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

[Liam R. Howlett]

* syzbot <syzbot+38a879f4a73497f2dfef@xxxxxxxxxxxxxxxxxxxxxxxxx> [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.

> 
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> ==================================================================
> 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'.
> 
> 
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