_noprof()
  split_free_pages()
  alloc_contig_range_noprof()  (compound path)

So it is possible that we arrived here via the page_pool network
allocator, which calls alloc_pages_bulk_noprof().

Then later, the page is passed into follow_page_pte().

thanks,
John Hubbard

> 
> =====================================================
> BUG: KMSAN: uninit-value in follow_page_pte+0x385/0x15b0 mm/gup.c:816
>  follow_page_pte+0x385/0x15b0 mm/gup.c:816
>  follow_pmd_mask mm/gup.c:915 [inline]
>  follow_pud_mask mm/gup.c:967 [inline]
>  follow_p4d_mask mm/gup.c:984 [inline]
>  follow_page_mask mm/gup.c:1023 [inline]
>  __get_user_pages+0x1524/0x5f00 mm/gup.c:1426
>  __get_user_pages_locked mm/gup.c:1692 [inline]
>  get_dump_page+0x142/0x570 mm/gup.c:2192
>  dump_user_range+0x26e/0x2230 fs/coredump.c:1366
>  elf_core_dump+0x697f/0x6c30 fs/binfmt_elf.c:2111
>  coredump_write+0x209c/0x2d30 fs/coredump.c:1049
>  do_coredump fs/coredump.c:1126 [inline]
>  vfs_coredump+0x7a08/0x8d90 fs/coredump.c:1200
>  get_signal+0x2068/0x29d0 kernel/signal.c:3019
>  arch_do_signal_or_restart+0x53/0xc00 arch/x86/kernel/signal.c:337
>  __exit_to_user_mode_loop kernel/entry/common.c:41 [inline]
>  exit_to_user_mode_loop kernel/entry/common.c:75 [inline]
>  __exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
>  irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:270 [inline]
>  irqentry_exit_to_user_mode include/linux/irq-entry-common.h:339 [inline]
>  irqentry_exit+0xef/0x6d0 kernel/entry/common.c:196
>  exc_page_fault+0x7e/0xb0 arch/x86/mm/fault.c:1530
>  asm_exc_page_fault+0x2b/0x30 arch/x86/include/asm/idtentry.h:618
> 
> Uninit was stored to memory at:
>  ext4_xattr_set_entry+0x1312/0x3440 fs/ext4/xattr.c:1740
>  ext4_xattr_block_set+0xc74/0x5040 fs/ext4/xattr.c:2030
>  ext4_xattr_set_handle+0x1e25/0x2b10 fs/ext4/xattr.c:2457
>  ext4_xattr_set+0x2ff/0x5a0 fs/ext4/xattr.c:2559
>  ext4_xattr_trusted_set+0x51/0x70 fs/ext4/xattr_trusted.c:38
>  __vfs_setxattr+0x742/0x840 fs/xattr.c:200
>  __vfs_setxattr_noperm+0x224/0xad0 fs/xattr.c:234
>  __vfs_setxattr_locked+0x43c/0x480 fs/xattr.c:295
>  vfs_setxattr+0x28d/0x650 fs/xattr.c:321
>  do_setxattr fs/xattr.c:636 [inline]
>  filename_setxattr+0x3a4/0xcc0 fs/xattr.c:665
>  path_setxattrat+0x6c8/0x7c0 fs/xattr.c:713
>  __do_sys_lsetxattr fs/xattr.c:754 [inline]
>  __se_sys_lsetxattr fs/xattr.c:750 [inline]
>  __x64_sys_lsetxattr+0x103/0x1c0 fs/xattr.c:750
>  x64_sys_call+0x3c8b/0x3e70 arch/x86/include/generated/asm/syscalls_64.h:190
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xc9/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> Uninit was created at:
>  __alloc_frozen_pages_noprof+0x6df/0xf50 mm/page_alloc.c:5263
>  alloc_pages_mpol+0x328/0x860 mm/mempolicy.c:2486
>  alloc_frozen_pages_noprof+0xf6/0x200 mm/mempolicy.c:2557
>  alloc_slab_page mm/slub.c:3075 [inline]
>  allocate_slab+0x1ea/0x1700 mm/slub.c:3248
>  new_slab mm/slub.c:3302 [inline]
>  ___slab_alloc+0x1048/0x38b0 mm/slub.c:4656
>  __slab_alloc+0xa3/0x180 mm/slub.c:4779
>  __slab_alloc_node mm/slub.c:4855 [inline]
>  slab_alloc_node mm/slub.c:5251 [inline]
>  __kmalloc_cache_noprof+0x9e0/0x1710 mm/slub.c:5775
>  kmalloc_noprof include/linux/slab.h:957 [inline]
>  kzalloc_noprof include/linux/slab.h:1094 [inline]
>  sctp_transport_new+0xb2/0xb60 net/sctp/transport.c:95
>  sctp_assoc_add_peer+0x440/0x22b0 net/sctp/associola.c:617
>  sctp_process_param net/sctp/sm_make_chunk.c:2546 [inline]
>  sctp_process_init+0x11b0/0x3bc0 net/sctp/sm_make_chunk.c:2366
>  sctp_cmd_process_init net/sctp/sm_sideeffect.c:712 [inline]
>  sctp_cmd_interpreter net/sctp/sm_sideeffect.c:1395 [inline]
>  sctp_side_effects net/sctp/sm_sideeffect.c:1204 [inline]
>  sctp_do_sm+0x1719/0x9b30 net/sctp/sm_sideeffect.c:1175
>  sctp_assoc_bh_rcv+0x88b/0xbc0 net/sctp/associola.c:1034
>  sctp_inq_push+0x29d/0x350 net/sctp/inqueue.c:88
>  sctp_backlog_rcv+0x3ca/0xd80 net/sctp/input.c:331
>  sk_backlog_rcv include/net/sock.h:1177 [inline]
>  __release_sock+0x3fb/0x7d0 net/core/sock.c:3213
>  release_sock+0x6b/0x260 net/core/sock.c:3795
>  sctp_wait_for_connect+0x451/0x810 net/sctp/socket.c:9393
>  __sctp_connect+0x1483/0x15a0 net/sctp/socket.c:1248
>  __sctp_setsockopt_connectx net/sctp/socket.c:1337 [inline]
>  sctp_getsockopt_connectx3+0x7e5/0x8a0 net/sctp/socket.c:1422
>  sctp_getsockopt+0xd9b/0x14c0 net/sctp/socket.c:8174
>  sock_common_getsockopt+0x9c/0xf0 net/core/sock.c:3946
>  do_sock_getsockopt+0x3a8/0x4c0 net/socket.c:2383
>  __sys_getsockopt net/socket.c:2412 [inline]
>  __do_sys_getsockopt net/socket.c:2419 [inline]
>  __se_sys_getsockopt net/socket.c:2416 [inline]
>  __x64_sys_getsockopt+0x32d/0x520 net/socket.c:2416
>  x64_sys_call+0x3a97/0x3e70 arch/x86/include/generated/asm/syscalls_64.h:56
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xc9/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> CPU: 0 UID: 0 PID: 9815 Comm: syz.4.1167 Not tainted syzkaller #0 PREEMPT(voluntary) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
> =====================================================
> 
> 
> ---
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