Re: [PATCH Part1 v5 23/38] x86/head/64: set up a startup %gs for stack protector
From: Borislav Petkov <bp@alien8.de>
Date: 2021-08-25 14:28:47
Also in:
kvm, linux-efi, linux-mm, lkml, platform-driver-x86
On Fri, Aug 20, 2021 at 10:19:18AM -0500, Brijesh Singh wrote:
From: Michael Roth <redacted>
As of commit 103a4908ad4d ("x86/head/64: Disable stack protection for
head$(BITS).o") kernel/head64.c is compiled with -fno-stack-protector
to allow a call to set_bringup_idt_handler(), which would otherwise
have stack protection enabled with CONFIG_STACKPROTECTOR_STRONG. While
sufficient for that case, this will still cause issues if we attempt to
call out to any external functions that were compiled with stack
protection enabled that in-turn make stack-protected calls, or if the
exception handlers set up by set_bringup_idt_handler() make calls to
stack-protected functions.
Subsequent patches for SEV-SNP CPUID validation support will introduce
both such cases. Attempting to disable stack protection for everything
in scope to address that is prohibitive since much of the code, like
SEV-ES #VC handler, is shared code that remains in use after boot and
could benefit from having stack protection enabled. Attempting to inline
calls is brittle and can quickly balloon out to library/helper code
where that's not really an option.
Instead, set up %gs to point a buffer that stack protector can use for
canary values when needed.
In doing so, it's likely we can stop using -no-stack-protector for
head64.c, but that hasn't been tested yet, and head32.c would need a
similar solution to be safe, so that is left as a potential follow-up.That...
quoted hunk ↗ jump to hunk
Signed-off-by: Michael Roth <redacted> Signed-off-by: Brijesh Singh <redacted> --- arch/x86/kernel/Makefile | 2 +- arch/x86/kernel/head64.c | 20 ++++++++++++++++++++ 2 files changed, 21 insertions(+), 1 deletion(-)diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile index 3e625c61f008..5abdfd0dbbc3 100644 --- a/arch/x86/kernel/Makefile +++ b/arch/x86/kernel/Makefile@@ -46,7 +46,7 @@ endif # non-deterministic coverage. KCOV_INSTRUMENT := n -CFLAGS_head$(BITS).o += -fno-stack-protector +CFLAGS_head32.o += -fno-stack-protector
... and that needs to be taken care of too.
quoted hunk ↗ jump to hunk
CFLAGS_irq.o := -I $(srctree)/$(src)/../include/asm/tracediff --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
That's "#ifdef". Below too.
Did you even build this with CONFIG_STACKPROTECTOR disabled?
Because if you did, you would've seen this:
arch/x86/kernel/head64.c:78:5: warning: "CONFIG_STACKPROTECTOR" is not defined, evaluates to 0 [-Wundef]
78 | #if CONFIG_STACKPROTECTOR
| ^~~~~~~~~~~~~~~~~~~~~
arch/x86/kernel/head64.c: In function ‘startup_64_setup_env’:
arch/x86/kernel/head64.c:613:35: error: ‘startup_gs_area’ undeclared (first use in this function)
613 | u64 gs_area = (u64)fixup_pointer(startup_gs_area, physbase);
| ^~~~~~~~~~~~~~~
arch/x86/kernel/head64.c:613:35: note: each undeclared identifier is reported only once for each function it appears in
arch/x86/kernel/head64.c:632:5: warning: "CONFIG_STACKPROTECTOR" is not defined, evaluates to 0 [-Wundef]
632 | #if CONFIG_STACKPROTECTOR
| ^~~~~~~~~~~~~~~~~~~~~
arch/x86/kernel/head64.c:613:6: warning: unused variable ‘gs_area’ [-Wunused-variable]
613 | u64 gs_area = (u64)fixup_pointer(startup_gs_area, physbase);
| ^~~~~~~
make[2]: *** [scripts/Makefile.build:271: arch/x86/kernel/head64.o] Error 1
make[1]: *** [scripts/Makefile.build:514: arch/x86/kernel] Error 2
make[1]: *** Waiting for unfinished jobs....
make: *** [Makefile:1851: arch/x86] Error 2
make: *** Waiting for unfinished jobs....
quoted hunk ↗ jump to hunk
+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
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette