Re: [syzbot] [mm?] KCSAN: data-race in __anon_vma_prepare / __vmf_anon_prepare (2)

[Lorenzo Stoakes]

This seems to be pretty much identical to the previous report, which we
concluded was benign [0].

I think I'll send a patch to add a data_race() annotation nto the read-side.

[0]:https://lore.kernel.org/linux-mm/6967c517.050a0220.150504.0007.GAE@xxxxxxxxxx/

On Fri, Jun 19, 2026 at 07:36:32PM -0700, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    5e2e14749c3d Merge tag 'landlock-7.2-rc1' of git://git.ker..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=10e5ccfe580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=b0ae27ce66b92956
> dashboard link: https://syzkaller.appspot.com/bug?extid=395b7abe9696862fc188
> compiler:       Debian clang version 22.1.6 (++20260514074242+fc4aad7b5db3-1~exp1~20260514074407.73), Debian LLD 22.1.6
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/a565c60a4762/disk-5e2e1474.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/e593a6eb0057/vmlinux-5e2e1474.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/d24b55020ea3/bzImage-5e2e1474.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+395b7abe9696862fc188@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> ==================================================================
> BUG: KCSAN: data-race in __anon_vma_prepare / __vmf_anon_prepare
>
> write to 0xffff88811a6a4d00 of 8 bytes by task 20911 on cpu 1:
>  __anon_vma_prepare+0x172/0x2f0 mm/rmap.c:212

WRITE: VMA under mmap read lock, page_table_lock:

	vma->anon_vma = anon_vma;

>  __vmf_anon_prepare+0x91/0x100 mm/memory.c:3829
>  hugetlb_no_page+0x1b8/0xf30 mm/hugetlb.c:5756
>  hugetlb_fault+0x608/0xda0 mm/hugetlb.c:-1
>  handle_mm_fault+0x1de9/0x2da0 mm/memory.c:6684
>  faultin_page mm/gup.c:1126 [inline]
>  __get_user_pages+0x129c/0x1f10 mm/gup.c:1428
>  populate_vma_page_range mm/gup.c:1860 [inline]
>  __mm_populate+0x242/0x390 mm/gup.c:1963
>  mm_populate include/linux/mm.h:4171 [inline]
>  vm_mmap_pgoff+0x23b/0x2d0 mm/util.c:586
>  ksys_mmap_pgoff+0x2c6/0x310 mm/mmap.c:606
>  x64_sys_call+0x14df/0x3020 arch/x86/include/generated/asm/syscalls_64.h:10
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> read to 0xffff88811a6a4d00 of 8 bytes by task 20901 on cpu 0:
>  __vmf_anon_prepare+0x26/0x100 mm/memory.c:3823

READ: VMA under VMA read lock:

	if (likely(vma->anon_vma))
		return 0;

It's benign because if we race here we'd just recheck in __anon_vma_prepare()
and the mm->page_table_lock would serialise for us.

>  hugetlb_no_page+0x1b8/0xf30 mm/hugetlb.c:5756
>  hugetlb_fault+0x608/0xda0 mm/hugetlb.c:-1
>  handle_mm_fault+0x1de9/0x2da0 mm/memory.c:6684
>  do_user_addr_fault+0x402/0x1060 arch/x86/mm/fault.c:1394
>  handle_page_fault arch/x86/mm/fault.c:1483 [inline]
>  exc_page_fault+0x62/0xa0 arch/x86/mm/fault.c:1536
>  asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:595
>  __put_user_nocheck_4+0x3/0x20 arch/x86/lib/putuser.S:97
>  ____sys_recvmsg+0x1a1/0x280 net/socket.c:2917
>  ___sys_recvmsg+0x11f/0x3a0 net/socket.c:2946
>  do_recvmmsg+0x1e5/0x560 net/socket.c:3041
>  __sys_recvmmsg net/socket.c:3115 [inline]
>  __do_sys_recvmmsg net/socket.c:3138 [inline]
>  __se_sys_recvmmsg net/socket.c:3131 [inline]
>  __x64_sys_recvmmsg+0xe5/0x170 net/socket.c:3131
>  x64_sys_call+0x80f/0x3020 arch/x86/include/generated/asm/syscalls_64.h:300
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> value changed: 0x0000000000000000 -> 0xffff8881020cec60
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 UID: 0 PID: 20901 Comm: syz.0.4316 Tainted: G        W           syzkaller #0 PREEMPT(lazy)
> Tainted: [W]=WARN
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
> ==================================================================
>
>
> ---
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Thanks, Lorenzo