linux-kernel - [PATCH] lkdtm: fix potential use after free

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Date:   Wed, 24 Apr 2019 17:59:52 +0800
From:   Weikang shi <swkhack@...il.com>
To:     keescook@...omium.org
Cc:     arnd@...db.de, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, swkhack <swkhack@...il.com>
Subject: [PATCH] lkdtm: fix potential use after free

From: swkhack <swkhack@...il.com>

The function lkdtm_READ_AFTER_FREE calls kfree(base) to free the memory
of base. However, following kfree(base),
it access the memory which base point to via base[offset]. This may result in a
use-after-free bug. This patch moves kfree(base) after the dereference.

Signed-off-by: swkhack <swkhack@...il.com>
---
 drivers/misc/lkdtm/heap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/misc/lkdtm/heap.c b/drivers/misc/lkdtm/heap.c
index 65026d7de..3e2f1580a 100644
--- a/drivers/misc/lkdtm/heap.c
+++ b/drivers/misc/lkdtm/heap.c
@@ -77,7 +77,6 @@ void lkdtm_READ_AFTER_FREE(void)
 	base[offset] = *val;
 	pr_info("Value in memory before free: %x\n", base[offset]);
 
-	kfree(base);
 
 	pr_info("Attempting bad read from freed memory\n");
 	saw = base[offset];
@@ -88,6 +87,7 @@ void lkdtm_READ_AFTER_FREE(void)
 	}
 	pr_info("Memory was not poisoned\n");
 
+	kfree(base);
 	kfree(val);
 }
 
-- 
2.17.1