Re: [PATCH v2 RESEND] perf top: Fix rare segfault in thread__comm_len()

[Arnaldo Carvalho de Melo]

Em Wed, Mar 15, 2023 at 02:42:17PM +0800, liuwenyu (D) escreveu:
> In thread__comm_len(),strlen() is called outside of the
> thread->comm_lock critical section,which may cause a UAF
> problems if comm__free() is called by the process_thread
> concurrently.

Applied, next time you resend a patch just to fix formatting, without
changing what the patch does, please collect tags provided for the
previous version, like the one from Namhyung:

Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>

I did it because I saw it for v1, so added now.

Thanks,

- Arnaldo
 
> backtrace of the core file is as follows:
> 
>     (gdb) bt
>     #0  __strlen_evex () at ../sysdeps/x86_64/multiarch/strlen-evex.S:77
>     #1  0x000055ad15d31de5 in thread__comm_len (thread=0x7f627d20e300) at util/thread.c:320
>     #2  0x000055ad15d4fade in hists__calc_col_len (h=0x7f627d295940, hists=0x55ad1772bfe0)
>         at util/hist.c:103
>     #3  hists__calc_col_len (hists=0x55ad1772bfe0, h=0x7f627d295940) at util/hist.c:79
>     #4  0x000055ad15d52c8c in output_resort (hists=hists@entry=0x55ad1772bfe0, prog=0x0,
>         use_callchain=false, cb=cb@entry=0x0, cb_arg=0x0) at util/hist.c:1926
>     #5  0x000055ad15d530a4 in evsel__output_resort_cb (evsel=evsel@entry=0x55ad1772bde0,
>         prog=prog@entry=0x0, cb=cb@entry=0x0, cb_arg=cb_arg@entry=0x0) at util/hist.c:1945
>     #6  0x000055ad15d53110 in evsel__output_resort (evsel=evsel@entry=0x55ad1772bde0,
>         prog=prog@entry=0x0) at util/hist.c:1950
>     #7  0x000055ad15c6ae9a in perf_top__resort_hists (t=t@entry=0x7ffcd9cbf4f0) at builtin-top.c:311
>     #8  0x000055ad15c6cc6d in perf_top__print_sym_table (top=0x7ffcd9cbf4f0) at builtin-top.c:346
>     #9  display_thread (arg=0x7ffcd9cbf4f0) at builtin-top.c:700
>     #10 0x00007f6282fab4fa in start_thread (arg=<optimized out>) at pthread_create.c:443
>     #11 0x00007f628302e200 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81
> 
> The reason is that strlen() get a pointer to a memory that has been freed.
> 
> The string pointer is stored in the structure comm_str, which corresponds
> to a rb_tree node,when the node is erased, the memory of the string is also freed.
> 
> In thread__comm_len(),it gets the pointer within the thread->comm_lock critical section,
> but passed to strlen() outside of the thread->comm_lock critical section, and the perf
> process_thread may called comm__free() concurrently, cause this segfault problem.
> 
> The process is as follows:
> 
> display_thread                                  process_thread
> --------------                                  --------------
> 
> thread__comm_len
>   -> thread__comm_str
>        # held the comm read lock
>     -> __thread__comm_str(thread)
>        # release the comm read lock
>                                                 thread__delete
>                                                      # held the comm write lock
>                                                   -> comm__free
>                                                     -> comm_str__put(comm->comm_str)
>                                                       -> zfree(&cs->str)
>                                                      # release the comm write lock
>       # The memory of the string pointed
>         to by comm has been free.
>     -> thread->comm_len = strlen(comm);
> 
> This patch expand the critical section range of thread->comm_lock in thread__comm_len(),
> to make strlen() called safe.
> 
> Signed-off-by: Wenyu Liu  <liuwenyu7@xxxxxxxxxx>
> ---
> v1 -> v2
>  - format the patch to be applied
> 
>  tools/perf/util/thread.c | 25 +++++++++++++++++++------
>  1 file changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/tools/perf/util/thread.c b/tools/perf/util/thread.c
> index e3e5427e1c3c..a2490a20eb56 100644
> --- a/tools/perf/util/thread.c
> +++ b/tools/perf/util/thread.c
> @@ -311,17 +311,30 @@ const char *thread__comm_str(struct thread *thread)
>  	return str;
>  }
>  
> +static int __thread__comm_len(struct thread *thread, const char *comm)
> +{
> +	if (!comm)
> +		return 0;
> +	thread->comm_len = strlen(comm);
> +
> +	return thread->comm_len;
> +}
> +
>  /* CHECKME: it should probably better return the max comm len from its comm list */
>  int thread__comm_len(struct thread *thread)
>  {
> -	if (!thread->comm_len) {
> -		const char *comm = thread__comm_str(thread);
> -		if (!comm)
> -			return 0;
> -		thread->comm_len = strlen(comm);
> +	int comm_len = thread->comm_len;
> +
> +	if (!comm_len) {
> +		const char *comm;
> +
> +		down_read(&thread->comm_lock);
> +		comm = __thread__comm_str(thread);
> +		comm_len = __thread__comm_len(thread, comm);
> +		up_read(&thread->comm_lock);
>  	}
>  
> -	return thread->comm_len;
> +	return comm_len;
>  }
>  
>  size_t thread__fprintf(struct thread *thread, FILE *fp)
> -- 
> 2.36.1

-- 

- Arnaldo