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mempolicy: fix show_numa_map() vs exec() + do_set_mempolicy() race 

9e7814404b "hold task->mempolicy while numa_maps scans." fixed the
race with the exiting task but this is not enough.

The current code assumes that get_vma_policy(task) should either see
task->mempolicy == NULL or it should be equal to ->task_mempolicy saved
by hold_task_mempolicy(), so we can never race with __mpol_put(). But
this can only work if we can't race with do_set_mempolicy(), and thus
we can't race with another do_set_mempolicy() or do_exit() after that.

However, do_set_mempolicy()->down_write(mmap_sem) can not prevent this
race. This task can exec, change it's ->mm, and call do_set_mempolicy()
after that; in this case they take 2 different locks.

Change hold_task_mempolicy() to use get_task_policy(), it never returns
NULL, and change show_numa_map() to use __get_vma_policy() or fall back
to proc_priv->task_mempolicy.

Note: this is the minimal fix, we will cleanup this code later. I think
hold_task_mempolicy() and release_task_mempolicy() should die, we can
move this logic into show_numa_map(). Or we can move get_task_policy()
outside of ->mmap_sem and !CONFIG_NUMA code at least.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Oleg Nesterov 2014-10-09 15:27:52 -07:00 committed by Linus Torvalds
parent 74d2c3a05c
commit 498f237178
1 changed files with 9 additions and 24 deletions
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29
fs/proc/task_mmu.c
View File

@ -87,32 +87,14 @@ unsigned long task_statm(struct mm_struct *mm,
#ifdef CONFIG_NUMA
/*
* These functions are for numa_maps but called in generic **maps seq_file
* ->start(), ->stop() ops.
*
* numa_maps scans all vmas under mmap_sem and checks their mempolicy.
* Each mempolicy object is controlled by reference counting. The problem here
* is how to avoid accessing dead mempolicy object.
*
* Because we're holding mmap_sem while reading seq_file, it's safe to access
* each vma's mempolicy, no vma objects will never drop refs to mempolicy.
*
* A task's mempolicy (task->mempolicy) has different behavior. task->mempolicy
* is set and replaced under mmap_sem but unrefed and cleared under task_lock().
* So, without task_lock(), we cannot trust get_vma_policy() because we cannot
* gurantee the task never exits under us. But taking task_lock() around
* get_vma_plicy() causes lock order problem.
*
* To access task->mempolicy without lock, we hold a reference count of an
* object pointed by task->mempolicy and remember it. This will guarantee
* that task->mempolicy points to an alive object or NULL in numa_maps accesses.
* Save get_task_policy() for show_numa_map().
*/
static void hold_task_mempolicy(struct proc_maps_private *priv)
{
struct task_struct *task = priv->task;
task_lock(task);
priv->task_mempolicy = task->mempolicy;
priv->task_mempolicy = get_task_policy(task);
mpol_get(priv->task_mempolicy);
task_unlock(task);
}
@ -1431,7 +1413,6 @@ static int show_numa_map(struct seq_file *m, void *v, int is_pid)
struct vm_area_struct *vma = v;
struct numa_maps *md = &numa_priv->md;
struct file *file = vma->vm_file;
struct task_struct *task = proc_priv->task;
struct mm_struct *mm = vma->vm_mm;
struct mm_walk walk = {};
struct mempolicy *pol;
@ -1451,9 +1432,13 @@ static int show_numa_map(struct seq_file *m, void *v, int is_pid)
walk.private = md;
walk.mm = mm;
pol = get_vma_policy(task, vma, vma->vm_start);
pol = __get_vma_policy(vma, vma->vm_start);
if (pol) {
mpol_to_str(buffer, sizeof(buffer), pol);
mpol_cond_put(pol);
} else {
mpol_to_str(buffer, sizeof(buffer), proc_priv->task_mempolicy);
}
seq_printf(m, "%08lx %s", vma->vm_start, buffer);