[PATCH] mm: fix NULL ptr deref when walking hugepages

[Sasha Levin <levinsasha928@gmail.com>]

A missing vlidation of the value returned by find_vma() could cause a NULL ptr
dereference when walking the pagetable.

This is triggerable from usermode by a simple user by trying to read a
page info out of /proc/pid/pagemap which doesn't exist.

Introduced by commit 025c5b24 ("thp: optimize away unnecessary page table
locking").

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
---
 fs/proc/task_mmu.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 3e564f0..885830b 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -820,7 +820,7 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 
 	/* find the first VMA at or above 'addr' */
 	vma = find_vma(walk->mm, addr);
-	if (pmd_trans_huge_lock(pmd, vma) == 1) {
+	if (vma && pmd_trans_huge_lock(pmd, vma) == 1) {
 		for (; addr != end; addr += PAGE_SIZE) {
 			unsigned long offset;
 
-- 
1.7.8.6

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