[PATCH 1/2] proc: check permissions earlier for /proc/*/wchan

[Alexey Dobriyan]

get_wchan() accesses stack page before permissions are checked,
let's not play this game.

Signed-off-by: Alexey Dobriyan <adobri...@gmail.com>
---

 fs/proc/base.c |   13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -391,14 +391,17 @@ static int proc_pid_wchan(struct seq_file *m, struct 
pid_namespace *ns,
        unsigned long wchan;
        char symname[KSYM_NAME_LEN];
 
-       wchan = get_wchan(task);
+       if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
+               goto print0;
 
-       if (wchan && ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)
-                       && !lookup_symbol_name(wchan, symname))
+       wchan = get_wchan(task);
+       if (wchan && !lookup_symbol_name(wchan, symname)) {
                seq_printf(m, "%s", symname);
-       else
-               seq_putc(m, '0');
+               return 0;
+       }
 
+print0:
+       seq_putc(m, '0');
        return 0;
 }
 #endif /* CONFIG_KALLSYMS */