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

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Message-ID: <20250805111903.3114427b@gandalf.local.home>
Date: Tue, 5 Aug 2025 11:19:03 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Pu Lehui <pulehui@...weicloud.com>
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 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.

>  	} 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