linux-kernel - [PATCH 3.18 18/50] strscpy: zero any trailing garbage bytes in the destination

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Date:   Fri,  4 Aug 2017 16:16:04 -0700
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Chris Metcalf <cmetcalf@...hip.com>
Subject: [PATCH 3.18 18/50] strscpy: zero any trailing garbage bytes in the destination

3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Chris Metcalf <cmetcalf@...hip.com>

commit 990486c8af044f89bddfbde1d1cf9fde449bedbf upstream.

It's possible that the destination can be shadowed in userspace
(as, for example, the perf buffers are now).  So we should take
care not to leak data that could be inspected by userspace.

Signed-off-by: Chris Metcalf <cmetcalf@...hip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 lib/string.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/lib/string.c
+++ b/lib/string.c
@@ -211,12 +211,13 @@ ssize_t strscpy(char *dest, const char *
 		unsigned long c, data;
 
 		c = *(unsigned long *)(src+res);
-		*(unsigned long *)(dest+res) = c;
 		if (has_zero(c, &data, &constants)) {
 			data = prep_zero_mask(c, data, &constants);
 			data = create_zero_mask(data);
+			*(unsigned long *)(dest+res) = c & zero_bytemask(data);
 			return res + find_zero(data);
 		}
+		*(unsigned long *)(dest+res) = c;
 		res += sizeof(unsigned long);
 		count -= sizeof(unsigned long);
 		max -= sizeof(unsigned long);