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Sun, 08 Jun 2025 11:23:38 -0700 (PDT) Received: from pc636 ([2001:9b1:d5a0:a500::800]) by smtp.gmail.com with ESMTPSA id 38308e7fff4ca-32ae1cad433sm7751971fa.53.2025.06.08.11.23.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 08 Jun 2025 11:23:37 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Sun, 8 Jun 2025 20:23:36 +0200 To: Kent Overstreet , "Paul E. 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Queue-Id: 84763180011 X-Rspamd-Server: rspam09 X-Stat-Signature: f57cn9p7ooxdwkxwhbug1tamcekiq9sd X-HE-Tag: 1749407020-165788 X-HE-Meta: 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 mbyUGQu7 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 +xFLmfmI s0v5DJZmilucauY7K6JJiO2zS0bL3es/+vgbegsjI6ux1rCohQ5XNhmLufFC3/lfOqvyl3Da9Oj0Yzob8ham7inWOTsAS3jQURmFNPvHrw8= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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