[PATCH] proc: Cleanup proc_fd_access_allowed

In process of getting proc_fd_access_allowed to work it has developed a few
warts.  In particular the special case that always allows introspection and
the special case to allow inspection of kernel threads.

The special case for introspection is needed for /proc/self/mem.

The special case for kernel threads really should be overridable
by security modules.

So consolidate these checks into ptrace.c:may_attach().

The check to always allow introspection is trivial.

The check to allow access to kernel threads, and zombies is a little
trickier.  mem_read and mem_write already verify an mm exists so it isn't
needed twice.  proc_fd_access_allowed only doesn't want a check to verify
task->mm exits, s it prevents all access to kernel threads.  So just move
the task->mm check into ptrace_attach where it is needed for practical
reasons.

I did a quick audit and none of the security modules in the kernel seem to
care if they are passed a task without an mm into security_ptrace.  So the
above move should be safe and it allows security modules to come up with
more restrictive policy.
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/proc/base.c

@@ -536,29 +536,15 @@ static int proc_fd_access_allowed(struct inode *inode)
 {
 	struct task_struct *task;
 	int allowed = 0;
-	/* Allow access to a task's file descriptors if either we may
-	 * use ptrace attach to the process and find out that
-	 * information, or if the task cannot possibly be ptraced
-	 * allow access if we have the proper capability.
+	/* Allow access to a task's file descriptors if it is us or we
+	 * may use ptrace attach to the process and find out that
+	 * information.
 	 */
 	task = get_proc_task(inode);
-	if (task == current)
-		allowed = 1;
-	if (task && !allowed) {
-		int alive;
-
-		task_lock(task);
-		alive = !!task->mm;
-		task_unlock(task);
-		if (alive)
-			/* For a living task obey ptrace_may_attach */
+	if (task) {
 		allowed = ptrace_may_attach(task);
-		else
-			/* For a special task simply check the capability */
-			allowed = capable(CAP_SYS_PTRACE);
-	}
-	if (task)
 		put_task_struct(task);
+	}
 	return allowed;
 }
 

kernel/ptrace.c

@@ -120,8 +120,18 @@ int ptrace_check_attach(struct task_struct *child, int kill)
 
 static int may_attach(struct task_struct *task)
 {
-	if (!task->mm)
-		return -EPERM;
+	/* May we inspect the given task?
+	 * This check is used both for attaching with ptrace
+	 * and for allowing access to sensitive information in /proc.
+	 *
+	 * ptrace_attach denies several cases that /proc allows
+	 * because setting up the necessary parent/child relationship
+	 * or halting the specified task is impossible.
+	 */
+	int dumpable = 0;
+	/* Don't let security modules deny introspection */
+	if (task == current)
+		return 0;
 	if (((current->uid != task->euid) ||
 	     (current->uid != task->suid) ||
 	     (current->uid != task->uid) ||
@@ -130,7 +140,9 @@ static int may_attach(struct task_struct *task)
 	     (current->gid != task->gid)) && !capable(CAP_SYS_PTRACE))
 		return -EPERM;
 	smp_rmb();
-	if (!task->mm->dumpable && !capable(CAP_SYS_PTRACE))
+	if (task->mm)
+		dumpable = task->mm->dumpable;
+	if (!dumpable && !capable(CAP_SYS_PTRACE))
 		return -EPERM;
 
 	return security_ptrace(current, task);
@@ -176,6 +188,8 @@ repeat:
 		goto repeat;
 	}
 
+	if (!task->mm)
+		goto bad;
 	/* the same process cannot be attached many times */
 	if (task->ptrace & PT_PTRACED)
 		goto bad;