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

On Wed 14-12-22 17:21:10, Mathieu Desnoyers wrote:

quoted

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@efficios.com>
Cc: Aneesh Kumar K.V <redacted>
Cc: Ben Widawsky <redacted>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Feng Tang <redacted>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Andrea Arcangeli <redacted>
Cc: Mel Gorman <redacted>
Cc: Mike Kravetz <redacted>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Vlastimil Babka <redacted>
Cc: Andi Kleen <redacted>
Cc: Dan Williams <redacted>
Cc: Huang Ying <redacted>
Cc: <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: stable@vger.kernel.org # 5.17+

Acked-by: Michal Hocko <mhocko@suse.com>
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.
---

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));