[PATCH 4.14 008/178] string.h: workaround for increased stack usage

[Greg Kroah-Hartman]

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Arnd Bergmann <arnd@xxxxxxxx>

commit 146734b091430c80d80bb96b1139a96fb4bc830e upstream.

The hardened strlen() function causes rather large stack usage in at
least one file in the kernel, in particular when CONFIG_KASAN is
enabled:

  drivers/media/usb/em28xx/em28xx-dvb.c: In function 'em28xx_dvb_init':
  drivers/media/usb/em28xx/em28xx-dvb.c:2062:1: error: the frame size of 3256 bytes is larger than 204 bytes [-Werror=frame-larger-than=]

Analyzing this problem led to the discovery that gcc fails to merge the
stack slots for the i2c_board_info[] structures after we strlcpy() into
them, due to the 'noreturn' attribute on the source string length check.

I reported this as a gcc bug, but it is unlikely to get fixed for gcc-8,
since it is relatively easy to work around, and it gets triggered
rarely.  An earlier workaround I did added an empty inline assembly
statement before the call to fortify_panic(), which works surprisingly
well, but is really ugly and unintuitive.

This is a new approach to the same problem, this time addressing it by
not calling the 'extern __real_strnlen()' function for string constants
where __builtin_strlen() is a compile-time constant and therefore known
to be safe.

We do this by checking if the last character in the string is a
compile-time constant '\0'.  If it is, we can assume that strlen() of
the string is also constant.

As a side-effect, this should also improve the object code output for
any other call of strlen() on a string constant.

[akpm@xxxxxxxxxxxxxxxxxxxx: add comment]
Link: http://lkml.kernel.org/r/20171205215143.3085755-1-arnd@xxxxxxxx
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365
Link: https://patchwork.kernel.org/patch/9980413/
Link: https://patchwork.kernel.org/patch/9974047/
Fixes: 6974f0c4555 ("include/linux/string.h: add the option of fortified string.h functions")
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Daniel Micay <danielmicay@xxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Martin Wilck <mwilck@xxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 include/linux/string.h |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -259,7 +259,10 @@ __FORTIFY_INLINE __kernel_size_t strlen(
 {
 	__kernel_size_t ret;
 	size_t p_size = __builtin_object_size(p, 0);
-	if (p_size == (size_t)-1)
+
+	/* Work around gcc excess stack consumption issue */
+	if (p_size == (size_t)-1 ||
+	    (__builtin_constant_p(p[p_size - 1]) && p[p_size - 1] == '\0'))
 		return __builtin_strlen(p);
 	ret = strnlen(p, p_size);
 	if (p_size <= ret)