[v5,11/14] arm64: efi: restore x18 if it was corrupted
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Message ID 20191105235608.107702-12-samitolvanen@google.com
State New
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Series
  • [v5,01/14] arm64: mm: avoid x18 in idmap_kpti_install_ng_mappings
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Commit Message

Sami Tolvanen Nov. 5, 2019, 11:56 p.m. UTC
If we detect a corrupted x18 and SCS is enabled, restore the register
before jumping back to instrumented code. This is safe, because the
wrapper is called with preemption disabled and a separate shadow stack
is used for interrupt handling.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
 arch/arm64/kernel/efi-rt-wrapper.S | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

Comments

Miguel Ojeda Nov. 6, 2019, 4:45 a.m. UTC | #1
On Wed, Nov 6, 2019 at 12:56 AM Sami Tolvanen <samitolvanen@google.com> wrote:
>
> If we detect a corrupted x18 and SCS is enabled, restore the register
> before jumping back to instrumented code. This is safe, because the
> wrapper is called with preemption disabled and a separate shadow stack
> is used for interrupt handling.

In case you do v6: I think putting the explanation about why this is
safe in the existing comment would be best given it is justifying a
subtlety of the code rather than the change itself. Ard?

Cheers,
Miguel
Ard Biesheuvel Nov. 7, 2019, 10:51 a.m. UTC | #2
On Wed, 6 Nov 2019 at 05:46, Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> On Wed, Nov 6, 2019 at 12:56 AM Sami Tolvanen <samitolvanen@google.com> wrote:
> >
> > If we detect a corrupted x18 and SCS is enabled, restore the register
> > before jumping back to instrumented code. This is safe, because the
> > wrapper is called with preemption disabled and a separate shadow stack
> > is used for interrupt handling.
>
> In case you do v6: I think putting the explanation about why this is
> safe in the existing comment would be best given it is justifying a
> subtlety of the code rather than the change itself. Ard?
>

Agreed, but only if you have to respin for other reasons.
Sami Tolvanen Nov. 7, 2019, 4:26 p.m. UTC | #3
On Thu, Nov 7, 2019 at 2:51 AM Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
>
> On Wed, 6 Nov 2019 at 05:46, Miguel Ojeda
> <miguel.ojeda.sandonis@gmail.com> wrote:
> >
> > On Wed, Nov 6, 2019 at 12:56 AM Sami Tolvanen <samitolvanen@google.com> wrote:
> > >
> > > If we detect a corrupted x18 and SCS is enabled, restore the register
> > > before jumping back to instrumented code. This is safe, because the
> > > wrapper is called with preemption disabled and a separate shadow stack
> > > is used for interrupt handling.
> >
> > In case you do v6: I think putting the explanation about why this is
> > safe in the existing comment would be best given it is justifying a
> > subtlety of the code rather than the change itself. Ard?
> >
>
> Agreed, but only if you have to respin for other reasons.

Sure, sounds good to me. I'll update the comment if other changes are needed.

Sami

Patch
diff mbox series

diff --git a/arch/arm64/kernel/efi-rt-wrapper.S b/arch/arm64/kernel/efi-rt-wrapper.S
index 3fc71106cb2b..945744f16086 100644
--- a/arch/arm64/kernel/efi-rt-wrapper.S
+++ b/arch/arm64/kernel/efi-rt-wrapper.S
@@ -34,5 +34,10 @@  ENTRY(__efi_rt_asm_wrapper)
 	ldp	x29, x30, [sp], #32
 	b.ne	0f
 	ret
-0:	b	efi_handle_corrupted_x18	// tail call
+0:
+#ifdef CONFIG_SHADOW_CALL_STACK
+	/* Restore x18 before returning to instrumented code. */
+	mov	x18, x2
+#endif
+	b	efi_handle_corrupted_x18	// tail call
 ENDPROC(__efi_rt_asm_wrapper)