[PATCH v2 1/4] lib/vsprintf.c: handle invalid format specifiers more robustly

[Rasmus Villemoes]

If we meet any invalid or unsupported format specifier, 'handling' it
by just printing it as a literal string is not safe: Presumably the
format string and the arguments passed gcc's type checking, but that
means something like sprintf(buf, "%n %pd", &intvar, dentry) would end
up interpreting &intvar as a struct dentry*.

When the offending specifier was %n it used to be at the end of the
format string, but we can't rely on that always being the case. Also,
gcc doesn't complain about some more or less exotic qualifiers (or
'length modifiers' in posix-speak) such as 'j' or 'q', but being
unrecognized by the kernel's printf implementation, they'd be
interpreted as unknown specifiers, and the rest of arguments would be
interpreted wrongly.

So let's complain about anything we don't understand, not just %n, and
stop pretending that we'd be able to make sense of the rest of the
format/arguments. If the offending specifier is in a printk() call we
unfortunately only get a "BUG: recent printk recursion!", but at least
direct users of the sprintf family will be caught.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
---
v2: add acked/reviewed-bys.

 lib/vsprintf.c | 31 +++++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 95cd63b43b99..f2590a80937f 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1769,14 +1769,14 @@ qualifier:
 
 	case 'n':
 		/*
-		 * Since %n poses a greater security risk than utility, treat
-		 * it as an invalid format specifier. Warn about its use so
-		 * that new instances don't get added.
+		 * Since %n poses a greater security risk than
+		 * utility, treat it as any other invalid or
+		 * unsupported format specifier.
 		 */
-		WARN_ONCE(1, "Please remove ignored %%n in '%s'\n", fmt);
 		/* Fall-through */
 
 	default:
+		WARN_ONCE(1, "Please remove unsupported %%%c in format string\n", *fmt);
 		spec->type = FORMAT_TYPE_INVALID;
 		return fmt - start;
 	}
@@ -1944,10 +1944,15 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
 			break;
 
 		case FORMAT_TYPE_INVALID:
-			if (str < end)
-				*str = '%';
-			++str;
-			break;
+			/*
+			 * Presumably the arguments passed gcc's type
+			 * checking, but there is no safe or sane way
+			 * for us to continue parsing the format and
+			 * fetching from the va_list; the remaining
+			 * specifiers and arguments would be out of
+			 * sync.
+			 */
+			goto out;
 
 		default:
 			switch (spec.type) {
@@ -1992,6 +1997,7 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
 		}
 	}
 
+out:
 	if (size > 0) {
 		if (str < end)
 			*str = '\0';
@@ -2189,9 +2195,10 @@ do {									\
 
 		switch (spec.type) {
 		case FORMAT_TYPE_NONE:
-		case FORMAT_TYPE_INVALID:
 		case FORMAT_TYPE_PERCENT_CHAR:
 			break;
+		case FORMAT_TYPE_INVALID:
+			goto out;
 
 		case FORMAT_TYPE_WIDTH:
 		case FORMAT_TYPE_PRECISION:
@@ -2253,6 +2260,7 @@ do {									\
 		}
 	}
 
+out:
 	return (u32 *)(PTR_ALIGN(str, sizeof(u32))) - bin_buf;
 #undef save_arg
 }
@@ -2375,12 +2383,14 @@ int bstr_printf(char *buf, size_t size, const char *fmt, const u32 *bin_buf)
 			break;
 
 		case FORMAT_TYPE_PERCENT_CHAR:
-		case FORMAT_TYPE_INVALID:
 			if (str < end)
 				*str = '%';
 			++str;
 			break;
 
+		case FORMAT_TYPE_INVALID:
+			goto out;
+
 		default: {
 			unsigned long long num;
 
@@ -2423,6 +2433,7 @@ int bstr_printf(char *buf, size_t size, const char *fmt, const u32 *bin_buf)
 		} /* switch(spec.type) */
 	} /* while(*fmt) */
 
+out:
 	if (size > 0) {
 		if (str < end)
 			*str = '\0';
-- 
2.1.3

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