From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C6538212560; Mon, 10 Nov 2025 09:13:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=193.142.43.55 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762766001; cv=none; b=k8hAytAMKSf9Mke/j4NH6A665Xt04ISarTLSE53qGtagEhX2J9PgjYoB631uwtsDSi2nRVjlsP2L9qJQydXQSNlYbQUGxY/LMOAqZYGaaKA4S1TjwMAHMvY0VyJ5ZpuxnBJuqLT+pBectYmIeWhtnreYiuGLc7cbr/UYZNKW700= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762766001; c=relaxed/simple; bh=HoHGOdSj7bTFhoiT0pF3v/Oy7LXbjZm8k5NEe753yd4=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=sa4rfaqJ4NR+AaLNP7qCeRP9DLSqY1Ud6rrY6KZUXF9xDcM1omO6x1cDpckvtpj5gWWA3PR5dNSXA64RtGMMo+OC7BMbrf0DTMLtZKL6ihwJkJ6QpKtXGgVZv16xF9Y165RT3Jn4H1wrvywX4ZMEFX6lBnr40ikrBhGkxhWLyp4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de; spf=pass smtp.mailfrom=linutronix.de; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=IeTpOANL; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=oAH1FF64; arc=none smtp.client-ip=193.142.43.55 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="IeTpOANL"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="oAH1FF64" From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1762765998; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=aWYulQfR0TreZlqe/qFy/nUmcaT6X+aYnci2sC0M8+s=; b=IeTpOANLyUhi255Pz3g+GevA/LeLToFvR9GF2uEztHAPEeKnyegJT3Hx3QBbSw6VWL+pCX KPmV+FYcX4exu56pt2rFkK9IVBVptW6wt88fQE/voW9NljT+MUXz4q1quD3uhoiaxmOvDZ kW1IIDMpoHlwWkc7+o4msV5t9Pb8rScnkzWoNd8ZeEtkcdpkAk6bA6X6QhPQNHeFyyucR5 LBLBwFz3cMuMKC6TCHnNCi5qTU8UwAKD2BbzxBom5/9H4XmT8FQpz9d3d3E7i2le3FHHUx djBrKgBBegfnxGC7fpOzxx1qO6r3Hu1XnnsQY2upt8YdYK2VfC8a/O2WlymhJg== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1762765998; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=aWYulQfR0TreZlqe/qFy/nUmcaT6X+aYnci2sC0M8+s=; b=oAH1FF64UEij1BWuPu1puzZO5DMShnO2Ri9sQ+M46IzSSjZp9jXlTuVFsi3DBPswFcgpav x7YvoAmfFmu0DdDQ== To: Petr Mladek Cc: Joanne Koong , "amurray @ thegoodpenguin . co . uk" , brauner@kernel.org, chao@kernel.org, djwong@kernel.org, jaegeuk@kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, syzkaller-bugs@googlegroups.com, Petr Mladek Subject: Re: [PATCH 1/2] printk_ringbuffer: Fix check of valid data size when blk_lpos overflows In-Reply-To: <20251107194720.1231457-2-pmladek@suse.com> References: <20251107194720.1231457-1-pmladek@suse.com> <20251107194720.1231457-2-pmladek@suse.com> Date: Mon, 10 Nov 2025 10:19:17 +0106 Message-ID: <87ms4uuu6q.fsf@jogness.linutronix.de> Precedence: bulk X-Mailing-List: linux-xfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain On 2025-11-07, Petr Mladek wrote: > The commit 67e1b0052f6bb8 ("printk_ringbuffer: don't needlessly wrap > data blocks around") allows to use the last 4 bytes of the ring buffer. > > But the check for the @data_size was not properly updated in get_data(). > It fails when "blk_lpos->next" overflows to "0". In this case: > > + is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next) > returns "false" because it checks "blk_lpos->next - 1". > > + "blk_lpos->begin < blk_lpos->next" fails because "blk_lpos->next" > is already 0. > > + is_blk_wrapped(data_ring, blk_lpos->begin + DATA_SIZE(data_ring), > blk_lpos->next) returns "false" because "begin_lpos" is from > the next wrap but "next_lpos - 1" is from the previous one. > > As a result, get_data() triggers the WARN_ON_ONCE() for "Illegal > block description", for example: > > [ 216.317316][ T7652] loop0: detected capacity change from 0 to 16 > ** 1 printk messages dropped ** > [ 216.327750][ T7652] ------------[ cut here ]------------ > [ 216.327789][ T7652] WARNING: kernel/printk/printk_ringbuffer.c:1278 at get_data+0x48a/0x840, CPU#1: syz.0.585/7652 > [ 216.327848][ T7652] Modules linked in: > [ 216.327907][ T7652] CPU: 1 UID: 0 PID: 7652 Comm: syz.0.585 Not tainted syzkaller #0 PREEMPT(full) > [ 216.327933][ T7652] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025 > [ 216.327953][ T7652] RIP: 0010:get_data+0x48a/0x840 > [ 216.327986][ T7652] Code: 83 c4 f8 48 b8 00 00 00 00 00 fc ff df 41 0f b6 04 07 84 c0 0f 85 ee 01 00 00 44 89 65 00 49 83 c5 08 eb 13 e8 a7 19 1f 00 90 <0f> 0b 90 eb 05 e8 9c 19 1f 00 45 31 ed 4c 89 e8 48 83 c4 28 5b 41 > [ 216.328007][ T7652] RSP: 0018:ffffc900035170e0 EFLAGS: 00010293 > [ 216.328029][ T7652] RAX: ffffffff81a1eee9 RBX: 00003fffffffffff RCX: ffff888033255b80 > [ 216.328048][ T7652] RDX: 0000000000000000 RSI: 00003fffffffffff RDI: 0000000000000000 > [ 216.328063][ T7652] RBP: 0000000000000012 R08: 0000000000000e55 R09: 000000325e213cc7 > [ 216.328079][ T7652] R10: 000000325e213cc7 R11: 00001de4c2000037 R12: 0000000000000012 > [ 216.328095][ T7652] R13: 0000000000000000 R14: ffffc90003517228 R15: 1ffffffff1bca646 > [ 216.328111][ T7652] FS: 00007f44eb8da6c0(0000) GS:ffff888125fda000(0000) knlGS:0000000000000000 > [ 216.328131][ T7652] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [ 216.328147][ T7652] CR2: 00007f44ea9722e0 CR3: 0000000066344000 CR4: 00000000003526f0 > [ 216.328168][ T7652] Call Trace: > [ 216.328178][ T7652] > [ 216.328199][ T7652] _prb_read_valid+0x672/0xa90 > [ 216.328328][ T7652] ? desc_read+0x1b8/0x3f0 > [ 216.328381][ T7652] ? __pfx__prb_read_valid+0x10/0x10 > [ 216.328422][ T7652] ? panic_on_this_cpu+0x32/0x40 > [ 216.328450][ T7652] prb_read_valid+0x3c/0x60 > [ 216.328482][ T7652] printk_get_next_message+0x15c/0x7b0 > [ 216.328526][ T7652] ? __pfx_printk_get_next_message+0x10/0x10 > [ 216.328561][ T7652] ? __lock_acquire+0xab9/0xd20 > [ 216.328595][ T7652] ? console_flush_all+0x131/0xb10 > [ 216.328621][ T7652] ? console_flush_all+0x478/0xb10 > [ 216.328648][ T7652] console_flush_all+0x4cc/0xb10 > [ 216.328673][ T7652] ? console_flush_all+0x131/0xb10 > [ 216.328704][ T7652] ? __pfx_console_flush_all+0x10/0x10 > [ 216.328748][ T7652] ? is_printk_cpu_sync_owner+0x32/0x40 > [ 216.328781][ T7652] console_unlock+0xbb/0x190 > [ 216.328815][ T7652] ? __pfx___down_trylock_console_sem+0x10/0x10 > [ 216.328853][ T7652] ? __pfx_console_unlock+0x10/0x10 > [ 216.328899][ T7652] vprintk_emit+0x4c5/0x590 > [ 216.328935][ T7652] ? __pfx_vprintk_emit+0x10/0x10 > [ 216.328993][ T7652] _printk+0xcf/0x120 > [ 216.329028][ T7652] ? __pfx__printk+0x10/0x10 > [ 216.329051][ T7652] ? kernfs_get+0x5a/0x90 > [ 216.329090][ T7652] _erofs_printk+0x349/0x410 > [ 216.329130][ T7652] ? __pfx__erofs_printk+0x10/0x10 > [ 216.329161][ T7652] ? __raw_spin_lock_init+0x45/0x100 > [ 216.329186][ T7652] ? __init_swait_queue_head+0xa9/0x150 > [ 216.329231][ T7652] erofs_fc_fill_super+0x1591/0x1b20 > [ 216.329285][ T7652] ? __pfx_erofs_fc_fill_super+0x10/0x10 > [ 216.329324][ T7652] ? sb_set_blocksize+0x104/0x180 > [ 216.329356][ T7652] ? setup_bdev_super+0x4c1/0x5b0 > [ 216.329385][ T7652] get_tree_bdev_flags+0x40e/0x4d0 > [ 216.329410][ T7652] ? __pfx_erofs_fc_fill_super+0x10/0x10 > [ 216.329444][ T7652] ? __pfx_get_tree_bdev_flags+0x10/0x10 > [ 216.329483][ T7652] vfs_get_tree+0x92/0x2b0 > [ 216.329512][ T7652] do_new_mount+0x302/0xa10 > [ 216.329537][ T7652] ? apparmor_capable+0x137/0x1b0 > [ 216.329576][ T7652] ? __pfx_do_new_mount+0x10/0x10 > [ 216.329605][ T7652] ? ns_capable+0x8a/0xf0 > [ 216.329637][ T7652] ? kmem_cache_free+0x19b/0x690 > [ 216.329682][ T7652] __se_sys_mount+0x313/0x410 > [ 216.329717][ T7652] ? __pfx___se_sys_mount+0x10/0x10 > [ 216.329836][ T7652] ? do_syscall_64+0xbe/0xfa0 > [ 216.329869][ T7652] ? __x64_sys_mount+0x20/0xc0 > [ 216.329901][ T7652] do_syscall_64+0xfa/0xfa0 > [ 216.329932][ T7652] ? lockdep_hardirqs_on+0x9c/0x150 > [ 216.329964][ T7652] ? entry_SYSCALL_64_after_hwframe+0x77/0x7f > [ 216.329988][ T7652] ? clear_bhb_loop+0x60/0xb0 > [ 216.330017][ T7652] entry_SYSCALL_64_after_hwframe+0x77/0x7f > [ 216.330040][ T7652] RIP: 0033:0x7f44ea99076a > [ 216.330080][ T7652] Code: d8 64 89 02 48 c7 c0 ff ff ff ff eb a6 e8 de 1a 00 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48 > [ 216.330100][ T7652] RSP: 002b:00007f44eb8d9e68 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5 > [ 216.330128][ T7652] RAX: ffffffffffffffda RBX: 00007f44eb8d9ef0 RCX: 00007f44ea99076a > [ 216.330146][ T7652] RDX: 0000200000000180 RSI: 00002000000001c0 RDI: 00007f44eb8d9eb0 > [ 216.330164][ T7652] RBP: 0000200000000180 R08: 00007f44eb8d9ef0 R09: 0000000000000000 > [ 216.330181][ T7652] R10: 0000000000000000 R11: 0000000000000246 R12: 00002000000001c0 > [ 216.330196][ T7652] R13: 00007f44eb8d9eb0 R14: 00000000000001a1 R15: 0000200000000080 > [ 216.330233][ T7652] > > Solve the problem by moving and fixing the sanity check. The problematic > if-else-if-else code will just distinguish three basic scenarios: > "regular" vs. "wrapped" vs. "too many times wrapped" block. > > The new sanity check is more precise. A valid "data_size" must be > lower than half of the data buffer size. Also it must not be zero at > this stage. It allows to catch problematic "data_size" even for wrapped > blocks. > > Closes: https://lore.kernel.org/all/69096836.a70a0220.88fb8.0006.GAE@google.com/ > Closes: https://lore.kernel.org/all/69078fb6.050a0220.29fc44.0029.GAE@google.com/ > Fixes: 67e1b0052f6bb82 ("printk_ringbuffer: don't needlessly wrap data blocks around") > Signed-off-by: Petr Mladek Reviewed-by: John Ogness Tested-by: John Ogness