[PATCH 3.2 149/164] __ptrace_may_access() should not deny sub-threads

[Ben Hutchings]

3.2.70-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Mark Grondona <mgrondona@xxxxxxxx>

commit 73af963f9f3036dffed55c3a2898598186db1045 upstream.

__ptrace_may_access() checks get_dumpable/ptrace_has_cap/etc if task !=
current, this can can lead to surprising results.

For example, a sub-thread can't readlink("/proc/self/exe") if the
executable is not readable.  setup_new_exec()->would_dump() notices that
inode_permission(MAY_READ) fails and then it does
set_dumpable(suid_dumpable).  After that get_dumpable() fails.

(It is not clear why proc_pid_readlink() checks get_dumpable(), perhaps we
could add PTRACE_MODE_NODUMPABLE)

Change __ptrace_may_access() to use same_thread_group() instead of "task
== current".  Any security check is pointless when the tasks share the
same ->mm.

Signed-off-by: Mark Grondona <mgrondona@xxxxxxxx>
Signed-off-by: Ben Woodard <woodard@xxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Cc: Sheng Yong <shengyong1@xxxxxxxxxx>
---
 kernel/ptrace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -225,7 +225,7 @@ int __ptrace_may_access(struct task_stru
 	 */
 	int dumpable = 0;
 	/* Don't let security modules deny introspection */
-	if (task == current)
+	if (same_thread_group(task, current))
 		return 0;
 	rcu_read_lock();
 	tcred = __task_cred(task);

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