linux-kernel - Re: [PATCH] tracing: Zero-initialize parser->buffer to avoid OOB access

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Message-ID: <63f8a8d8-9f5e-49ac-8420-0a6dff638ea1@huaweicloud.com>
Date: Wed, 6 Aug 2025 14:59:00 +0800
From: Pu Lehui <pulehui@...weicloud.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: mhiramat@...nel.org, mathieu.desnoyers@...icios.com,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing: Zero-initialize parser->buffer to avoid OOB
 access



On 2025/8/6 11:14, Pu Lehui wrote:
> 
> 
> On 2025/8/5 23:19, Steven Rostedt wrote:
>> On Tue,  5 Aug 2025 15:12:03 +0000
>> Pu Lehui <pulehui@...weicloud.com> wrote:
>>
>>> From: Pu Lehui <pulehui@...wei.com>
>>>
>>> When the length of the string written to set_ftrace_filter exceeds
>>> FTRACE_BUFF_MAX, the following KASAN alarm will be triggered:
>>>
>>> BUG: KASAN: slab-out-of-bounds in strsep+0x18c/0x1b0
>>> Read of size 1 at addr ffff0000d00bd5ba by task ash/165
>>>
>>> CPU: 1 UID: 0 PID: 165 Comm: ash Not tainted 
>>> 6.16.0-g6bcdbd62bd56-dirty #3 NONE
>>> Hardware name: linux,dummy-virt (DT)
>>> Call trace:
>>>   show_stack+0x34/0x50 (C)
>>>   dump_stack_lvl+0xa0/0x158
>>>   print_address_description.constprop.0+0x88/0x398
>>>   print_report+0xb0/0x280
>>>   kasan_report+0xa4/0xf0
>>>   __asan_report_load1_noabort+0x20/0x30
>>>   strsep+0x18c/0x1b0
>>>   ftrace_process_regex.isra.0+0x100/0x2d8
>>>   ftrace_regex_release+0x484/0x618
>>>   __fput+0x364/0xa58
>>>   ____fput+0x28/0x40
>>>   task_work_run+0x154/0x278
>>>   do_notify_resume+0x1f0/0x220
>>>   el0_svc+0xec/0xf0
>>>   el0t_64_sync_handler+0xa0/0xe8
>>>   el0t_64_sync+0x1ac/0x1b0
>>>
>>> The reason is that trace_get_user will fail when processing a string
>>> longer than FTRACE_BUFF_MAX, but not set the end of parser->buffer
>>> to 0. Then an oob access will be triggered in ftrace_regex_release->
>>> ftrace_process_regex->strsep->strpbrk. We can solve this problem by
>>> zero-initializing parser->buffer.
>>>
>>> Fixes: 8c9af478c06b ("ftrace: Handle commands when closing 
>>> set_ftrace_filter file")
>>
>> Fix one bug then create another!!!
>>
>>> Signed-off-by: Pu Lehui <pulehui@...wei.com>
>>> ---
>>>   kernel/trace/trace.c | 5 +----
>>>   1 file changed, 1 insertion(+), 4 deletions(-)
>>>
>>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>>> index 4283ed4e8f59..97e66cbd3617 100644
>>> --- a/kernel/trace/trace.c
>>> +++ b/kernel/trace/trace.c
>>> @@ -1776,7 +1776,7 @@ int trace_parser_get_init(struct trace_parser 
>>> *parser, int size)
>>>   {
>>>       memset(parser, 0, sizeof(*parser));
>>> -    parser->buffer = kmalloc(size, GFP_KERNEL);
>>> +    parser->buffer = kzalloc(size, GFP_KERNEL);
>>>       if (!parser->buffer)
>>>           return 1;
>>> @@ -1860,13 +1860,10 @@ int trace_get_user(struct trace_parser 
>>> *parser, const char __user *ubuf,
>>>       /* We either got finished input or we have to wait for another 
>>> call. */
>>>       if (isspace(ch) || !ch) {
>>> -        parser->buffer[parser->idx] = 0;
>>>           parser->cont = false;
>>>       } else if (parser->idx < parser->size - 1) {
>>>           parser->cont = true;
>>>           parser->buffer[parser->idx++] = ch;
>>> -        /* Make sure the parsed string always terminates with '\0'. */
>>> -        parser->buffer[parser->idx] = 0;
>>
>> The returned buffer is expected to end with a '0'. This now removes 
>> that if
>> the idx is the length of the parser.
> 
> Hi Steven,
> 
> Not really, if the idx is the length of the parser, which also mean that 
> the length of user string is parser->size, then it will goto fail branch 
> [0]. And with this patch, it will also terminates with '\0'.
> 
> Link: 
> https://elixir.bootlin.com/linux/v6.16/source/kernel/trace/trace.c#L1903 
> [0]
> 

Hi Steven,

After a while of thinking, I think we should limit the use of 
parser->buffer when trace_get_user fails to parse. And I sent a new 
version [0]. Hope that will be fine.

Link: 
https://lore.kernel.org/all/20250806070109.1320165-1-pulehui@huaweicloud.com/ 
[0]

>>
>>>       } else {
>>>           return -EINVAL;
>>>       }
>>
>> The real fix would be:
>>
>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> index d0b1964648c1..422224a55f1d 100644
>> --- a/kernel/trace/trace.c
>> +++ b/kernel/trace/trace.c
>> @@ -815,7 +815,7 @@ int trace_pid_write(struct trace_pid_list 
>> *filtered_pids,
>>       loff_t pos;
>>       pid_t pid;
>> -    if (trace_parser_get_init(&parser, PID_BUF_SIZE + 1))
>> +    if (trace_parser_get_init(&parser, PID_BUF_SIZE))
>>           return -ENOMEM;
>>       /*
>> @@ -1776,7 +1776,7 @@ int trace_parser_get_init(struct trace_parser 
>> *parser, int size)
>>   {
>>       memset(parser, 0, sizeof(*parser));
>> -    parser->buffer = kmalloc(size, GFP_KERNEL);
>> +    parser->buffer = kmalloc(size + 1, GFP_KERNEL);
>>       if (!parser->buffer)
>>           return 1;
>> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
>> index 05447b958a1a..a3ce88a48947 100644
>> --- a/kernel/trace/trace_events.c
>> +++ b/kernel/trace/trace_events.c
>> @@ -1507,7 +1507,7 @@ ftrace_event_write(struct file *file, const char 
>> __user *ubuf,
>>       if (ret < 0)
>>           return ret;
>> -    if (trace_parser_get_init(&parser, EVENT_BUF_SIZE + 1))
>> +    if (trace_parser_get_init(&parser, EVENT_BUF_SIZE))
>>           return -ENOMEM;
>>       read = trace_get_user(&parser, ubuf, cnt, ppos);
>>
>> -- Steve