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The commit67e1b0052f("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] <TASK> [ 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] </TASK> 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:67e1b0052f("printk_ringbuffer: don't needlessly wrap data blocks around") Reviewed-by: John Ogness <john.ogness@linutronix.de> Tested-by: John Ogness <john.ogness@linutronix.de> Link: https://patch.msgid.link/20251107194720.1231457-2-pmladek@suse.com Signed-off-by: Petr Mladek <pmladek@suse.com>
Linux kernel
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