linux-kernel - Re: [syzbot] [mm?] KCSAN: data-race in mtree_range_walk / rcu_segcblist

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Message-ID: <CANpmjNORLNHsWamYPU=rS7ftjZndchiEYHXznUS_RyR1edvrCA@mail.gmail.com>
Date: Fri, 21 Jun 2024 17:28:17 +0200
From: Marco Elver <elver@...gle.com>
To: liam.howlett@...cle.com
Cc: syzbot <syzbot+9bb7d0f2fdb4229b9d67@...kaller.appspotmail.com>, 
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	lstoakes@...il.com, syzkaller-bugs@...glegroups.com, vbabka@...e.cz, 
	RCU <rcu@...r.kernel.org>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [syzbot] [mm?] KCSAN: data-race in mtree_range_walk /
 rcu_segcblist_enqueue (2)

[+Cc rcu folks]

On Fri, 21 Jun 2024 at 15:29, syzbot
<syzbot+9bb7d0f2fdb4229b9d67@...kaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    50736169ecc8 Merge tag 'for-6.10-rc4-tag' of git://git.ker..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=164ec02a980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=704451bc2941bcb0
> dashboard link: https://syzkaller.appspot.com/bug?extid=9bb7d0f2fdb4229b9d67
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/e4cbed12fec1/disk-50736169.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/d50b5dcae4cd/vmlinux-50736169.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/f2c14c5fcce2/bzImage-50736169.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+9bb7d0f2fdb4229b9d67@...kaller.appspotmail.com
>
> ==================================================================
> BUG: KCSAN: data-race in mtree_range_walk / rcu_segcblist_enqueue
>
> write to 0xffff888104077308 of 8 bytes by task 12265 on cpu 1:
>  rcu_segcblist_enqueue+0x67/0xb0 kernel/rcu/rcu_segcblist.c:345
>  rcutree_enqueue kernel/rcu/tree.c:2940 [inline]
>  call_rcu_core kernel/rcu/tree.c:2957 [inline]
>  __call_rcu_common kernel/rcu/tree.c:3093 [inline]
>  call_rcu+0x1bd/0x430 kernel/rcu/tree.c:3176
>  ma_free_rcu lib/maple_tree.c:197 [inline]
>  mas_free lib/maple_tree.c:1304 [inline]
>  mas_replace_node+0x2f8/0x440 lib/maple_tree.c:1741
>  mas_wr_node_store lib/maple_tree.c:3956 [inline]
>  mas_wr_modify+0x2bc3/0x3c90 lib/maple_tree.c:4189
>  mas_wr_store_entry+0x250/0x390 lib/maple_tree.c:4229
>  mas_store_prealloc+0x151/0x2b0 lib/maple_tree.c:5485
>  vma_iter_store mm/internal.h:1398 [inline]
>  vma_complete+0x3a7/0x760 mm/mmap.c:535
>  __split_vma+0x623/0x690 mm/mmap.c:2440
>  split_vma mm/mmap.c:2466 [inline]
>  vma_modify+0x198/0x1f0 mm/mmap.c:2507
>  vma_modify_flags include/linux/mm.h:3347 [inline]
>  mprotect_fixup+0x335/0x610 mm/mprotect.c:637
>  do_mprotect_pkey+0x673/0x9a0 mm/mprotect.c:820
>  __do_sys_mprotect mm/mprotect.c:841 [inline]
>  __se_sys_mprotect mm/mprotect.c:838 [inline]
>  __x64_sys_mprotect+0x48/0x60 mm/mprotect.c:838
>  x64_sys_call+0x26f5/0x2d70 arch/x86/include/generated/asm/syscalls_64.h:11
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> read to 0xffff888104077308 of 8 bytes by task 12266 on cpu 0:
>  mtree_range_walk+0x140/0x460 lib/maple_tree.c:2774
>  mas_state_walk lib/maple_tree.c:3678 [inline]
>  mas_walk+0x16e/0x320 lib/maple_tree.c:4909
>  lock_vma_under_rcu+0x84/0x260 mm/memory.c:5840
>  do_user_addr_fault arch/x86/mm/fault.c:1329 [inline]
>  handle_page_fault arch/x86/mm/fault.c:1481 [inline]
>  exc_page_fault+0x150/0x650 arch/x86/mm/fault.c:1539
>  asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:623
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 PID: 12266 Comm: syz-executor.3 Not tainted 6.10.0-rc4-syzkaller-00148-g50736169ecc8 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/07/2024
> ==================================================================

This is not an ordinary data race. I suspect this to be an incorrect
use of RCU, resulting in some kind of use-after-free / type-confusion.

The access within rcu_segcblist_enqueue() is to maple_node::rcu (at
offset 8 into maple_node). The racing access in mtree_range_walk() is
to either maple_node::mr64::pivot[0] or maple_node::ma64::pivot[0]
(both also offset 8 into maple_node).