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

[Marco Elver <elver@google.com>]

On Sat, 18 Apr 2026 at 01:51, 'Liam R. Howlett' via syzkaller-bugs
<syzkaller-bugs@googlegroups.com> wrote:
>
> * syzbot <syzbot+38a879f4a73497f2dfef@syzkaller.appspotmail.com> [260417 05:12]:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    1d51b370a0f8 Merge tag 'jfs-7.1' of github.com:kleikamp/li..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=117dc4ce580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=7f207c4b1fbf85a3
> > dashboard link: https://syzkaller.appspot.com/bug?extid=38a879f4a73497f2dfef
> > 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.
>
> ... and you won't.  This will work unless we tear aligned unsigned long
> writes/reads.
>
> I'm debating marking these as data_race().  Marking them all as
> READ_ONCE and this one write as WRITE_ONCE.  It seems overkill for
> something that won't happen.
>
> Alternatively, I can move the slot store fast path to need an
> allocation, but that's worse.

The writer:

>   rcu_assign_pointer(slots[offset + 1], wr_mas->entry);
>   wr_mas->pivots[offset] = mas->index - 1;  // <-- stores pivots[offset]

The reader races here:

>   if (pivots[offset] >= mas->index) {   // <-- load pivots[offset]
>       max = pivots[offset];  // <-- load pivots[offset] again
>       break;
>   }

The compiler is free to reload them as written. What if there's a
concurrent update between the first and second load?

> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/e08ff8d2b0e5/disk-1d51b370.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/c11d4b098bbf/vmlinux-1d51b370.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/6a4691f32e3d/bzImage-1d51b370.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+38a879f4a73497f2dfef@syzkaller.appspotmail.com
> >
> > ==================================================================
> > BUG: KCSAN: data-race in mas_wr_store_entry / mtree_range_walk
> >
> > write to 0xffff888104f71d08 of 8 bytes by task 4757 on cpu 0:
> >  mas_wr_slot_store lib/maple_tree.c:3232 [inline]
> >  mas_wr_store_entry+0x3405/0x5ad0 lib/maple_tree.c:3528
> >  mas_store_prealloc+0x43e/0x690 lib/maple_tree.c:4936
> >  vma_iter_store_overwrite mm/vma.h:616 [inline]
> >  commit_merge+0x6a1/0x720 mm/vma.c:766
> >  vma_expand+0x301/0x460 mm/vma.c:1219
> >  vma_merge_new_range+0x29c/0x320 mm/vma.c:1112
> >  __mmap_region mm/vma.c:2766 [inline]
> >  mmap_region+0x1073/0x2110 mm/vma.c:2856
> >  do_mmap+0x9b2/0xbd0 mm/mmap.c:560
> >  vm_mmap_pgoff+0x183/0x2d0 mm/util.c:581
> >  ksys_mmap_pgoff+0xc1/0x310 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+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >
> > read to 0xffff888104f71d08 of 8 bytes by task 4759 on cpu 1:
> >  mtree_range_walk+0x1a6/0x490 lib/maple_tree.c:2032
> >  mas_state_walk lib/maple_tree.c:2952 [inline]
> >  mas_walk+0x1cc/0x370 lib/maple_tree.c:4366
> >  lock_vma_under_rcu+0xc9/0x210 mm/mmap_lock.c:304
> >  do_user_addr_fault+0x232/0x1050 arch/x86/mm/fault.c:1325
> >  handle_page_fault arch/x86/mm/fault.c:1474 [inline]
> >  exc_page_fault+0x62/0xa0 arch/x86/mm/fault.c:1527
> >  asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
> >
> > value changed: 0x00007f68dc2a5fff -> 0x00007f68dc284fff
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 1 UID: 0 PID: 4759 Comm: syz.5.348 Not tainted syzkaller #0 PREEMPT(full)
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
> > ==================================================================
> > netlink: 64 bytes leftover after parsing attributes in process `syz.5.348'.
> >
> >
> > ---
> > This report is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
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> >
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> >
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> >
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