[PATCH v2] security: disable FORTIFY_SOURCE on clang

["Jason A. Donenfeld" <Jason@zx2c4.com>]

clang-10 has a broken optimization stage that doesn't allow the
compiler to prove at compile time that certain memcpys are within
bounds, and thus the outline memcpy is always called, resulting in
horrific performance, and in some cases, excessive stack frame growth.
Here's a simple reproducer:

    typedef unsigned long size_t;
    void *c(void *dest, const void *src, size_t n) __asm__("memcpy");
    extern inline __attribute__((gnu_inline)) void *memcpy(void *dest, const void *src, size_t n) { return c(dest, src, n); }
    void blah(char *a)
    {
      unsigned long long b[10], c[10];
      int i;

      memcpy(b, a, sizeof(b));
      for (i = 0; i < 10; ++i)
        c[i] = b[i] ^ b[9 - i];
      for (i = 0; i < 10; ++i)
        b[i] = c[i] ^ a[i];
      memcpy(a, b, sizeof(b));
    }

Compile this with clang-9 and clang-10 and observe:

zx2c4@thinkpad /tmp/curve25519-hacl64-stack-frame-size-test $ clang-10 -Wframe-larger-than=0 -O3 -c b.c -o c10.o
b.c:5:6: warning: stack frame size of 104 bytes in function 'blah' [-Wframe-larger-than=]
void blah(char *a)
     ^
1 warning generated.
zx2c4@thinkpad /tmp/curve25519-hacl64-stack-frame-size-test $ clang-9 -Wframe-larger-than=0 -O3 -c b.c -o c9.o

Looking at the disassembly of c10.o and c9.o, one can see that c9.o is
properly optimized in the obvious way one would expect, while c10.o has
blown up and includes extern calls to memcpy.

But actually, for versions of clang earlier than 10, fortify source
mostly does nothing. So, between being broken and doing nothing, it
probably doesn't make sense to pretend to offer this option. So, this
commit just disables it entirely when compiling with clang.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: LKML <linux-kernel@vger.kernel.org>
Cc: clang-built-linux <clang-built-linux@googlegroups.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: George Burgess <gbiv@google.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Link: https://bugs.llvm.org/show_bug.cgi?id=45802
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 security/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/security/Kconfig b/security/Kconfig
index cd3cc7da3a55..76bcfb3eb16f 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -191,6 +191,7 @@ config HARDENED_USERCOPY_PAGESPAN
 config FORTIFY_SOURCE
 	bool "Harden common str/mem functions against buffer overflows"
 	depends on ARCH_HAS_FORTIFY_SOURCE
+	depends on !CC_IS_CLANG
 	help
 	  Detect overflows of buffers in common string and memory functions
 	  where the compiler can determine and validate the buffer sizes.
-- 
2.26.2