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

[Namhyung Kim]

Hello,

On Fri, Mar 3, 2023 at 2:02 AM liuwenyu (D) <liuwenyu7@xxxxxxxxxx> wrote:
>
> 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.
>
> 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>

Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>

Thanks,
Namhyung


> ---
>   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
>