[PATCH] x86/build/64: Prevent native builds from generating SSE/AVX code

["Antoni Kuczyński" <antoni.kucz@gmail.com>]

With CONFIG_X86_NATIVE_CPU=y, GCC is invoked with -march=native after
the -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -mno-sse4a flags
that keep FP/SIMD code out of the kernel. The GCC driver expands
-march=native into an explicit list of ISA options for the build host
(e.g. "-march=ivybridge ... -msse4.2 -mavx ..."), and because these come
later on the command line they override the earlier -mno-* flags.

As a result, on any build host with AVX the compiler is free to use
SSE/AVX registers throughout the kernel, for instance to zero or copy
structures. A trivial reproducer:

  struct s { long a, b, c, d; };
  void f(struct s *p) { struct s z = {0}; *p = z; }

  $ gcc -mno-sse -mno-sse2 -mno-avx -march=native -Os -c t.c
  vpxor   %xmm0,%xmm0,%xmm0
  vmovdqu %ymm0,(%rdi)

On an Ivy Bridge machine (i5-3317U, GCC 14.2) a v7.3 kernel built this
way resets immediately after the bootloader hands over control. Booting
the same image in QEMU with "-d int,cpu_reset -no-reboot" shows:

  v=06 (#UD) IP=0010:000000000125e47b
  v=0d (#GP) -> v=08 (#DF) -> Triple fault

  __pi_startup_64_load_idt+0xb:
  c5 f9 ef c0    vpxor  %xmm0,%xmm0,%xmm0

The startup code executes an AVX instruction before CR4.OSXSAVE/XCR0
are set up and before an IDT is installed, so the #UD escalates to a
triple fault. Even past early boot, compiler-generated SIMD code would
silently clobber user space FPU state, since the kernel does not save
it on kernel entry outside of kernel_fpu_begin()/kernel_fpu_end().

Re-apply the -mno-* flags after -march=native, the same way EGPR use
is already suppressed for native builds.

Fixes: ea1dcca1de12 ("x86/kbuild/64: Add the CONFIG_X86_NATIVE_CPU option to locally optimize the kernel with '-march=native'")
Cc: stable@vger.kernel.org
Signed-off-by: Antoni Kuczyński <antoni.kucz@gmail.com>
---

Notes:
    Tested: v7.3 + this patch, CONFIG_X86_NATIVE_CPU=y, GCC 14.2.0 (Debian),
    boots on i5-3317U. arch/x86/boot/startup/gdt_idt.o contains no xmm/ymm
    instructions after the change (previously 2).

 arch/x86/Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index b7e08519b8c1..2c0575fd7bbc 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -161,6 +161,9 @@ else
 
 ifdef CONFIG_X86_NATIVE_CPU
         KBUILD_CFLAGS += -march=native
+        # -march=native expands to explicit -msse/-mavx/... flags that
+        # override the earlier -mno-* ones; disable FP/SIMD code again.
+        KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -mno-sse4a
         # Prevent the compiler from generating EGPR use. The kernel is
         # not yet prepared for general in-kernel use.
         KBUILD_CFLAGS += $(call cc-option,-mno-apx-features=egpr)
-- 
2.47.3