[PATCH 10/12] perf record: Fix segfault when running with suid and kptr_restrict is 1

From: Arnaldo Carvalho de Melo
Date: Fri Nov 25 2016 - 10:13:20 EST


From: Wang Nan <wangnan0@xxxxxxxxxx>

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 aecff69a510d..420ada9de22f 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);

--
2.7.4