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: 8409cca705 ("userns: allow ptrace from non-init user namespaces")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman 2016-10-13 21:23:16 -05:00
parent 9c763584b7
commit bfedb58925
4 changed files with 20 additions and 18 deletions

View file

@ -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;

View file

@ -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);

View file

@ -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);

View file

@ -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)
};