[PATCH] printk: ringbuffer: Wrong data pointer when appending small string

From: Petr Mladek
Date: Wed Oct 14 2020 - 13:50:57 EST

data_realloc() returns wrong data pointer when the block is wrapped and
the size is not increased. It might happen when pr_cont() wants to
add only few characters and there is already a space for them because
of alignment.

It might cause writing outsite the buffer. It has been detected by LTP
tests with KASAN enabled:

[ 221.921944] oom-kill:constraint=CONSTRAINT_MEMCG,nodemask=(null),cpuset=c,mems_allowed=0,oom_memcg=/0,task_memcg=in
[ 221.922108] ==================================================================
[ 221.922111] BUG: KASAN: global-out-of-bounds in vprintk_store+0x362/0x3d0
[ 221.922112] Write of size 2 at addr ffffffffba51dbcd by task
memcg_test_1/11282
[ 221.922113]
[ 221.922114] CPU: 1 PID: 11282 Comm: memcg_test_1 Not tainted
5.9.0-next-20201013 #1
[ 221.922116] Hardware name: Supermicro SYS-5019S-ML/X11SSH-F, BIOS
2.0b 07/27/2017
[ 221.922116] Call Trace:
[ 221.922117] dump_stack+0xa4/0xd9
[ 221.922118] print_address_description.constprop.0+0x21/0x210
[ 221.922119] ? _raw_write_lock_bh+0xe0/0xe0
[ 221.922120] ? vprintk_store+0x362/0x3d0
[ 221.922121] kasan_report.cold+0x37/0x7c
[ 221.922122] ? vprintk_store+0x362/0x3d0
[ 221.922123] check_memory_region+0x18c/0x1f0
[ 221.922124] memcpy+0x3c/0x60
[ 221.922125] vprintk_store+0x362/0x3d0
[ 221.922125] ? __ia32_sys_syslog+0x50/0x50
[ 221.922126] ? _raw_spin_lock_irqsave+0x9b/0x100
[ 221.922127] ? _raw_spin_lock_irq+0xf0/0xf0
[ 221.922128] ? __kasan_check_write+0x14/0x20
[ 221.922129] vprintk_emit+0x8d/0x1f0
[ 221.922130] vprintk_default+0x1d/0x20
[ 221.922131] vprintk_func+0x5a/0x100
[ 221.922132] printk+0xb2/0xe3
[ 221.922133] ? swsusp_write.cold+0x189/0x189
[ 221.922134] ? kernfs_vfs_xattr_set+0x60/0x60
[ 221.922134] ? _raw_write_lock_bh+0xe0/0xe0
[ 221.922135] ? trace_hardirqs_on+0x38/0x100
[ 221.922136] pr_cont_kernfs_path.cold+0x49/0x4b
[ 221.922137] mem_cgroup_print_oom_context.cold+0x74/0xc3
[ 221.922138] dump_header+0x340/0x3bf
[ 221.922139] oom_kill_process.cold+0xb/0x10
[ 221.922140] out_of_memory+0x1e9/0x860
[ 221.922141] ? oom_killer_disable+0x210/0x210
[ 221.922142] mem_cgroup_out_of_memory+0x198/0x1c0
[ 221.922143] ? mem_cgroup_count_precharge_pte_range+0x250/0x250
[ 221.922144] try_charge+0xa9b/0xc50
[ 221.922145] ? arch_stack_walk+0x9e/0xf0
[ 221.922146] ? memory_high_write+0x230/0x230
[ 221.922146] ? avc_has_extended_perms+0x830/0x830
[ 221.922147] ? stack_trace_save+0x94/0xc0
[ 221.922148] ? stack_trace_consume_entry+0x90/0x90
[ 221.922149] __memcg_kmem_charge+0x73/0x120
[ 221.922150] ? cred_has_capability+0x10f/0x200
[ 221.922151] ? mem_cgroup_can_attach+0x260/0x260
[ 221.922152] ? selinux_sb_eat_lsm_opts+0x2f0/0x2f0
[ 221.922153] ? obj_cgroup_charge+0x16b/0x220
[ 221.922154] ? kmem_cache_alloc+0x78/0x4c0
[ 221.922155] obj_cgroup_charge+0x122/0x220
[ 221.922156] ? vm_area_alloc+0x20/0x90
[ 221.922156] kmem_cache_alloc+0x78/0x4c0
[ 221.922157] vm_area_alloc+0x20/0x90
[ 221.922158] mmap_region+0x3ed/0x9a0
[ 221.922159] ? cap_mmap_addr+0x1d/0x80
[ 221.922160] do_mmap+0x3ee/0x720
[ 221.922161] vm_mmap_pgoff+0x16a/0x1c0
[ 221.922162] ? randomize_stack_top+0x90/0x90
[ 221.922163] ? copy_page_range+0x1980/0x1980
[ 221.922163] ksys_mmap_pgoff+0xab/0x350
[ 221.922164] ? find_mergeable_anon_vma+0x110/0x110
[ 221.922165] ? __audit_syscall_entry+0x1a6/0x1e0
[ 221.922166] __x64_sys_mmap+0x8d/0xb0
[ 221.922167] do_syscall_64+0x38/0x50
[ 221.922168] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 221.922169] RIP: 0033:0x7fe8f5e75103
[ 221.922172] Code: 54 41 89 d4 55 48 89 fd 53 4c 89 cb 48 85 ff 74
56 49 89 d9 45 89 f8 45 89 f2 44 89 e2 4c 89 ee 48 89 ef b8 09 00 00
00 0f 05 <48> 3d 00 f0 ff ff 77 7d 5b 5d 41 5c 41 5d 41 5e 41 5f c3 66
2e 0f
[ 221.922173] RSP: 002b:00007ffd38c90198 EFLAGS: 00000246 ORIG_RAX:
0000000000000009
[ 221.922175] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fe8f5e75103
[ 221.922176] RDX: 0000000000000003 RSI: 0000000000001000 RDI: 0000000000000000
[ 221.922178] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
[ 221.922179] R10: 0000000000002022 R11: 0000000000000246 R12: 0000000000000003
[ 221.922180] R13: 0000000000001000 R14: 0000000000002022 R15: 0000000000000000
[ 221.922181]
[ 213O[ 221.922182] The buggy address belongs to the variable:
[ 221.922183] clear_seq+0x2d/0x40
[ 221.922183]
[ 221.922184] Memory state around the buggy address:
[ 221.922185] ffffffffba51da80: 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00
[ 221.922187] ffffffffba51db00: 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00
[ 221.922188] >ffffffffba51db80: f9 f9 f9 f9 00 f9 f9 f9 f9 f9 f9 f9
00 f9 f9 f9
[ 221.922189] ^
[ 221.922190] ffffffffba51dc00: f9 f9 f9 f9 00 f9 f9 f9 f9 f9 f9 f9
00 f9 f9 f9
[ 221.922191] ffffffffba51dc80: f9 f9 f9 f9 01 f9 f9 f9 f9 f9 f9 f9
00 f9 f9 f9
[ 221.922193] ==================================================================
[ 221.922194] Disabling lock debugging due to kernel taint
[ 221.922196] ,task=memcg_test_1,pid=11280,uid=0
[ 221.922205] Memory cgroup out of memory: Killed process 11280

Link: https://lore.kernel.org/r/CA+G9fYt46oC7-BKryNDaaXPJ9GztvS2cs_7GjYRjanRi4+ryCQ@xxxxxxxxxxxxxx
Fixes: 4cfc7258f876a7feba673ac ("printk: ringbuffer: add finalization/extension support")
Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
kernel/printk/printk_ringbuffer.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 2493348a1631..24a960a89aa8 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -1125,7 +1125,10 @@ static char *data_realloc(struct printk_ringbuffer *rb,

/* If the data block does not increase, there is nothing to do. */
if (head_lpos - next_lpos < DATA_SIZE(data_ring)) {
- blk = to_block(data_ring, blk_lpos->begin);
+ if (wrapped)
+ blk = to_block(data_ring, 0);
+ else
+ blk = to_block(data_ring, blk_lpos->begin);
return &blk->data[0];
}

--
2.26.2