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Date: Mon, 16 Jan 2012 13:56:55 +1100
From: Christopher Yeoh <cyeoh@....ibm.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: linux-kernel@...r.kernel.org,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
David Howells <dhowells@...hat.com>
Subject: Re: [PATCH] Fix race in process_vm_rw_core
On Sat, 14 Jan 2012 18:58:29 +0100
Oleg Nesterov <oleg@...hat.com> wrote:
> On 01/14, Christopher Yeoh wrote:
> >
> > On Fri, 13 Jan 2012 17:04:42 +0100
> > Oleg Nesterov <oleg@...hat.com> wrote:
> > > On 01/13, Christopher Yeoh wrote:
> > > > ...
> > > > +struct mm_struct *get_check_task_mm(struct task_struct *task,
> > > > unsigned int mode) +{
> > > > + struct mm_struct *mm;
> > > > + int err;
> > > > +
> > > > + err =
> > > > mutex_lock_killable(&task->signal->cred_guard_mutex);
> > > > + if (err)
> > > > + return ERR_PTR(err);
> > > > +
> > > > + task_lock(task);
> > > > + if (__ptrace_may_access(task, mode)) {
> > > > + mm = ERR_PTR(-EACCES);
> > > > + goto out;
> > > > + }
> > >
> > > Probably you should check "mm != current->mm" before
> > > __ptrace_may_access(), otherwise this changes the rules for,
> > > say, /proc/pid/maps.
> >
> > __ptrace_may_access has a check for task == current already -
> > Is that sufficient?
> >
> > /* Don't let security modules deny introspection */
> > if (task == current)
> > return 0;
>
> I don't think this is sufficient in the multithreaded or CLONE_VM
> case, task_cred/etc is per-thread.
>
> It is not that I think that this "current->mm != mm" check is
> important, in fact personally I think it shouldn't exist.
>
> But we shouldn't add the subtle and not documented behavioural
> change, and obviously process_vm_rw() has no security problems if mm
> == current->mm.
>
Ok, updated patch below:
- adds the "current->mm != mm" check
- removes EXPORT_SYMBOL_GPL for get_check_task_mm
fs/proc/base.c | 17 +----------------
include/linux/sched.h | 4 ++++
kernel/fork.c | 30 ++++++++++++++++++++++++++++++
mm/process_vm_access.c | 20 ++++++--------------
4 files changed, 41 insertions(+), 30 deletions(-)
Signed-off-by: Chris Yeoh <yeohc@....ibm.com>
Cc: stable@...r.kernel.org
---
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 851ba3d..094d650 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -254,22 +254,7 @@ static struct mm_struct *check_mem_permission(struct task_struct *task)
struct mm_struct *mm_for_maps(struct task_struct *task)
{
- struct mm_struct *mm;
- int err;
-
- err = mutex_lock_killable(&task->signal->cred_guard_mutex);
- if (err)
- return ERR_PTR(err);
-
- mm = get_task_mm(task);
- if (mm && mm != current->mm &&
- !ptrace_may_access(task, PTRACE_MODE_READ)) {
- mmput(mm);
- mm = ERR_PTR(-EACCES);
- }
- mutex_unlock(&task->signal->cred_guard_mutex);
-
- return mm;
+ return get_check_task_mm(task, PTRACE_MODE_READ);
}
static int proc_pid_cmdline(struct task_struct *task, char * buffer)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1c4f3e9..8a64cae 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2235,6 +2235,10 @@ static inline void mmdrop(struct mm_struct * mm)
extern void mmput(struct mm_struct *);
/* Grab a reference to a task's mm, if it is not already going away */
extern struct mm_struct *get_task_mm(struct task_struct *task);
+/* Grab a reference to a task's mm, if it is not already going away
+ and ptrace_may_access with the mode parameter passed to it succeeds */
+extern struct mm_struct *get_check_task_mm(struct task_struct *task,
+ unsigned int mode);
/* Remove the current tasks stale references to the old mm_struct */
extern void mm_release(struct task_struct *, struct mm_struct *);
/* Allocate a new mm structure and copy contents from tsk->mm */
diff --git a/kernel/fork.c b/kernel/fork.c
index da4a6a1..b6c193a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -644,6 +644,36 @@ struct mm_struct *get_task_mm(struct task_struct *task)
}
EXPORT_SYMBOL_GPL(get_task_mm);
+struct mm_struct *get_check_task_mm(struct task_struct *task, unsigned int mode)
+{
+ struct mm_struct *mm;
+ int err;
+
+ err = mutex_lock_killable(&task->signal->cred_guard_mutex);
+ if (err)
+ return ERR_PTR(err);
+
+ task_lock(task);
+ mm = task->mm;
+ if (mm != current->mm && __ptrace_may_access(task, mode)) {
+ mm = ERR_PTR(-EACCES);
+ goto out;
+ }
+
+ if (mm) {
+ if (task->flags & PF_KTHREAD)
+ mm = NULL;
+ else
+ atomic_inc(&mm->mm_users);
+ }
+
+out:
+ task_unlock(task);
+ mutex_unlock(&task->signal->cred_guard_mutex);
+
+ return mm;
+}
+
/* Please note the differences between mmput and mm_release.
* mmput is called whenever we stop holding onto a mm_struct,
* error success whatever.
diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c
index e920aa3..aa8009d 100644
--- a/mm/process_vm_access.c
+++ b/mm/process_vm_access.c
@@ -298,23 +298,15 @@ static ssize_t process_vm_rw_core(pid_t pid, const struct iovec *lvec,
goto free_proc_pages;
}
- task_lock(task);
- if (__ptrace_may_access(task, PTRACE_MODE_ATTACH)) {
- task_unlock(task);
- rc = -EPERM;
- goto put_task_struct;
- }
- mm = task->mm;
-
- if (!mm || (task->flags & PF_KTHREAD)) {
- task_unlock(task);
- rc = -EINVAL;
+ mm = get_check_task_mm(task, PTRACE_MODE_ATTACH);
+ if (!mm || IS_ERR(mm)) {
+ if (!mm)
+ rc = -EINVAL;
+ else
+ rc = -EPERM;
goto put_task_struct;
}
- atomic_inc(&mm->mm_users);
- task_unlock(task);
-
for (i = 0; i < riovcnt && iov_l_curr_idx < liovcnt; i++) {
rc = process_vm_rw_single_vec(
(unsigned long)rvec[i].iov_base, rvec[i].iov_len,
--
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