[tip:perf/core] perf record: Fix segfault when running with suid and kptr_restrict is 1

[tip-bot for Wang Nan]

Commit-ID:  3dbe46c5245f61328797738c6a0a6cd4bf921f61
Gitweb:     http://git.kernel.org/tip/3dbe46c5245f61328797738c6a0a6cd4bf921f61
Author:     Wang Nan <wangnan0@xxxxxxxxxx>
AuthorDate: Tue, 15 Nov 2016 04:05:45 +0000
Committer:  Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
CommitDate: Fri, 25 Nov 2016 11:11:10 -0300

perf record: Fix segfault when running with suid and kptr_restrict is 1

Before this patch perf panics if kptr_restrict is set to 1 and perf is
owned by root with suid set:

  $ whoami
  wangnan
  $ ls -l ./perf
  -rwsr-xr-x 1 root root 19781908 Sep 21 19:29 /home/wangnan/perf
  $ cat /proc/sys/kernel/kptr_restrict
  1
  $ cat /proc/sys/kernel/perf_event_paranoid
  -1
  $ ./perf record -a
  Segmentation fault (core dumped)
  $

The reason is that perf assumes it is allowed to read kptr from
/proc/kallsyms when euid is root, but in fact the kernel doesn't allow
reading kptr when euid and uid do not match with each other:

  $ cp /bin/cat .
  $ sudo chown root:root ./cat
  $ sudo chmod u+s ./cat
  $ cat /proc/kallsyms | grep do_fork
  0000000000000000 T _do_fork          <--- kptr is hidden even euid is root
  $ sudo cat /proc/kallsyms | grep do_fork
  ffffffff81080230 T _do_fork

See lib/vsprintf.c for kernel side code.

This patch fixes this problem by checking both uid and euid.

Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
Tested-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxx>
Cc: He Kuang <hekuang@xxxxxxxxxx>
Cc: Zefan Li <lizefan@xxxxxxxxxx>
Cc: pi3orama@xxxxxxx
Link: http://lkml.kernel.org/r/20161115040617.69788-3-wangnan0@xxxxxxxxxx
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
---
 tools/perf/util/symbol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
index aecff69..420ada9 100644
--- a/tools/perf/util/symbol.c
+++ b/tools/perf/util/symbol.c
@@ -1962,7 +1962,7 @@ static bool symbol__read_kptr_restrict(void)
 		char line[8];
 
 		if (fgets(line, sizeof(line), fp) != NULL)
-			value = (geteuid() != 0) ?
+			value = ((geteuid() != 0) || (getuid() != 0)) ?
 					(atoi(line) != 0) :
 					(atoi(line) == 2);