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Message-ID: <lsq.1514480744.13273983@decadent.org.uk>
Date: Thu, 28 Dec 2017 17:05:44 +0000
From: Ben Hutchings <ben@...adent.org.uk>
To: linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC: akpm@...ux-foundation.org, "Cyrill Gorcunov" <gorcunov@...nvz.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
"Kees Cook" <keescook@...omium.org>
Subject: [PATCH 3.16 196/204] mm: Add a user_ns owner to mm_struct and fix
ptrace permission checks
3.16.52-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: "Eric W. Biederman" <ebiederm@...ssion.com>
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@...omium.org>
Tested-by: Cyrill Gorcunov <gorcunov@...nvz.org>
Fixes: 8409cca70561 ("userns: allow ptrace from non-init user namespaces")
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
include/linux/mm_types.h | 1 +
kernel/fork.c | 9 ++++++---
kernel/ptrace.c | 26 +++++++++++---------------
mm/init-mm.c | 2 ++
4 files changed, 20 insertions(+), 18 deletions(-)
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -419,6 +419,7 @@ 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 *exe_file;
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -527,7 +527,8 @@ static void mm_init_aio(struct mm_struct
#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)
{
atomic_set(&mm->mm_users, 1);
atomic_set(&mm->mm_count, 1);
@@ -551,6 +552,7 @@ static struct mm_struct *mm_init(struct
if (likely(!mm_alloc_pgd(mm))) {
mmu_notifier_mm_init(mm);
+ mm->user_ns = get_user_ns(user_ns);
return mm;
}
@@ -588,7 +590,7 @@ struct mm_struct *mm_alloc(void)
memset(mm, 0, sizeof(*mm));
mm_init_cpumask(mm);
- return mm_init(mm, current);
+ return mm_init(mm, current, current_user_ns());
}
/*
@@ -603,6 +605,7 @@ 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);
@@ -822,7 +825,7 @@ static struct mm_struct *dup_mm(struct t
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
mm->pmd_huge_pte = NULL;
#endif
- if (!mm_init(mm, tsk))
+ if (!mm_init(mm, tsk, mm->user_ns))
goto fail_nomem;
if (init_new_context(tsk, mm))
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -230,7 +230,7 @@ static int ptrace_has_cap(struct user_na
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;
@@ -281,16 +281,11 @@ static int __ptrace_may_access(struct ta
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);
}
@@ -341,6 +336,11 @@ static int ptrace_attach(struct task_str
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;
@@ -354,10 +354,6 @@ static int ptrace_attach(struct task_str
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);
--- 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)
};
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