[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>

1 files changed, 6 insertions, 20 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index f38da6bda269..773469703c62 100644
--- a/fs/proc/base.c
+++ b/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 */
-                        allowed = ptrace_may_attach(task);
-                else
-                        /* For a special task simply check the capability */
-                        allowed = capable(CAP_SYS_PTRACE);
-        }
-        if (task)
+        if (task) {
+                allowed = ptrace_may_attach(task);
                 put_task_struct(task);
+        }
         return allowed;
 }