mm: Add a user_ns owner to mm_struct and fix ptrace permission checks

commit bfedb589252c01fa505ac9f6f2a3d5d68d707ef4 upstream.

During exec dumpable is cleared if the file that is being executed is
not readable by the user executing the file.  A bug in
ptrace_may_access allows reading the file if the executable happens to
enter into a subordinate user namespace (aka clone(CLONE_NEWUSER),
unshare(CLONE_NEWUSER), or setns(fd, CLONE_NEWUSER).

This problem is fixed with only necessary userspace breakage by adding
a user namespace owner to mm_struct, captured at the time of exec, so
it is clear in which user namespace CAP_SYS_PTRACE must be present in
to be able to safely give read permission to the executable.

The function ptrace_may_access is modified to verify that the ptracer
has CAP_SYS_ADMIN in task->mm->user_ns instead of task->cred->user_ns.
This ensures that if the task changes it's cred into a subordinate
user namespace it does not become ptraceable.

The function ptrace_attach is modified to only set PT_PTRACE_CAP when
CAP_SYS_PTRACE is held over task->mm->user_ns.  The intent of
PT_PTRACE_CAP is to be a flag to note that whatever permission changes
the task might go through the tracer has sufficient permissions for
it not to be an issue.  task->cred->user_ns is always the same
as or descendent of mm->user_ns.  Which guarantees that having
CAP_SYS_PTRACE over mm->user_ns is the worst case for the tasks
credentials.

To prevent regressions mm->dumpable and mm->user_ns are not considered
when a task has no mm.  As simply failing ptrace_may_attach causes
regressions in privileged applications attempting to read things
such as /proc/<pid>/stat

Acked-by: Kees Cook <keescook@chromium.org>
Tested-by: Cyrill Gorcunov <gorcunov@openvz.org>
Fixes: 8409cca70561 ("userns: allow ptrace from non-init user namespaces")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 17 insertions, 18 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 997ac1d584f7..ba8a01564985 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -745,7 +745,8 @@ static void mm_init_owner(struct mm_struct *mm, struct task_struct *p)
 #endif
 }
 
-static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
+static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
+        struct user_namespace *user_ns)
 {
         mm->mmap = NULL;
         mm->mm_rb = RB_ROOT;
@@ -785,6 +786,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
         if (init_new_context(p, mm))
                 goto fail_nocontext;
 
+        mm->user_ns = get_user_ns(user_ns);
         return mm;
 
 fail_nocontext:
@@ -830,7 +832,7 @@ struct mm_struct *mm_alloc(void)
                 return NULL;
 
         memset(mm, 0, sizeof(*mm));
-        return mm_init(mm, current);
+        return mm_init(mm, current, current_user_ns());
 }
 
 /*
@@ -845,6 +847,7 @@ void __mmdrop(struct mm_struct *mm)
         destroy_context(mm);
         mmu_notifier_mm_destroy(mm);
         check_mm(mm);
+        put_user_ns(mm->user_ns);
         free_mm(mm);
 }
 EXPORT_SYMBOL_GPL(__mmdrop);
@@ -1126,7 +1129,7 @@ static struct mm_struct *dup_mm(struct task_struct *tsk)
 
         memcpy(mm, oldmm, sizeof(*mm));
 
-        if (!mm_init(mm, tsk))
+        if (!mm_init(mm, tsk, mm->user_ns))
                 goto fail_nomem;
 
         err = dup_mmap(mm, oldmm);
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index e6474f7272ec..282821557183 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -220,7 +220,7 @@ static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
 static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
 {
         const struct cred *cred = current_cred(), *tcred;
-        int dumpable = 0;
+        struct mm_struct *mm;
         kuid_t caller_uid;
         kgid_t caller_gid;
 
@@ -271,16 +271,11 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
         return -EPERM;
 ok:
         rcu_read_unlock();
-        smp_rmb();
-        if (task->mm)
-                dumpable = get_dumpable(task->mm);
-        rcu_read_lock();
-        if (dumpable != SUID_DUMP_USER &&
-            !ptrace_has_cap(__task_cred(task)->user_ns, mode)) {
-                rcu_read_unlock();
-                return -EPERM;
-        }
-        rcu_read_unlock();
+        mm = task->mm;
+        if (mm &&
+            ((get_dumpable(mm) != SUID_DUMP_USER) &&
+             !ptrace_has_cap(mm->user_ns, mode)))
+            return -EPERM;
 
         return security_ptrace_access_check(task, mode);
 }
@@ -331,6 +326,11 @@ static int ptrace_attach(struct task_struct *task, long request,
 
         task_lock(task);
         retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS);
+        if (!retval) {
+                struct mm_struct *mm = task->mm;
+                if (mm && ns_capable(mm->user_ns, CAP_SYS_PTRACE))
+                        flags |= PT_PTRACE_CAP;
+        }
         task_unlock(task);
         if (retval)
                 goto unlock_creds;
@@ -344,10 +344,6 @@ static int ptrace_attach(struct task_struct *task, long request,
 
         if (seize)
                 flags |= PT_SEIZED;
-        rcu_read_lock();
-        if (ns_capable(__task_cred(task)->user_ns, CAP_SYS_PTRACE))
-                flags |= PT_PTRACE_CAP;
-        rcu_read_unlock();
         task->ptrace = flags;
 
         __ptrace_link(task, current);