[PATCH 4.14 008/178] string.h: workaround for increased stack usage
From: Greg Kroah-Hartman
Date: Mon Dec 18 2017 - 11:12:23 EST
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
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]
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(-)
@@ -259,7 +259,10 @@ __FORTIFY_INLINE __kernel_size_t strlen(
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'))
ret = strnlen(p, p_size);
if (p_size <= ret)