netdev - [RFC 2/3] vsprintf: print <no-symbol> if symbol not found

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Message-Id: <1511821819-5496-3-git-send-email-me@tobin.cc>
Date:   Tue, 28 Nov 2017 09:30:18 +1100
From:   "Tobin C. Harding" <me@...in.cc>
To:     kernel-hardening@...ts.openwall.com
Cc:     "Tobin C. Harding" <me@...in.cc>, linux-kernel@...r.kernel.org,
        Network Development <netdev@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tycho Andersen <tycho@...ho.ws>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Petr Mladek <pmladek@...e.com>, Baoquan He <bhe@...hat.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Ian Abbott <abbotti@....co.uk>,
        Niklas Söderlund 
        <niklas.soderlund+renesas@...natech.se>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Larry Finger <Larry.Finger@...inger.net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Joe Perches <joe@...ches.com>,
        William Roberts <william.c.roberts@...el.com>,
        Rob Herring <robh@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Subject: [RFC 2/3] vsprintf: print <no-symbol> if symbol not found

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.

Future users of vsprintf may wish to know, after a call that uses
specifier %p[sSB], whether or not a symbol was found. The actual
sanitized string should be contained (isolated) within the vsprintf.c
therefore we should provide a predicate function. This also allows the
sanitized string to be updated at a later stage with minimal risk to
calling code.

Print '<no-symbol>' for printk specifier %s[sSB] if no symbol is
found. Provide predicate function string_is_no_symbol().

Signed-off-by: Tobin C. Harding <me@...in.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 1746bae94d41..01e18a8c63fd 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -665,6 +665,8 @@ 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)
@@ -672,6 +674,7 @@ 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')
@@ -680,11 +683,14 @@ 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
@@ -692,6 +698,12 @@ 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