Re: [PATCH v2] cgroup: align the comm length with TASK_COMM_LEN

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 23 Sep 2022 15:51:05 +0800
Kassey Li <quic_yingangl@quicinc.com> wrote:

> __string could get a dst string with length less than
> TASK_COMM_LEN.
> 
> A task->comm may change that can cause out of bounds access
> for the dst string buffer, e.g in the call trace of below:
> 
> Call trace:
> 
>     dump_backtrace.cfi_jt+0x0/0x4
>     show_stack+0x14/0x1c
>     dump_stack+0xa0/0xd8
>     die_callback+0x248/0x24c
>     notify_die+0x7c/0xf8
>     die+0xac/0x290
>     die_kernel_fault+0x88/0x98
>     die_kernel_fault+0x0/0x98
>     do_page_fault+0xa0/0x544
>     do_mem_abort+0x60/0x10c
>     el1_da+0x1c/0xc4
>     trace_event_raw_event_cgroup_migrate+0x124/0x170
>     cgroup_attach_task+0x2e8/0x41c
>     __cgroup1_procs_write+0x114/0x1ec
>     cgroup1_tasks_write+0x10/0x18
>     cgroup_file_write+0xa4/0x208
>     kernfs_fop_write+0x1f0/0x2f4
>     __vfs_write+0x5c/0x200
>     vfs_write+0xe0/0x1a0
>     ksys_write+0x74/0xdc
>     __arm64_sys_write+0x18/0x20
>     el0_svc_common+0xc0/0x1a4
>     el0_svc_compat_handler+0x18/0x20
>     el0_svc_compat+0x8/0x2c
> 
> Change it as arrary with same length TASK_COMM_LEN,
> This idea is from commit d1eb650ff413 ("tracepoint: Move signal sending
> tracepoint to events/signal.h").

This does not make sense. What exactly is the bug here?

__string() will do a strlen(task->comm) + 1 to allocate on the ring buffer.
It should not be less that task->comm. The above stack dump does not show
what happened.

This looks like another bug and I do not see how this patch addresses
the issue.

-- Steve

> 
> Signed-off-by: Kassey Li <quic_yingangl@quicinc.com>
> ---
>  include/trace/events/cgroup.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/trace/events/cgroup.h b/include/trace/events/cgroup.h
> index dd7d7c9efecd..b4ef0ffa38a4 100644
> --- a/include/trace/events/cgroup.h
> +++ b/include/trace/events/cgroup.h
> @@ -130,7 +130,7 @@ DECLARE_EVENT_CLASS(cgroup_migrate,
>  		__field(	u64,		dst_id			)
>  		__field(	int,		pid			)
>  		__string(	dst_path,	path			)
> -		__string(	comm,		task->comm		)
> +		__array(char, comm, TASK_COMM_LEN)
>  	),
>  
>  	TP_fast_assign(
> @@ -139,12 +139,12 @@ DECLARE_EVENT_CLASS(cgroup_migrate,
>  		__entry->dst_level = dst_cgrp->level;
>  		__assign_str(dst_path, path);
>  		__entry->pid = task->pid;
> -		__assign_str(comm, task->comm);
> +		memcpy(__entry->comm, task->comm, TASK_COMM_LEN);
>  	),
>  
>  	TP_printk("dst_root=%d dst_id=%llu dst_level=%d dst_path=%s pid=%d comm=%s",
>  		  __entry->dst_root, __entry->dst_id, __entry->dst_level,
> -		  __get_str(dst_path), __entry->pid, __get_str(comm))
> +		  __get_str(dst_path), __entry->pid, __entry->comm)
>  );
>  
>  DEFINE_EVENT(cgroup_migrate, cgroup_attach_task,