From: Michael Roth <michael.roth@xxxxxxx>
As of commit 103a4908ad4d ("x86/head/64: Disable stack protection for
head$(BITS).o") kernel/head64.c is compiled with -fno-stack-protector
to allow a call to set_bringup_idt_handler(), which would otherwise
have stack protection enabled with CONFIG_STACKPROTECTOR_STRONG. While
sufficient for that case, this will still cause issues if we attempt to
call out to any external functions that were compiled with stack
protection enabled that in-turn make stack-protected calls, or if the
exception handlers set up by set_bringup_idt_handler() make calls to
stack-protected functions.
Subsequent patches for SEV-SNP CPUID validation support will introduce
both such cases. Attempting to disable stack protection for everything
in scope to address that is prohibitive since much of the code, like
SEV-ES #VC handler, is shared code that remains in use after boot and
could benefit from having stack protection enabled. Attempting to inline
calls is brittle and can quickly balloon out to library/helper code
where that's not really an option.
Instead, set up %gs to point a buffer that stack protector can use for
canary values when needed.
In doing so, it's likely we can stop using -no-stack-protector for
head64.c, but that hasn't been tested yet, and head32.c would need a
similar solution to be safe, so that is left as a potential follow-up.
Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
---
arch/x86/kernel/Makefile | 2 +-
arch/x86/kernel/head64.c | 20 ++++++++++++++++++++
2 files changed, 21 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 3e625c61f008..5abdfd0dbbc3 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -46,7 +46,7 @@ endif
# non-deterministic coverage.
KCOV_INSTRUMENT := n
-CFLAGS_head$(BITS).o += -fno-stack-protector
+CFLAGS_head32.o += -fno-stack-protector
CFLAGS_irq.o := -I $(srctree)/$(src)/../include/asm/trace
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index a1711c4594fa..f1b76a54c84e 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -74,6 +74,11 @@ static struct desc_struct startup_gdt[GDT_ENTRIES] = {
[GDT_ENTRY_KERNEL_DS] = GDT_ENTRY_INIT(0xc093, 0, 0xfffff),
};
+/* For use by stack protector code before switching to virtual addresses */
+#if CONFIG_STACKPROTECTOR
+static char startup_gs_area[64];
+#endif
+
/*
* Address needs to be set at runtime because it references the startup_gdt
* while the kernel still uses a direct mapping.
@@ -605,6 +610,8 @@ void early_setup_idt(void)
*/
void __head startup_64_setup_env(unsigned long physbase)
{
+ u64 gs_area = (u64)fixup_pointer(startup_gs_area, physbase);
+
/* Load GDT */
startup_gdt_descr.address = (unsigned long)fixup_pointer(startup_gdt, physbase);
native_load_gdt(&startup_gdt_descr);
@@ -614,5 +621,18 @@ void __head startup_64_setup_env(unsigned long physbase)
"movl %%eax, %%ss\n"
"movl %%eax, %%es\n" : : "a"(__KERNEL_DS) : "memory");
+ /*
+ * GCC stack protection needs a place to store canary values. The
+ * default is %gs:0x28, which is what the kernel currently uses.
+ * Point GS base to a buffer that can be used for this purpose.
+ * Note that newer GCCs now allow this location to be configured,
+ * so if we change from the default in the future we need to ensure
+ * that this buffer overlaps whatever address ends up being used.
+ */
+#if CONFIG_STACKPROTECTOR
+ asm volatile("movl %%eax, %%gs\n" : : "a"(__KERNEL_DS) : "memory");
+ native_wrmsr(MSR_GS_BASE, gs_area, gs_area >> 32);
+#endif
+
startup_64_load_idt(physbase);
}
--
2.17.1
