linux-kernel - [PATCH RFC 2/9] s390/gmap: don't unconditionally call pte_unmap_unlock() in __gmap

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Date:   Thu,  9 Sep 2021 16:59:38 +0200
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-s390@...r.kernel.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, David Hildenbrand <david@...hat.com>
Subject: [PATCH RFC 2/9] s390/gmap: don't unconditionally call pte_unmap_unlock() in __gmap_zap()

... otherwise we will try unlocking a spinlock that was never locked via a
garbage pointer.

At the time we reach this code path, we usually successfully looked up
a PGSTE already; however, evil user space could have manipulated the VMA
layout in the meantime and triggered removal of the page table.

Fixes: 1e133ab296f3 ("s390/mm: split arch/s390/mm/pgtable.c")
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 arch/s390/mm/gmap.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
index b6b56cd4ca64..9023bf3ced89 100644
--- a/arch/s390/mm/gmap.c
+++ b/arch/s390/mm/gmap.c
@@ -690,9 +690,10 @@ void __gmap_zap(struct gmap *gmap, unsigned long gaddr)
 
 		/* Get pointer to the page table entry */
 		ptep = get_locked_pte(gmap->mm, vmaddr, &ptl);
-		if (likely(ptep))
+		if (likely(ptep)) {
 			ptep_zap_unused(gmap->mm, vmaddr, ptep, 0);
-		pte_unmap_unlock(ptep, ptl);
+			pte_unmap_unlock(ptep, ptl);
+		}
 	}
 }
 EXPORT_SYMBOL_GPL(__gmap_zap);
-- 
2.31.1