Re: [RFC PATCH] mm/mempolicy: Fix memory leak in set_mempolicy_home_node system call

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On 2022-12-15 02:51, Michal Hocko wrote:
On Wed 14-12-22 17:21:10, Mathieu Desnoyers wrote:
When encountering any vma in the range with policy other than MPOL_BIND
or MPOL_PREFERRED_MANY, an error is returned without issuing a mpol_put
on the policy just allocated with mpol_dup().

This allows arbitrary users to leak kernel memory.

Fixes: c6018b4b2549 ("mm/mempolicy: add set_mempolicy_home_node syscall")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
Cc: Ben Widawsky <ben.widawsky@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Feng Tang <feng.tang@xxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Huang Ying <ying.huang@xxxxxxxxx>
Cc: <linux-api@xxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # 5.17+

Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks for catching this!

Btw. looking at the code again it seems rather pointless to duplicate
the policy just to throw it away anyway. A slightly bigger diff but this
looks more reasonable to me. What do you think? I can also send it as a
clean up on top of your fix.

I think it would be best if this comes as a cleanup on top of my fix. The diff is larger than the minimal change needed to fix the leak in stable branches.

Your approach looks fine, except for the vma_policy(vma) -> old change already spotted by Aneesh.

Thanks,

Mathieu

---
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 61aa9aedb728..918cdc8a7f0c 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1489,7 +1489,7 @@ SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, le
  {
  	struct mm_struct *mm = current->mm;
  	struct vm_area_struct *vma;
-	struct mempolicy *new;
+	struct mempolicy *new. *old;
  	unsigned long vmstart;
  	unsigned long vmend;
  	unsigned long end;
@@ -1521,30 +1521,28 @@ SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, le
  		return 0;
  	mmap_write_lock(mm);
  	for_each_vma_range(vmi, vma, end) {
-		vmstart = max(start, vma->vm_start);
-		vmend   = min(end, vma->vm_end);
-		new = mpol_dup(vma_policy(vma));
-		if (IS_ERR(new)) {
-			err = PTR_ERR(new);
-			break;
-		}
-		/*
-		 * Only update home node if there is an existing vma policy
-		 */
-		if (!new)
-			continue;
-
  		/*
  		 * If any vma in the range got policy other than MPOL_BIND
  		 * or MPOL_PREFERRED_MANY we return error. We don't reset
  		 * the home node for vmas we already updated before.
  		 */
-		if (new->mode != MPOL_BIND && new->mode != MPOL_PREFERRED_MANY) {
+		old = vma_policy(vma);
+		if (!old)
+			continue;
+		if (old->mode != MPOL_BIND && old->mode != MPOL_PREFERRED_MANY) {
  			err = -EOPNOTSUPP;
  			break;
  		}
+ new = mpol_dup(vma_policy(vma));
+		if (IS_ERR(new)) {
+			err = PTR_ERR(new);
+			break;
+		}
+
  		new->home_node = home_node;
+		vmstart = max(start, vma->vm_start);
+		vmend   = min(end, vma->vm_end);
  		err = mbind_range(mm, vmstart, vmend, new);
  		mpol_put(new);
  		if (err)

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com




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