vsprintf: Limit the length of inlined error messages

The inlined error messages must be used carefully because
they need to fit into the given buffer.

Handle them using a custom wrapper that makes people aware
of the problem. Also define a reasonable hard limit to
avoid a completely insane usage.

Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Link: http://lkml.kernel.org/r/20190417115350.20479-11-pmladek@suse.com
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Tobin C . Harding" <me@tobin.cc>
Cc: Joe Perches <joe@perches.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

lib/vsprintf.c

@@ -612,6 +612,21 @@ static char *string_nocheck(char *buf, char *end, const char *s,
 	return widen_string(buf, len, end, spec);
 }
 
+/* Be careful: error messages must fit into the given buffer. */
+static char *error_string(char *buf, char *end, const char *s,
+			  struct printf_spec spec)
+{
+	/*
+	 * Hard limit to avoid a completely insane messages. It actually
+	 * works pretty well because most error messages are in
+	 * the many pointer format modifiers.
+	 */
+	if (spec.precision == -1)
+		spec.precision = 2 * sizeof(void *);
+
+	return string_nocheck(buf, end, s, spec);
+}
+
 /*
  * This is not a fool-proof test. 99% of the time that this will fault is
  * due to a bad pointer, not one that crosses into bad memory. Just test
@@ -638,7 +653,7 @@ static int check_pointer(char **buf, char *end, const void *ptr,
 
 	err_msg = check_pointer_msg(ptr);
 	if (err_msg) {
-		*buf = string_nocheck(*buf, end, err_msg, spec);
+		*buf = error_string(*buf, end, err_msg, spec);
 		return -EFAULT;
 	}
 
@@ -741,7 +756,7 @@ static char *ptr_to_id(char *buf, char *end, const void *ptr,
 	if (static_branch_unlikely(&not_filled_random_ptr_key)) {
 		spec.field_width = 2 * sizeof(ptr);
 		/* string length must be less than default_width */
-		return string_nocheck(buf, end, str, spec);
+		return error_string(buf, end, str, spec);
 	}
 
 #ifdef CONFIG_64BIT
@@ -777,7 +792,7 @@ char *restricted_pointer(char *buf, char *end, const void *ptr,
 		if (in_irq() || in_serving_softirq() || in_nmi()) {
 			if (spec.field_width == -1)
 				spec.field_width = 2 * sizeof(ptr);
-			return string_nocheck(buf, end, "pK-error", spec);
+			return error_string(buf, end, "pK-error", spec);
 		}
 
 		/*
@@ -1511,12 +1526,12 @@ char *ip_addr_string(char *buf, char *end, const void *ptr,
 		case AF_INET6:
 			return ip6_addr_string_sa(buf, end, &sa->v6, spec, fmt);
 		default:
-			return string_nocheck(buf, end, "(einval)", spec);
+			return error_string(buf, end, "(einval)", spec);
 		}}
 	}
 
 	err_fmt_msg = fmt[0] == 'i' ? "(%pi?)" : "(%pI?)";
-	return string_nocheck(buf, end, err_fmt_msg, spec);
+	return error_string(buf, end, err_fmt_msg, spec);
 }
 
 static noinline_for_stack
@@ -1653,7 +1668,7 @@ char *netdev_bits(char *buf, char *end, const void *addr,
 		size = sizeof(netdev_features_t);
 		break;
 	default:
-		return string_nocheck(buf, end, "(%pN?)", spec);
+		return error_string(buf, end, "(%pN?)", spec);
 	}
 
 	return special_hex_number(buf, end, num, size);
@@ -1765,7 +1780,7 @@ char *time_and_date(char *buf, char *end, void *ptr, struct printf_spec spec,
 	case 'R':
 		return rtc_str(buf, end, (const struct rtc_time *)ptr, spec, fmt);
 	default:
-		return string_nocheck(buf, end, "(%ptR?)", spec);
+		return error_string(buf, end, "(%ptR?)", spec);
 	}
 }
 
@@ -1774,7 +1789,7 @@ char *clock(char *buf, char *end, struct clk *clk, struct printf_spec spec,
 	    const char *fmt)
 {
 	if (!IS_ENABLED(CONFIG_HAVE_CLK))
-		return string_nocheck(buf, end, "(%pC?)", spec);
+		return error_string(buf, end, "(%pC?)", spec);
 
 	if (check_pointer(&buf, end, clk, spec))
 		return buf;
@@ -1785,7 +1800,7 @@ char *clock(char *buf, char *end, struct clk *clk, struct printf_spec spec,
 #ifdef CONFIG_COMMON_CLK
 		return string(buf, end, __clk_get_name(clk), spec);
 #else
-		return string_nocheck(buf, end, "(%pC?)", spec);
+		return error_string(buf, end, "(%pC?)", spec);
 #endif
 	}
 }
@@ -1843,7 +1858,7 @@ char *flags_string(char *buf, char *end, void *flags_ptr,
 		names = gfpflag_names;
 		break;
 	default:
-		return string_nocheck(buf, end, "(%pG?)", spec);
+		return error_string(buf, end, "(%pG?)", spec);
 	}
 
 	return format_flags(buf, end, flags, names);
@@ -1899,7 +1914,7 @@ char *device_node_string(char *buf, char *end, struct device_node *dn,
 	str_spec.field_width = -1;
 
 	if (!IS_ENABLED(CONFIG_OF))
-		return string_nocheck(buf, end, "(%pOF?)", spec);
+		return error_string(buf, end, "(%pOF?)", spec);
 
 	if (check_pointer(&buf, end, dn, spec))
 		return buf;
@@ -1978,7 +1993,7 @@ static char *kobject_string(char *buf, char *end, void *ptr,
 		return device_node_string(buf, end, ptr, spec, fmt + 1);
 	}
 
-	return string_nocheck(buf, end, "(%pO?)", spec);
+	return error_string(buf, end, "(%pO?)", spec);
 }
 
 /*