syzbot reported this bug:
==================================================================
BUG: KASAN: slab-out-of-bounds in trace_seq_to_buffer kernel/trace/trace.c:1830 [inline]
BUG: KASAN: slab-out-of-bounds in tracing_splice_read_pipe+0x6be/0xdd0 kernel/trace/trace.c:6822
Write of size 4507 at addr ffff888032b6b000 by task syz.2.320/7260

CPU: 1 UID: 0 PID: 7260 Comm: syz.2.320 Not tainted 6.15.0-rc1-syzkaller-00301-g3bde70a2c827 #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:408 [inline]
 print_report+0xc3/0x670 mm/kasan/report.c:521
 kasan_report+0xe0/0x110 mm/kasan/report.c:634
 check_region_inline mm/kasan/generic.c:183 [inline]
 kasan_check_range+0xef/0x1a0 mm/kasan/generic.c:189
 __asan_memcpy+0x3c/0x60 mm/kasan/shadow.c:106
 trace_seq_to_buffer kernel/trace/trace.c:1830 [inline]
 tracing_splice_read_pipe+0x6be/0xdd0 kernel/trace/trace.c:6822
 ....
==================================================================

It has been reported that trace_seq_to_buffer() tries to copy more data
than PAGE_SIZE to buf. Therefore, to prevent this, we should use the
smaller of trace_seq_used(&iter->seq) and PAGE_SIZE as an argument.

Reported-by: syzbot+c8cd2d2c412b868263fb@syzkaller.appspotmail.com
Fixes: 3c56819b14b0 ("tracing: splice support for tracing_pipe")
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
---
v2: Corrected the correct location as suggested by the maintainer
---
 kernel/trace/trace.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 8ddf6b17215c..170c31da61e0 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6784,7 +6784,7 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
 	};
 	ssize_t ret;
 	size_t rem;
-	unsigned int i;
+	unsigned int i, copy_len;
 
 	if (splice_grow_spd(pipe, &spd))
 		return -ENOMEM;
@@ -6818,16 +6818,18 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
 
 		rem = tracing_fill_pipe_page(rem, iter);
 
+		copy_len = trace_seq_used(&iter->seq);
+
 		/* Copy the data into the page, so we can start over. */
 		ret = trace_seq_to_buffer(&iter->seq,
 					  page_address(spd.pages[i]),
-					  trace_seq_used(&iter->seq));
+					  min(copy_len, PAGE_SIZE));
 		if (ret < 0) {
 			__free_page(spd.pages[i]);
 			break;
 		}
 		spd.partial[i].offset = 0;
-		spd.partial[i].len = trace_seq_used(&iter->seq);
+		spd.partial[i].len = min(copy_len, PAGE_SIZE);
 
 		trace_seq_init(&iter->seq);
 	}
--

On Tue, 22 Apr 2025 00:28:50 +0900
Jeongjun Park <aha310510@gmail.com> wrote:

> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -6784,7 +6784,7 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
>  	};
>  	ssize_t ret;
>  	size_t rem;
> -	unsigned int i;
> +	unsigned int i, copy_len;

FYI, I don't care for variables to be on the same line unless they are
related. As "i" and "copy_len" are not related, the should be separate
declarations.

	unsigned int copy_len;
	unsigned int i;

>  
>  	if (splice_grow_spd(pipe, &spd))
>  		return -ENOMEM;
> @@ -6818,16 +6818,18 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
>  
>  		rem = tracing_fill_pipe_page(rem, iter);
>  
> +		copy_len = trace_seq_used(&iter->seq);

Why not have the min here?

		copy_len = min(trace_seq_used(&iter->seq), PAGE_SIZE);

??

> +
>  		/* Copy the data into the page, so we can start over. */
>  		ret = trace_seq_to_buffer(&iter->seq,
>  					  page_address(spd.pages[i]),
> -					  trace_seq_used(&iter->seq));
> +					  min(copy_len, PAGE_SIZE));
>  		if (ret < 0) {
>  			__free_page(spd.pages[i]);
>  			break;
>  		}
>  		spd.partial[i].offset = 0;
> -		spd.partial[i].len = trace_seq_used(&iter->seq);
> +		spd.partial[i].len = min(copy_len, PAGE_SIZE);

And actually, len should equal ret as that's how much was copied.

-- Steve


>  
>  		trace_seq_init(&iter->seq);
>  	}
> --