[kernel-hardening] [PATCH 2/3] vsprintf: print <no-symbol> if symbol not found

["Tobin C. Harding" <me@tobin.cc>]

Depends on: commit bd6b239cdbb2 ("kallsyms: don't leak address when
symbol not found")

Currently vsprintf for specifiers %p[SsB] relies on the behaviour of
kallsyms (sprint_symbol()) and prints the actual address if a symbol is
not found. Previous patch changes this behaviour so tha sprint_symbol()
returns an error if symbol not found. With this patch in place we can
print a sanitized message '<no-symbol>' instead of leaking the address.

Print '<no-symbol>' for printk specifier %s[sSB] if no symbol is found.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
---
 include/linux/kernel.h |  2 ++
 lib/vsprintf.c         | 18 +++++++++++++++---
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index ce51455e2adf..89e8ce79c2d1 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -460,6 +460,8 @@ char *kvasprintf(gfp_t gfp, const char *fmt, va_list args);
 extern __printf(2, 0)
 const char *kvasprintf_const(gfp_t gfp, const char *fmt, va_list args);
 
+extern int string_is_no_symbol(const char *s);
+
 extern __scanf(2, 3)
 int sscanf(const char *, const char *, ...);
 extern __scanf(2, 0)
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 01c3957b2de6..c112b0980ead 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -667,6 +667,8 @@ char *bdev_name(char *buf, char *end, struct block_device *bdev,
 }
 #endif
 
+#define PRINTK_NO_SYMBOL_STR "<no-symbol>"
+
 static noinline_for_stack
 char *symbol_string(char *buf, char *end, void *ptr,
 		    struct printf_spec spec, const char *fmt)
@@ -674,6 +676,7 @@ char *symbol_string(char *buf, char *end, void *ptr,
 	unsigned long value;
 #ifdef CONFIG_KALLSYMS
 	char sym[KSYM_SYMBOL_LEN];
+	int ret;
 #endif
 
 	if (fmt[1] == 'R')
@@ -682,11 +685,14 @@ char *symbol_string(char *buf, char *end, void *ptr,
 
 #ifdef CONFIG_KALLSYMS
 	if (*fmt == 'B')
-		sprint_backtrace(sym, value);
+		ret = sprint_backtrace(sym, value);
 	else if (*fmt != 'f' && *fmt != 's')
-		sprint_symbol(sym, value);
+		ret = sprint_symbol(sym, value);
 	else
-		sprint_symbol_no_offset(sym, value);
+		ret = sprint_symbol_no_offset(sym, value);
+
+	if (ret == -1)
+		strcpy(sym, PRINTK_NO_SYMBOL_STR);
 
 	return string(buf, end, sym, spec);
 #else
@@ -694,6 +700,12 @@ char *symbol_string(char *buf, char *end, void *ptr,
 #endif
 }
 
+int string_is_no_symbol(const char *s)
+{
+	return !!strstr(s, PRINTK_NO_SYMBOL_STR);
+}
+EXPORT_SYMBOL(string_is_no_symbol);
+
 static noinline_for_stack
 char *resource_string(char *buf, char *end, struct resource *res,
 		      struct printf_spec spec, const char *fmt)
-- 
2.7.4