[PATCH v2 6/9] procfs: add permission checks on the file's opener of /proc/*/stat

Djalal Harouni
2013-10-01T20:33:04Z

Some fields of the /proc/*/stat are sensitive fields that need
appropriate protection.

However, /proc file descriptors can be passed to a more privileged
process (e.g. a suid-exec) which will pass the classic
ptrace_may_access() permission check during read().

To prevent it, use proc_same_open_cred() to detect if current's cred
have changed between ->open() and ->read(), if so, call
proc_allow_access() to check if the original file's opener had enough
permissions to read these sensitive fields. This will prevent passing
file descriptors to a more privileged process to leak data.

The patch also adds a previously missing signal->cred_guard_mutex lock.

This patch does not break userspace since it only hides the fields that
were supposed to be protected.

Cc: Kees Cook <keescook@chromium.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
---
fs/proc/array.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index cbd0f1b..f034e05 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -394,7 +394,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
char state;
pid_t ppid = 0, pgid = -1, sid = -1;
int num_threads = 0;
- int permitted;
+ int permitted = 0;
struct mm_struct *mm;
unsigned long long start_time;
unsigned long cmin_flt = 0, cmaj_flt = 0;
@@ -404,10 +404,22 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
unsigned long rsslim = 0;
char tcomm[sizeof(task->comm)];
unsigned long flags;
+ struct file *file = m->private;
+ int same_cred = proc_same_open_cred(file->f_cred);
+ unsigned int ptrace_mode = PTRACE_MODE_READ | PTRACE_MODE_NOAUDIT;

state = *get_task_state(task);
vsize = eip = esp = 0;
- permitted = ptrace_may_access(task, PTRACE_MODE_READ | PTRACE_MODE_NOAUDIT);
+
+ if (!mutex_lock_killable(&task->signal->cred_guard_mutex)) {
+ permitted = ptrace_may_access(task, ptrace_mode);
+ if (permitted && !same_cred)
+ permitted = proc_allow_access(file->f_cred,
+ task, ptrace_mode);
+
+ mutex_unlock(&task->signal->cred_guard_mutex);
+ }
+
mm = get_task_mm(task);
if (mm) {
vsize = task_vsize(mm);
--
1.7.11.7