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McKenney" Cc: "Paul E. McKenney" , syzbot , akpm@linux-foundation.org, josh@joshtriplett.org, linux-bcachefs@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, rcu@vger.kernel.org, syzkaller-bugs@googlegroups.com Subject: Re: [syzbot] [rcu?] [bcachefs?] BUG: unable to handle kernel NULL pointer dereference in rcu_core (3) Message-ID: References: <67a2b20a.050a0220.50516.0003.GAE@google.com> <9694d40a-072e-47c2-a950-3b258bbe04f5@paulmck-laptop> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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@syzkaller.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: > > > > > > 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: f = rhp->func; debug_rcu_head_callback(rhp); WRITE_ONCE(rhp->func, (rcu_callback_t)0L); f(rhp); 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. -- Uladzislau Rezki