Date:   Mon, 16 Nov 2020 14:32:52 +0000
From:   Richard Fitzgerald <rf@...nsource.cirrus.com>
To:     <pmladek@...e.com>, <rostedt@...dmis.org>,
        <sergey.senozhatsky@...il.com>
CC:     <linux-kernel@...r.kernel.org>, <patches@...nsource.cirrus.com>,
        Richard Fitzgerald <rf@...nsource.cirrus.com>
Subject: [PATCH] lib: vsprintf: Fix handling of number field widths in vsscanf

The existing code attempted to handle numbers by doing a strto[u]l(),
ignoring the field width, and then bitshifting the field out of the
converted value. If the string contains a run of valid digits longer
than will fit in a long or long long, this would overflow and no amount
of bitshifting can recover the correct value.

This patch fixes vsscanf to obey number field widths.

A new _parse_integer_limit() is added that takes a limit for the number
of characters to parse. A length of INT_MAX is effectively unlimited, as
we are not likely to need parsing of digit strings >INT_MAX length.

The number field conversion in vsscanf is changed to use this new
_parse_integer_limit() function so that field widths are obeyed when
parsing the number. Note also that the conversion is always done as a
long long - as there's currently no overflow checking there is no point
implementing separate long and long long conversions.

Signed-off-by: Richard Fitzgerald <rf@...nsource.cirrus.com>
---
 lib/kstrtox.c  | 14 ++++++++---
 lib/kstrtox.h  |  2 ++
 lib/vsprintf.c | 65 +++++++++++++++++++++++++++-----------------------
 3 files changed, 48 insertions(+), 33 deletions(-)

diff --git a/lib/kstrtox.c b/lib/kstrtox.c
index a14ccf905055..9867501a4ab0 100644
--- a/lib/kstrtox.c
+++ b/lib/kstrtox.c
@@ -39,20 +39,23 @@ const char *_parse_integer_fixup_radix(const char *s, unsigned int *base)
 
 /*
  * Convert non-negative integer string representation in explicitly given radix
- * to an integer.
+ * to an integer. The maximum number of characters to convert can be given.
+ * A character limit of INT_MAX is effectively unlimited since a string that
+ * long is unreasonable.
  * Return number of characters consumed maybe or-ed with overflow bit.
  * If overflow occurs, result integer (incorrect) is still returned.
  *
  * Don't you dare use this function.
  */
-unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long *p)
+unsigned int _parse_integer_limit(const char *s, unsigned int base, unsigned long long *p,
+				  int max_chars)
 {
 	unsigned long long res;
 	unsigned int rv;
 
 	res = 0;
 	rv = 0;
-	while (1) {
+	for (; max_chars > 0; max_chars--) {
 		unsigned int c = *s;
 		unsigned int lc = c | 0x20; /* don't tolower() this line */
 		unsigned int val;
@@ -82,6 +85,11 @@ unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long
 	return rv;
 }
 
+unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long *p)
+{
+	return _parse_integer_limit(s, base, p, INT_MAX);
+}
+
 static int _kstrtoull(const char *s, unsigned int base, unsigned long long *res)
 {
 	unsigned long long _res;
diff --git a/lib/kstrtox.h b/lib/kstrtox.h
index 3b4637bcd254..4e5dfa3437dc 100644
--- a/lib/kstrtox.h
+++ b/lib/kstrtox.h
@@ -4,6 +4,8 @@
 
 #define KSTRTOX_OVERFLOW	(1U << 31)
 const char *_parse_integer_fixup_radix(const char *s, unsigned int *base);
+unsigned int _parse_integer_limit(const char *s, unsigned int base,
+				  unsigned long long *res, int max_chars);
 unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long *res);
 
 #endif
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 14c9a6af1b23..8ec47b5da2cb 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -53,6 +53,25 @@
 #include <linux/string_helpers.h>
 #include "kstrtox.h"
 
+static unsigned long long simple_strntoull(const char *startp, int max_chars,
+					   char **endp, unsigned int base)
+{
+	const char *cp;
+	unsigned long long result;
+	unsigned int rv;
+
+	cp = _parse_integer_fixup_radix(startp, &base);
+	max_chars -= (cp - startp);
+	rv = _parse_integer_limit(cp, base, &result, max_chars);
+	/* FIXME */
+	cp += (rv & ~KSTRTOX_OVERFLOW);
+
+	if (endp)
+		*endp = (char *)cp;
+
+	return result;
+}
+
 /**
  * simple_strtoull - convert a string to an unsigned long long
  * @cp: The start of the string
@@ -63,18 +82,7 @@
  */
 unsigned long long simple_strtoull(const char *cp, char **endp, unsigned int base)
 {
-	unsigned long long result;
-	unsigned int rv;
-
-	cp = _parse_integer_fixup_radix(cp, &base);
-	rv = _parse_integer(cp, base, &result);
-	/* FIXME */
-	cp += (rv & ~KSTRTOX_OVERFLOW);
-
-	if (endp)
-		*endp = (char *)cp;
-
-	return result;
+	return simple_strntoull(cp, INT_MAX, endp, base);
 }
 EXPORT_SYMBOL(simple_strtoull);
 
@@ -126,6 +134,15 @@ long long simple_strtoll(const char *cp, char **endp, unsigned int base)
 }
 EXPORT_SYMBOL(simple_strtoll);
 
+static long long simple_strntoll(const char *cp, int max_chars, char **endp,
+				 unsigned int base)
+{
+	if (*cp == '-')
+		return -simple_strntoull(cp + 1, max_chars - 1, endp, base);
+
+	return simple_strntoull(cp, max_chars, endp, base);
+}
+
 static noinline_for_stack
 int skip_atoi(const char **s)
 {
@@ -3444,25 +3461,13 @@ int vsscanf(const char *buf, const char *fmt, va_list args)
 			break;
 
 		if (is_sign)
-			val.s = qualifier != 'L' ?
-				simple_strtol(str, &next, base) :
-				simple_strtoll(str, &next, base);
+			val.s = simple_strntoll(str,
+						field_width > 0 ? field_width : INT_MAX,
+						&next, base);
 		else
-			val.u = qualifier != 'L' ?
-				simple_strtoul(str, &next, base) :
-				simple_strtoull(str, &next, base);
-
-		if (field_width > 0 && next - str > field_width) {
-			if (base == 0)
-				_parse_integer_fixup_radix(str, &base);
-			while (next - str > field_width) {
-				if (is_sign)
-					val.s = div_s64(val.s, base);
-				else
-					val.u = div_u64(val.u, base);
-				--next;
-			}
-		}
+			val.u = simple_strntoull(str,
+						 field_width > 0 ? field_width : INT_MAX,
+						 &next, base);
 
 		switch (qualifier) {
 		case 'H':	/* that's 'hh' in format */
-- 
2.20.1

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