[PATCH] fix put_user under mmap_sem in sys_get_mempolicy()

[PATCH] fix put_user under mmap_sem in sys_get_mempolicy()

[Oleg Nesterov]

Hello.

sys_get_mempolicy() accesses user memory with mmap_sem held.
If I understand correctly, this can cause deadlock:

sys_get_mempolicy:		Another thread, same mm:

down_read(mmap_sem);
				down_write(mmap_sem);
put_user();
do_page_fault:
down_read(mmap_sem);

Compile tested only, I have no NUMA machine.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 2.6.11-rc1/mm/mempolicy.c~	Wed Jan 12 11:44:55 2005
+++ 2.6.11-rc1/mm/mempolicy.c	Fri Jan 21 17:41:47 2005
@@ -482,26 +482,38 @@ asmlinkage long sys_get_mempolicy(int __
 				  unsigned long maxnode,
 				  unsigned long addr, unsigned long flags)
 {
-	int err, pval;
-	struct mm_struct *mm = current->mm;
-	struct vm_area_struct *vma = NULL;
+	int err, pval = 0; /* make compiler happy */
 	struct mempolicy *pol = current->mempolicy;
 
 	if (flags & ~(unsigned long)(MPOL_F_NODE|MPOL_F_ADDR))
 		return -EINVAL;
 	if (nmask != NULL && maxnode < MAX_NUMNODES)
 		return -EINVAL;
+
 	if (flags & MPOL_F_ADDR) {
+		struct mm_struct *mm = current->mm;
+		struct vm_area_struct *vma;
+
+		err = 0;
 		down_read(&mm->mmap_sem);
 		vma = find_vma_intersection(mm, addr, addr+1);
-		if (!vma) {
-			up_read(&mm->mmap_sem);
-			return -EFAULT;
+		if (!vma)
+			err = -EFAULT;
+		else {
+			if (vma->vm_ops && vma->vm_ops->get_policy)
+				pol = vma->vm_ops->get_policy(vma, addr);
+			else
+				pol = vma->vm_policy;
+
+			if (flags & MPOL_F_NODE) {
+				pval = lookup_node(mm, addr);
+				if (pval < 0)
+					err = pval;
+			}
 		}
-		if (vma->vm_ops && vma->vm_ops->get_policy)
-			pol = vma->vm_ops->get_policy(vma, addr);
-		else
-			pol = vma->vm_policy;
+		up_read(&mm->mmap_sem);
+		if (err)
+			goto out;
 	} else if (addr)
 		return -EINVAL;
 
@@ -509,17 +521,14 @@ asmlinkage long sys_get_mempolicy(int __
 		pol = &default_policy;
 
 	if (flags & MPOL_F_NODE) {
-		if (flags & MPOL_F_ADDR) {
-			err = lookup_node(mm, addr);
-			if (err < 0)
+		if (!(flags & MPOL_F_ADDR)) {
+			if (pol == current->mempolicy &&
+					pol->policy == MPOL_INTERLEAVE) {
+				pval = current->il_next;
+			} else {
+				err = -EINVAL;
 				goto out;
-			pval = err;
-		} else if (pol == current->mempolicy &&
-				pol->policy == MPOL_INTERLEAVE) {
-			pval = current->il_next;
-		} else {
-			err = -EINVAL;
-			goto out;
+			}
 		}
 	} else
 		pval = pol->policy;
@@ -534,10 +543,7 @@ asmlinkage long sys_get_mempolicy(int __
 		get_zonemask(pol, nodes);
 		err = copy_nodes_to_user(nmask, maxnode, nodes, sizeof(nodes));
 	}
-
- out:
-	if (vma)
-		up_read(&current->mm->mmap_sem);
+out:
 	return err;
 }

Re: [PATCH] fix put_user under mmap_sem in sys_get_mempolicy()

[Andi Kleen]

On Fri, Jan 21, 2005 at 05:51:07PM +0300, Oleg Nesterov wrote:
> Hello.
> 
> sys_get_mempolicy() accesses user memory with mmap_sem held.
> If I understand correctly, this can cause deadlock:
> 
> sys_get_mempolicy:		Another thread, same mm:
> 
> down_read(mmap_sem);
> 				down_write(mmap_sem);
> put_user();
> do_page_fault:
> down_read(mmap_sem);

Hrm. Normal Linux policy seems to ignore this potential
and rare deadlock and using nesting safely (e.g. it's been 
known for a long time for the tasklist rw spinlock, but
nobody really cares and it doesn't seem to be hit by users). 
Do you really think it is likely to happen?
> 
> Compile tested only, I have no NUMA machine.

It's hard to figure out what your patch actually does because
of all the gratious white space changes.

I suppose this simpler patch has the same effect (also untested).


diff -u linux-2.6.11-rc1-bk4/mm/mempolicy.c-o linux-2.6.11-rc1-bk4/mm/mempolicy.c
--- linux-2.6.11-rc1-bk4/mm/mempolicy.c-o	2005-01-14 10:12:27.000000000 +0100
+++ linux-2.6.11-rc1-bk4/mm/mempolicy.c	2005-01-21 15:26:12.000000000 +0100
@@ -485,7 +485,7 @@
 	int err, pval;
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma = NULL;
-	struct mempolicy *pol = current->mempolicy;
+	struct mempolicy *pol = current->mempolicy, *pol2 = NULL;
 
 	if (flags & ~(unsigned long)(MPOL_F_NODE|MPOL_F_ADDR))
 		return -EINVAL;
@@ -502,6 +502,10 @@
 			pol = vma->vm_ops->get_policy(vma, addr);
 		else
 			pol = vma->vm_policy;
+		pol2 = mpol_copy(pol);
+		up_read(&mm->mmap_sem);
+		if (IS_ERR(pol2)) 
+			return PTR_ERR(pol2);
 	} else if (addr)
 		return -EINVAL;
 
@@ -536,8 +540,7 @@
 	}
 
  out:
-	if (vma)
-		up_read(&current->mm->mmap_sem);
+	mpol_free(pol2);
 	return err;
 }
 

Re: [PATCH] fix put_user under mmap_sem in sys_get_mempolicy()

[Oleg Nesterov]

Andi Kleen wrote:
>
> I suppose this simpler patch has the same effect (also untested).
>
> 	if (flags & ~(unsigned long)(MPOL_F_NODE|MPOL_F_ADDR))
> 		return -EINVAL;
>@@ -502,6 +502,10 @@
> 			pol = vma->vm_ops->get_policy(vma, addr);
> 		else
> 			pol = vma->vm_policy;
>+		pol2 = mpol_copy(pol);
>+		up_read(&mm->mmap_sem);
>+		if (IS_ERR(pol2)) 
>+			return PTR_ERR(pol2);
>

I don't think so. With MPOL_F_ADDR|MPOL_F_NODE sys_get_mempolicy
calls lookup_node()->get_user_pages() few lines below, so we can't
up_read(&mm->mmap_sem) here.

> It's hard to figure out what your patch actually does because
> of all the gratious white space changes.

For your convenience here is the code with the patch applied.

	if (flags & MPOL_F_ADDR) {
		struct mm_struct *mm = current->mm;
		struct vm_area_struct *vma;

		err = 0;
		down_read(&mm->mmap_sem);
		vma = find_vma_intersection(mm, addr, addr+1);
		if (!vma)
			err = -EFAULT;
		else {
			if (vma->vm_ops && vma->vm_ops->get_policy)
				pol = vma->vm_ops->get_policy(vma, addr);
			else
				pol = vma->vm_policy;

			if (flags & MPOL_F_NODE) {
				pval = lookup_node(mm, addr);
				if (pval < 0)
					err = pval;
			}
		}
		up_read(&mm->mmap_sem);
		if (err)
			goto out;
	} else if (addr)
		return -EINVAL;

	if (!pol)
		pol = &default_policy;

	if (flags & MPOL_F_NODE) {
		if (!(flags & MPOL_F_ADDR)) {
			if (pol == current->mempolicy &&
					pol->policy == MPOL_INTERLEAVE) {
				pval = current->il_next;
			} else {
				err = -EINVAL;
				goto out;
			}
		}
	} else
		pval = pol->policy;

Re: [PATCH] fix put_user under mmap_sem in sys_get_mempolicy()

[Andi Kleen]

On Fri, Jan 21, 2005 at 07:01:55PM +0300, Oleg Nesterov wrote:
> Andi Kleen wrote:
> >
> > I suppose this simpler patch has the same effect (also untested).
> >
> > 	if (flags & ~(unsigned long)(MPOL_F_NODE|MPOL_F_ADDR))
> > 		return -EINVAL;
> >@@ -502,6 +502,10 @@
> > 			pol = vma->vm_ops->get_policy(vma, addr);
> > 		else
> > 			pol = vma->vm_policy;
> >+		pol2 = mpol_copy(pol);
> >+		up_read(&mm->mmap_sem);
> >+		if (IS_ERR(pol2)) 
> >+			return PTR_ERR(pol2);
> >
> 
> I don't think so. With MPOL_F_ADDR|MPOL_F_NODE sys_get_mempolicy
> calls lookup_node()->get_user_pages() few lines below, so we can't
> up_read(&mm->mmap_sem) here.

True.

> 
> > It's hard to figure out what your patch actually does because
> > of all the gratious white space changes.
> 
> For your convenience here is the code with the patch applied.

Looks reasonable. 

-Andi

P.S.: Again if you really care about these class of deadlocks take a look at
tasklist_lock.