linux-kernel - Re: [syzbot] [rcu?] [bcachefs?] BUG: unable to handle kernel NULL pointer dereference in rcu

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Message-ID: <602bb1be-f4a4-4194-803f-856e95711870@paulmck-laptop>
Date: Sun, 8 Jun 2025 17:25:05 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Uladzislau Rezki <urezki@...il.com>
Cc: Kent Overstreet <kent.overstreet@...ux.dev>,
	syzbot <syzbot+80e5d6f453f14a53383a@...kaller.appspotmail.com>,
	akpm@...ux-foundation.org, josh@...htriplett.org,
	linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, rcu@...r.kernel.org,
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [rcu?] [bcachefs?] BUG: unable to handle kernel NULL
 pointer dereference in rcu_core (3)

On Sun, Jun 08, 2025 at 08:23:36PM +0200, Uladzislau Rezki wrote:
> On Sun, Jun 08, 2025 at 11:26:28AM -0400, Kent Overstreet wrote:
> > On Wed, Feb 05, 2025 at 06:56:19AM -0800, Paul E. McKenney wrote:
> > > On Tue, Feb 04, 2025 at 04:34:18PM -0800, syzbot wrote:
> > > > Hello,
> > > > 
> > > > syzbot found the following issue on:
> > > > 
> > > > HEAD commit:    0de63bb7d919 Merge tag 'pull-fix' of git://git.kernel.org/..
> > > > git tree:       upstream
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=10faf5f8580000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=1909f2f0d8e641ce
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=80e5d6f453f14a53383a
> > > > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16b69d18580000
> > > > 
> > > > Downloadable assets:
> > > > disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7feb34a89c2a/non_bootable_disk-0de63bb7.raw.xz
> > > > vmlinux: https://storage.googleapis.com/syzbot-assets/1142009a30a7/vmlinux-0de63bb7.xz
> > > > kernel image: https://storage.googleapis.com/syzbot-assets/5d9e46a8998d/bzImage-0de63bb7.xz
> > > > mounted in repro: https://storage.googleapis.com/syzbot-assets/526692501242/mount_0.gz
> > > > 
> > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > Reported-by: syzbot+80e5d6f453f14a53383a@...kaller.appspotmail.com
> > > > 
> > > >  slab radix_tree_node start ffff88803bf382c0 pointer offset 24 size 576
> > > > BUG: kernel NULL pointer dereference, address: 0000000000000000
> > > > #PF: supervisor instruction fetch in kernel mode
> > > > #PF: error_code(0x0010) - not-present page
> > > > PGD 0 P4D 0 
> > > > Oops: Oops: 0010 [#1] PREEMPT SMP KASAN NOPTI
> > > > CPU: 0 UID: 0 PID: 5705 Comm: syz-executor Not tainted 6.14.0-rc1-syzkaller-00020-g0de63bb7d919 #0
> > > > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> > > > RIP: 0010:0x0
> > > > Code: Unable to access opcode bytes at 0xffffffffffffffd6.
> > > > RSP: 0018:ffffc90000007bd8 EFLAGS: 00010246
> > > > RAX: dffffc0000000000 RBX: 1ffff110077e705c RCX: 23438dd059a4b100
> > > > RDX: 0000000000000100 RSI: 0000000000000000 RDI: ffff88803bf382d8
> > > > RBP: ffffc90000007e10 R08: ffffffff819f146c R09: 1ffff11003f8519a
> > > > R10: dffffc0000000000 R11: 0000000000000000 R12: ffffffff81a6d507
> > > > R13: ffff88803bf382e0 R14: 0000000000000000 R15: ffff88803bf382d8
> > > > FS:  0000555567992500(0000) GS:ffff88801fc00000(0000) knlGS:0000000000000000
> > > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > CR2: ffffffffffffffd6 CR3: 000000004da38000 CR4: 0000000000352ef0
> > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > > Call Trace:
> > > >  <IRQ>
> > > >  rcu_do_batch kernel/rcu/tree.c:2546 [inline]
> > > 
> > > The usual way that this happens is that someone clobbers the rcu_head
> > > structure of something that has been passed to call_rcu().  The most
> > > popular way of clobbering this structure is to pass the same something to
> > > call_rcu() twice in a row, but other creative arrangements are possible.
> > > 
> > > Building your kernel with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y can usually
> > > spot invoking call_rcu() twice in a row.
> > 
> > I don't think it's that - syzbot's .config already has that enabled.
> > KASAN, too.
> > 
> > And the only place we do call_rcu() is from rcu_pending.c, where we've
> > got a rearming rcu callback - but we track whether it's outstanding, and
> > we do all relevant operations with a lock held.
> > 
> > And we only use rcu_pending.c with SRCU, not regular RCU.
> > 
> > We do use kfree_rcu() in a few places (all boring, I expect), but that
> > doesn't (generally?) use the rcu callback list.
> >
> Right, kvfree_rcu() does not intersect with regular callbacks, it has
> its own path. 
> 
> It looks like the problem is here:
> 
> <snip>
>   f = rhp->func;
>   debug_rcu_head_callback(rhp);
>   WRITE_ONCE(rhp->func, (rcu_callback_t)0L);
>   f(rhp);
> <snip>
> 
> we do not check if callback, "f", is a NULL. If it is, the kernel bug
> is triggered right away. For example:
> 
> call_rcu(&rh, NULL);
> 
> @Paul, do you think it makes sense to narrow callers which apparently
> pass NULL as a callback? To me it seems the case of this bug. But we
> do not know the source.
> 
> It would give at least a stack-trace of caller which passes a NULL.

Adding a check for NULL func passed to __call_rcu_common(), you mean?

That wouldn't hurt, and would either (as you say) catch the culprit
or show that the problem is elsewhere.

							Thanx, Paul