[PATCH] proc: make proc_fd_permission() thread-friendly

[Oleg Nesterov]

proc_fd_permission() says "process can still access /proc/self/fd
after it has executed a setuid()", but the "task_pid() = proc_pid()
check only helps if the task is group leader, /proc/self points to
/proc/leader-pid.

Change this check to use task_tgid() so that the whole process can
access /proc/self/fd.

Note: CLONE_THREAD doesn't require CLONE_FILES so task->files can
differ, but I don't think this can lead to any security problem.

Test-case:

	void *tfunc(void *arg)
	{
		assert(opendir("/proc/self/fd"));
		return NULL;
	}

	int main(void)
	{
		pthread_t t;
		pthread_create(&t, NULL, tfunc, NULL);
		pthread_join(t, NULL);
		return 0;
	}

fails if, say, this executable is not readable and suid_dumpable = 0.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 fs/proc/fd.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 0ff80f9..985ea88 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -286,7 +286,7 @@ int proc_fd_permission(struct inode *inode, int mask)
 	int rv = generic_permission(inode, mask);
 	if (rv == 0)
 		return 0;
-	if (task_pid(current) == proc_pid(inode))
+	if (task_tgid(current) == proc_pid(inode))
 		rv = 0;
 	return rv;
 }
-- 
1.5.5.1


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