[RFT PATCH v2 15/23] x86/boot: Provide __pti_set_user_pgtbl() to startup code

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

The SME encryption startup code populates page tables using the ordinary
set_pXX() helpers, and in a PTI build, these will call out to
__pti_set_user_pgtbl() to manipulate the shadow copy of the page tables
for user space.

This is unneeded for the startup code, which only manipulates the
swapper page tables, and so this call could be avoided in this
particular case. So instead of exposing the ordinary
__pti_set_user_pgtblt() to the startup code after its gets confined into
its own symbol space, provide an alternative which just returns pgd,
which is always correct in the startup context.

Annotate it as __weak for now, this will be dropped in a subsequent
patch.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/boot/startup/sme.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/boot/startup/sme.c b/arch/x86/boot/startup/sme.c
index 7fc6a689cefe..1e9f1f5a753c 100644
--- a/arch/x86/boot/startup/sme.c
+++ b/arch/x86/boot/startup/sme.c
@@ -557,3 +557,12 @@ void __head sme_enable(struct boot_params *bp)
 	cc_vendor	= CC_VENDOR_AMD;
 	cc_set_mask(me_mask);
 }
+
+#ifdef CONFIG_MITIGATION_PAGE_TABLE_ISOLATION
+/* Local version for startup code, which never operates on user page tables */
+__weak
+pgd_t __pti_set_user_pgtbl(pgd_t *pgdp, pgd_t pgd)
+{
+	return pgd;
+}
+#endif
-- 
2.49.0.906.g1f30a19c02-goog