lib: vsprintf: Fix handling of number field widths in vsscanf

[ Upstream commit 900fdc4573766dd43b847b4f54bd4a1ee2bc7360 ]

The existing code attempted to handle numbers by doing a strto[u]l(),
ignoring the field width, and then repeatedly dividing to extract the
field out of the full 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 dividing can recover the correct value.

This patch fixes vsscanf() to obey number field widths when parsing
the number.

A new _parse_integer_limit() is added that takes a limit for the number
of characters to parse. The number field conversion in vsscanf is changed
to use this new function.

If a number starts with a radix prefix, the field width  must be long
enough for at last one digit after the prefix. If not, it will be handled
like this:

 sscanf("0x4", "%1i", &i): i=0, scanning continues with the 'x'
 sscanf("0x4", "%2i", &i): i=0, scanning continues with the '4'

This is consistent with the observed behaviour of userland sscanf.

Note that this patch does NOT fix the problem of a single field value
overflowing the target type. So for example:

  sscanf("123456789abcdef", "%x", &i);

Will not produce the correct result because the value obviously overflows
INT_MAX. But sscanf will report a successful conversion.

Note that where a very large number is used to mean "unlimited", the value
INT_MAX is used for consistency with the behaviour of vsnprintf().

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210514161206.30821-2-rf@opensource.cirrus.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/lib/kstrtox.c b/lib/kstrtox.c
index 661a1e807bd1ac89616ee1eff4ac395c05d617d8..1a02b87b19c7e976b7cebcfab22fee34b82a06c2 100644 (file)
--- a/lib/kstrtox.c
+++ b/lib/kstrtox.c
@@ -39,20 +39,22 @@ 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. A maximum of max_chars characters will be converted.
+ *
  * 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,
+                                 size_t max_chars)
 {
        unsigned long long res;
        unsigned int rv;
 
        res = 0;
        rv = 0;
-       while (1) {
+       while (max_chars--) {
                unsigned int c = *s;
                unsigned int lc = c | 0x20; /* don't tolower() this line */
                unsigned int val;
@@ -82,6 +84,11 @@ 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 3b4637bcd2540d7c4244363016c8d75ec604fd98..158c400ca8658e0d44c195a80a5628edb507ca5e 100644 (file)
--- 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,
+                                 size_t 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 4a990f3fd345820766a49a34e29c499e243af7b4..83b164707e5c963797afd73a46c8474a0901f5da 100644 (file)
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -46,6 +46,31 @@
 #include <linux/string_helpers.h>
 #include "kstrtox.h"
 
+static unsigned long long simple_strntoull(const char *startp, size_t max_chars,
+                                          char **endp, unsigned int base)
+{
+       const char *cp;
+       unsigned long long result = 0ULL;
+       size_t prefix_chars;
+       unsigned int rv;
+
+       cp = _parse_integer_fixup_radix(startp, &base);
+       prefix_chars = cp - startp;
+       if (prefix_chars < max_chars) {
+               rv = _parse_integer_limit(cp, base, &result, max_chars - prefix_chars);
+               /* FIXME */
+               cp += (rv & ~KSTRTOX_OVERFLOW);
+       } else {
+               /* Field too short for prefix + digit, skip over without converting */
+               cp = startp + max_chars;
+       }
+
+       if (endp)
+               *endp = (char *)cp;
+
+       return result;
+}
+
 /**
  * simple_strtoull - convert a string to an unsigned long long
  * @cp: The start of the string
@@ -56,18 +81,7 @@
  */
 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);
 
@@ -102,6 +116,21 @@ long simple_strtol(const char *cp, char **endp, unsigned int base)
 }
 EXPORT_SYMBOL(simple_strtol);
 
+static long long simple_strntoll(const char *cp, size_t max_chars, char **endp,
+                                unsigned int base)
+{
+       /*
+        * simple_strntoull() safely handles receiving max_chars==0 in the
+        * case cp[0] == '-' && max_chars == 1.
+        * If max_chars == 0 we can drop through and pass it to simple_strntoull()
+        * and the content of *cp is irrelevant.
+        */
+       if (*cp == '-' && max_chars > 0)
+               return -simple_strntoull(cp + 1, max_chars - 1, endp, base);
+
+       return simple_strntoull(cp, max_chars, endp, base);
+}
+
 /**
  * simple_strtoll - convert a string to a signed long long
  * @cp: The start of the string
@@ -112,10 +141,7 @@ EXPORT_SYMBOL(simple_strtol);
  */
 long long simple_strtoll(const char *cp, char **endp, unsigned int base)
 {
-       if (*cp == '-')
-               return -simple_strtoull(cp + 1, endp, base);
-
-       return simple_strtoull(cp, endp, base);
+       return simple_strntoll(cp, INT_MAX, endp, base);
 }
 EXPORT_SYMBOL(simple_strtoll);
 
@@ -2943,25 +2969,13 @@ 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 */