Message-ID: <986db0e1-bc7c-47a5-9d8d-28dbaa3c1a1a@lucifer.local>
Date: Wed, 16 Jul 2025 20:38:33 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: syzbot <syzbot+ebfd0e44b5c11034e1eb@...kaller.appspotmail.com>
Cc: Liam.Howlett@...cle.com, akpm@...ux-foundation.org, jannh@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, pfalcato@...e.de,
        syzkaller-bugs@...glegroups.com, vbabka@...e.cz
Subject: Re: [syzbot] [mm?] KASAN: slab-use-after-free Read in mas_next_slot
 (2)

On Wed, Jul 16, 2025 at 08:04:03PM +0100, Lorenzo Stoakes wrote:
> OK looks very much like the removal in v2 of the resets on unmap were a mistake.
>
> Working on a fix for this.

Fix at https://lore.kernel.org/linux-mm/4fbf4271-6ab9-49c0-b30f-c8716bf19f09@lucifer.local/

This will get squashed into the commit so I didn't include the tags below as
they'd be eliminated anyway.

Note that I was able to make the reproducer more reliable by introducing an
rcu_barrier() after unmap, as suggested by Liam.

Cheers, Lorenzo

>
> On Wed, Jul 16, 2025 at 10:55:35AM -0700, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    0be23810e32e Add linux-next specific files for 20250714
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=11a9a7d4580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=adc3ea2bfe31343b
> > dashboard link: https://syzkaller.appspot.com/bug?extid=ebfd0e44b5c11034e1eb
> > compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11d0658c580000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15dd858c580000
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/13b5be5048fe/disk-0be23810.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/3d2b3b2ceddf/vmlinux-0be23810.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/c7e5fbf3efa6/bzImage-0be23810.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+ebfd0e44b5c11034e1eb@...kaller.appspotmail.com
> >
> > ==================================================================
> > BUG: KASAN: slab-use-after-free in ma_dead_node lib/maple_tree.c:575 [inline]
> > BUG: KASAN: slab-use-after-free in mas_rewalk_if_dead lib/maple_tree.c:4415 [inline]
> > BUG: KASAN: slab-use-after-free in mas_next_slot+0x185/0xcf0 lib/maple_tree.c:4697
> > Read of size 8 at addr ffff8880755dc600 by task syz.0.656/6830
> >
> > CPU: 1 UID: 0 PID: 6830 Comm: syz.0.656 Not tainted 6.16.0-rc6-next-20250714-syzkaller #0 PREEMPT(full)
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
> > Call Trace:
> >  <TASK>
> >  dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
> >  print_address_description mm/kasan/report.c:378 [inline]
> >  print_report+0xca/0x230 mm/kasan/report.c:480
> >  kasan_report+0x118/0x150 mm/kasan/report.c:593
> >  ma_dead_node lib/maple_tree.c:575 [inline]
> >  mas_rewalk_if_dead lib/maple_tree.c:4415 [inline]
> >  mas_next_slot+0x185/0xcf0 lib/maple_tree.c:4697
> >  mas_find+0xb0e/0xd30 lib/maple_tree.c:6062
> >  vma_find include/linux/mm.h:855 [inline]
> >  remap_move mm/mremap.c:1819 [inline]
> >  do_mremap mm/mremap.c:1904 [inline]
> >  __do_sys_mremap mm/mremap.c:1968 [inline]
> >  __se_sys_mremap+0xaff/0xef0 mm/mremap.c:1936
> >  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >  do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > RIP: 0033:0x7f4fecf8e929
> > Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
> > RSP: 002b:00007fff93ea4718 EFLAGS: 00000246 ORIG_RAX: 0000000000000019
> > RAX: ffffffffffffffda RBX: 00007f4fed1b5fa0 RCX: 00007f4fecf8e929
> > RDX: 0000000000600002 RSI: 0000000000600002 RDI: 0000200000000000
> > RBP: 00007f4fed010b39 R08: 0000200000a00000 R09: 0000000000000000
> > R10: 0000000000000007 R11: 0000000000000246 R12: 0000000000000000
> > R13: 00007f4fed1b5fa0 R14: 00007f4fed1b5fa0 R15: 0000000000000005
> >  </TASK>
> >
> > Allocated by task 6830:
> >  kasan_save_stack mm/kasan/common.c:47 [inline]
> >  kasan_save_track+0x3e/0x80 mm/kasan/common.c:68
> >  unpoison_slab_object mm/kasan/common.c:319 [inline]
> >  __kasan_slab_alloc+0x6c/0x80 mm/kasan/common.c:345
> >  kasan_slab_alloc include/linux/kasan.h:250 [inline]
> >  slab_post_alloc_hook mm/slub.c:4180 [inline]
> >  slab_alloc_node mm/slub.c:4229 [inline]
> >  kmem_cache_alloc_noprof+0x1c1/0x3c0 mm/slub.c:4236
> >  mt_alloc_one lib/maple_tree.c:176 [inline]
> >  mas_alloc_nodes+0x2e9/0x8e0 lib/maple_tree.c:1255
> >  mas_node_count_gfp lib/maple_tree.c:1337 [inline]
> >  mas_preallocate+0x3ad/0x6f0 lib/maple_tree.c:5537
> >  vma_iter_prealloc mm/vma.h:463 [inline]
> >  __split_vma+0x2fa/0xa00 mm/vma.c:528
> >  vms_gather_munmap_vmas+0x2de/0x12b0 mm/vma.c:1359
> >  __mmap_prepare mm/vma.c:2361 [inline]
> >  __mmap_region mm/vma.c:2653 [inline]
> >  mmap_region+0x724/0x20c0 mm/vma.c:2741
> >  do_mmap+0xc45/0x10d0 mm/mmap.c:561
> >  vm_mmap_pgoff+0x2a6/0x4d0 mm/util.c:579
> >  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >  do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >
> > Freed by task 23:
> >  kasan_save_stack mm/kasan/common.c:47 [inline]
> >  kasan_save_track+0x3e/0x80 mm/kasan/common.c:68
> >  kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:576
> >  poison_slab_object mm/kasan/common.c:247 [inline]
> >  __kasan_slab_free+0x62/0x70 mm/kasan/common.c:264
> >  kasan_slab_free include/linux/kasan.h:233 [inline]
> >  slab_free_hook mm/slub.c:2417 [inline]
> >  slab_free mm/slub.c:4680 [inline]
> >  kmem_cache_free+0x18f/0x400 mm/slub.c:4782
> >  rcu_do_batch kernel/rcu/tree.c:2584 [inline]
> >  rcu_core+0xca8/0x1710 kernel/rcu/tree.c:2840
> >  handle_softirqs+0x283/0x870 kernel/softirq.c:579
> >  run_ksoftirqd+0x9b/0x100 kernel/softirq.c:968
> >  smpboot_thread_fn+0x53f/0xa60 kernel/smpboot.c:160
> >  kthread+0x70e/0x8a0 kernel/kthread.c:463
> >  ret_from_fork+0x3f9/0x770 arch/x86/kernel/process.c:148
> >  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> >
> > Last potentially related work creation:
> >  kasan_save_stack+0x3e/0x60 mm/kasan/common.c:47
> >  kasan_record_aux_stack+0xbd/0xd0 mm/kasan/generic.c:548
> >  __call_rcu_common kernel/rcu/tree.c:3102 [inline]
> >  call_rcu+0x157/0x9c0 kernel/rcu/tree.c:3222
> >  mas_wr_node_store lib/maple_tree.c:3893 [inline]
> >  mas_wr_store_entry+0x1f1b/0x25b0 lib/maple_tree.c:4104
> >  mas_store_prealloc+0xb00/0xf60 lib/maple_tree.c:5510
> >  vma_iter_store_new mm/vma.h:509 [inline]
> >  vma_complete+0x224/0xae0 mm/vma.c:354
> >  __split_vma+0x8a6/0xa00 mm/vma.c:568
> >  vms_gather_munmap_vmas+0x2de/0x12b0 mm/vma.c:1359
> >  do_vmi_align_munmap+0x25d/0x420 mm/vma.c:1527
> >  do_vmi_munmap+0x253/0x2e0 mm/vma.c:1584
> >  do_munmap+0xe1/0x140 mm/mmap.c:1071
> >  mremap_to+0x304/0x7b0 mm/mremap.c:1367
> >  remap_move mm/mremap.c:1861 [inline]
> >  do_mremap mm/mremap.c:1904 [inline]
> >  __do_sys_mremap mm/mremap.c:1968 [inline]
> >  __se_sys_mremap+0xa0b/0xef0 mm/mremap.c:1936
> >  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >  do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >
> > The buggy address belongs to the object at ffff8880755dc600
> >  which belongs to the cache maple_node of size 256
> > The buggy address is located 0 bytes inside of
> >  freed 256-byte region [ffff8880755dc600, ffff8880755dc700)
> >
> > The buggy address belongs to the physical page:
> > page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x755dc
> > head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> > flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
> > page_type: f5(slab)
> > raw: 00fff00000000040 ffff88801a491000 dead000000000122 0000000000000000
> > raw: 0000000000000000 0000000080100010 00000000f5000000 0000000000000000
> > head: 00fff00000000040 ffff88801a491000 dead000000000122 0000000000000000
> > head: 0000000000000000 0000000080100010 00000000f5000000 0000000000000000
> > head: 00fff00000000001 ffffea0001d57701 00000000ffffffff 00000000ffffffff
> > head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000002
> > page dumped because: kasan: bad access detected
> > page_owner tracks the page as allocated
> > page last allocated via order 1, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 6828, tgid 6828 (cmp), ts 120765032919, free_ts 112542256570
> >  set_page_owner include/linux/page_owner.h:32 [inline]
> >  post_alloc_hook+0x240/0x2a0 mm/page_alloc.c:1851
> >  prep_new_page mm/page_alloc.c:1859 [inline]
> >  get_page_from_freelist+0x21e4/0x22c0 mm/page_alloc.c:3858
> >  __alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5148
> >  alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2416
> >  alloc_slab_page mm/slub.c:2487 [inline]
> >  allocate_slab+0x8a/0x370 mm/slub.c:2655
> >  new_slab mm/slub.c:2709 [inline]
> >  ___slab_alloc+0xbeb/0x1410 mm/slub.c:3891
> >  __slab_alloc mm/slub.c:3981 [inline]
> >  __slab_alloc_node mm/slub.c:4056 [inline]
> >  slab_alloc_node mm/slub.c:4217 [inline]
> >  kmem_cache_alloc_noprof+0x283/0x3c0 mm/slub.c:4236
> >  mt_alloc_one lib/maple_tree.c:176 [inline]
> >  mas_alloc_nodes+0x2e9/0x8e0 lib/maple_tree.c:1255
> >  mas_node_count_gfp lib/maple_tree.c:1337 [inline]
> >  mas_preallocate+0x3ad/0x6f0 lib/maple_tree.c:5537
> >  vma_iter_prealloc mm/vma.h:463 [inline]
> >  commit_merge+0x1fd/0x700 mm/vma.c:753
> >  vma_expand+0x40c/0x7e0 mm/vma.c:1158
> >  vma_merge_new_range+0x6a3/0x860 mm/vma.c:1095
> >  __mmap_region mm/vma.c:2666 [inline]
> >  mmap_region+0xd46/0x20c0 mm/vma.c:2741
> >  do_mmap+0xc45/0x10d0 mm/mmap.c:561
> >  vm_mmap_pgoff+0x2a6/0x4d0 mm/util.c:579
> >  ksys_mmap_pgoff+0x51f/0x760 mm/mmap.c:607
> > page last free pid 5955 tgid 5955 stack trace:
> >  reset_page_owner include/linux/page_owner.h:25 [inline]
> >  free_pages_prepare mm/page_alloc.c:1395 [inline]
> >  __free_frozen_pages+0xbc4/0xd30 mm/page_alloc.c:2895
> >  __slab_free+0x303/0x3c0 mm/slub.c:4591
> >  qlink_free mm/kasan/quarantine.c:163 [inline]
> >  qlist_free_all+0x97/0x140 mm/kasan/quarantine.c:179
> >  kasan_quarantine_reduce+0x148/0x160 mm/kasan/quarantine.c:286
> >  __kasan_slab_alloc+0x22/0x80 mm/kasan/common.c:329
> >  kasan_slab_alloc include/linux/kasan.h:250 [inline]
> >  slab_post_alloc_hook mm/slub.c:4180 [inline]
> >  slab_alloc_node mm/slub.c:4229 [inline]
> >  kmem_cache_alloc_noprof+0x1c1/0x3c0 mm/slub.c:4236
> >  getname_flags+0xb8/0x540 fs/namei.c:146
> >  getname include/linux/fs.h:2914 [inline]
> >  do_sys_openat2+0xbc/0x1c0 fs/open.c:1429
> >  do_sys_open fs/open.c:1450 [inline]
> >  __do_sys_openat fs/open.c:1466 [inline]
> >  __se_sys_openat fs/open.c:1461 [inline]
> >  __x64_sys_openat+0x138/0x170 fs/open.c:1461
> >  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >  do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >
> > Memory state around the buggy address:
> >  ffff8880755dc500: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> >  ffff8880755dc580: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> > >ffff8880755dc600: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >                    ^
> >  ffff8880755dc680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >  ffff8880755dc700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> > ==================================================================
> >
> >
> > ---
> > This report is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@...glegroups.com.
> >
> > syzbot will keep track of this issue. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> >
> > If the report is already addressed, let syzbot know by replying with:
> > #syz fix: exact-commit-title
> >
> > If you want syzbot to run the reproducer, reply with:
> > #syz test: git://repo/address.git branch-or-commit-hash
> > If you attach or paste a git patch, syzbot will apply it before testing.
> >
> > If you want to overwrite report's subsystems, reply with:
> > #syz set subsystems: new-subsystem
> > (See the list of subsystem names on the web dashboard)
> >
> > If the report is a duplicate of another one, reply with:
> > #syz dup: exact-subject-of-another-report
> >
> > If you want to undo deduplication, reply with:
> > #syz undup

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