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From:   Weikang shi <swkhack@gmail.com>
To:     keescook@chromium.org
Cc:     arnd@arndb.de, gregkh@linuxfoundation.org,
        linux-kernel@vger.kernel.org, swkhack <swkhack@gmail.com>
Subject: [PATCH] lkdtm: fix potential use after free
Date:   Wed, 24 Apr 2019 18:21:03 +0800
Message-Id: <20190424102103.11816-1-swkhack@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: swkhack <swkhack@gmail.com>

The function lkdtm_WRITE_AFTER_FREE calls kfree(base) to free the memory
of base. However, following kfree(base),
it write the memory which base point to via base[offset] = 0x0abcdef0. This may result in a
use-after-free bug. This patch moves kfree(base) after the write.

Signed-off-by: swkhack <swkhack@gmail.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..0b9141525 100644
--- a/drivers/misc/lkdtm/heap.c
+++ b/drivers/misc/lkdtm/heap.c
@@ -40,8 +40,8 @@ void lkdtm_WRITE_AFTER_FREE(void)
 	pr_info("Allocated memory %p-%p\n", base, &base[offset * 2]);
 	pr_info("Attempting bad write to freed memory at %p\n",
 		&base[offset]);
-	kfree(base);
 	base[offset] = 0x0abcdef0;
+	kfree(base);
 	/* Attempt to notice the overwrite. */
 	again = kmalloc(len, GFP_KERNEL);
 	kfree(again);
-- 
2.17.1