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

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

Cc: stable@vger.kernel.org
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>

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 4a8acedf4b7d914b8474773328d09adb3d6bc2c4..08d947fc4c59dac845652dee2ad1c98aac2f2626 100644 (file)
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -473,6 +473,7 @@ struct mm_struct {
         */
        struct task_struct __rcu *owner;
 #endif
+       struct user_namespace *user_ns;
 
        /* store ref to file /proc/<pid>/exe symlink points to */
        struct file __rcu *exe_file;

diff --git a/kernel/fork.c b/kernel/fork.c
index 997ac1d584f76b0e42551975bdae960e61c0807e..ba8a01564985f2e6e3556c34c6c16744c3b29a86 100644 (file)
--- 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 e6474f7272ec2ce96c95532ea906da113acd5354..282821557183f3d58f8433dddbf2e2609e2ed6ee 100644 (file)
--- 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);

diff --git a/mm/init-mm.c b/mm/init-mm.c
index a56a851908d245248803a602f68d5833cf1f8412..975e49f00f34c1b282dee5840740effb9a999bed 100644 (file)
--- a/mm/init-mm.c
+++ b/mm/init-mm.c
@@ -6,6 +6,7 @@
 #include <linux/cpumask.h>
 
 #include <linux/atomic.h>
+#include <linux/user_namespace.h>
 #include <asm/pgtable.h>
 #include <asm/mmu.h>
 
@@ -21,5 +22,6 @@ struct mm_struct init_mm = {
        .mmap_sem       = __RWSEM_INITIALIZER(init_mm.mmap_sem),
        .page_table_lock =  __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
        .mmlist         = LIST_HEAD_INIT(init_mm.mmlist),
+       .user_ns        = &init_user_ns,
        INIT_MM_CONTEXT(init_mm)
 };