mempolicy: fix show_numa_map() vs exec() + do_set_mempolicy() race (498f2371) · Commits · 戴 / test

"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>

fs/proc/task_mmu.c

+9 −24

Original line number	Diff line number	Diff line
		@@ -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);

Admin message