linux-kernel - [PATCH 03/13] lib/vsprintf.c: eliminate potential race in string()

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Message-Id: <1445373013-20207-4-git-send-email-linux@rasmusvillemoes.dk>
Date:	Tue, 20 Oct 2015 22:30:03 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: [PATCH 03/13] lib/vsprintf.c: eliminate potential race in string()

If the string corresponding to a %s specifier can change under us, we
might end up copying a \0 byte to the output buffer. There might be
callers who expect the output buffer to contain a genuine C string
whose length is exactly the snprintf return value (assuming truncation
hasn't happened or has been checked for).

We can avoid this by only passing over the source string once,
stopping the first time we meet a nul byte (or when we reach the given
precision), and then letting widen_string() handle left/right space
padding.

Cc: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 lib/vsprintf.c | 28 +++++++++-------------------
 1 file changed, 9 insertions(+), 19 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index a0145b5d114e..ba00eab1e703 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -557,32 +557,22 @@ char *widen_string(char *buf, int n, char *end, struct printf_spec spec)
 static noinline_for_stack
 char *string(char *buf, char *end, const char *s, struct printf_spec spec)
 {
-	int len, i;
+	int len = 0;
+	size_t lim = spec.precision;
 
 	if ((unsigned long)s < PAGE_SIZE)
 		s = "(null)";
 
-	len = strnlen(s, spec.precision);
-
-	if (!(spec.flags & LEFT)) {
-		while (len < spec.field_width--) {
-			if (buf < end)
-				*buf = ' ';
-			++buf;
-		}
-	}
-	for (i = 0; i < len; ++i) {
-		if (buf < end)
-			*buf = *s;
-		++buf; ++s;
-	}
-	while (len < spec.field_width--) {
+	while (lim--) {
+		char c = *s++;
+		if (!c)
+			break;
 		if (buf < end)
-			*buf = ' ';
+			*buf = c;
 		++buf;
+		++len;
 	}
-
-	return buf;
+	return widen_string(buf, len, end, spec);
 }
 
 static noinline_for_stack
-- 
2.6.1

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