On 03/13, Oleg Nesterov wrote:
>
> Yes. But it seems that use_mm() and unuse_mm() should invalidate vmacache too.
>
> Suppose that a kernel thread T does, say,
>
> use_mm(foreign_mm);
> get_user(...);
> unuse_mm();
>
> This can trigger a fault and populate T->vmacache[]. If this code is called
> again vmacache_find() can use the stale entries.
>
> Or, assuming that only a kernel thread can do use_mm(), we can change
> vmacache_valid() to also check !PF_KTHREAD.
Yes, I think we should check PF_KTHREAD, because
> Hmm. Another problem is that use_mm() doesn't take ->mmap_sem and thus
> it can race with vmacache_flush_all()...
this also closes this race. use_mm() users should not use vmacache at all.
> Finally. Shouldn't vmacache_update() check current->mm == mm as well?
> What if access_remote_vm/get_user_pages trigger find_vma() ??? Unless
> I missed something this is not theoretical at all and can lead to the
> corrupted vmacache, no?
Looks like a real problem or I am totally confused. I think we need
something like below (uncompiled).
Oleg.
--- x/mm/vmacache.c
+++ x/mm/vmacache.c
@@ -30,20 +30,24 @@ void vmacache_flush_all(struct mm_struct
rcu_read_unlock();
}
+static bool vmacache_valid_mm(mm)
+{
+ return current->mm == mm && !(current->flags & PF_KTHREAD);
+}
+
void vmacache_update(unsigned long addr, struct vm_area_struct *newvma)
{
- int idx = VMACACHE_HASH(addr);
- current->vmacache[idx] = newvma;
+ if (vmacache_valid_mm(newvma->vm_mm))
+ current->vmacache[VMACACHE_HASH(addr)] = newvma;
}
static bool vmacache_valid(struct mm_struct *mm)
{
- struct task_struct *curr = current;
-
- if (mm != curr->mm)
+ if (!vmacache_valid_mm(mm))
return false;
if (mm->vmacache_seqnum != curr->vmacache_seqnum) {
+ struct task_struct *curr = current;
/*
* First attempt will always be invalid, initialize
* the new cache for this task here.
--
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