[PATCH v6 24/76] x86/boot/compressed/64: Unmap GHCB page before booting the kernel

Joerg Roedel
2020-08-24T09:14:54Z

From: Joerg Roedel <jroedel@suse.de>

Force a page-fault on any further accesses to the GHCB page when they
shouldn't happen anymore. This will catch the bugs where a #VC exception
is raised when no one is expected anymore.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/r/20200724160336.5435-24-joro@8bytes.org
---
arch/x86/boot/compressed/ident_map_64.c | 17 +++++++++++++++--
arch/x86/boot/compressed/misc.h | 6 ++++++
arch/x86/boot/compressed/sev-es.c | 14 ++++++++++++++
3 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
index 05742f641a06..063a60edcf99 100644
--- a/arch/x86/boot/compressed/ident_map_64.c
+++ b/arch/x86/boot/compressed/ident_map_64.c
@@ -298,6 +298,11 @@ int set_page_encrypted(unsigned long address)
return set_clr_page_flags(&mapping_info, address, _PAGE_ENC, 0);
}

+int set_page_non_present(unsigned long address)
+{
+ return set_clr_page_flags(&mapping_info, address, 0, _PAGE_PRESENT);
+}
+
static void do_pf_error(const char *msg, unsigned long error_code,
unsigned long address, unsigned long ip)
{
@@ -316,8 +321,14 @@ static void do_pf_error(const char *msg, unsigned long error_code,

void do_boot_page_fault(struct pt_regs *regs, unsigned long error_code)
{
- unsigned long address = native_read_cr2() & PMD_MASK;
- unsigned long end = address + PMD_SIZE;
+ unsigned long address = native_read_cr2();
+ unsigned long end;
+ bool ghcb_fault;
+
+ ghcb_fault = sev_es_check_ghcb_fault(address);
+
+ address &= PMD_MASK;
+ end = address + PMD_SIZE;

/*
* Check for unexpected error codes. Unexpected are:
@@ -327,6 +338,8 @@ void do_boot_page_fault(struct pt_regs *regs, unsigned long error_code)
*/
if (error_code & (X86_PF_PROT | X86_PF_USER | X86_PF_RSVD))
do_pf_error("Unexpected page-fault:", error_code, address, regs->ip);
+ else if (ghcb_fault)
+ do_pf_error("Page-fault on GHCB page:", error_code, address, regs->ip);

/*
* Error code is sane - now identity map the 2M region around
diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index 9995c70ca813..c0e0ffeee50a 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -100,6 +100,7 @@ static inline void choose_random_location(unsigned long input,
#ifdef CONFIG_X86_64
extern int set_page_decrypted(unsigned long address);
extern int set_page_encrypted(unsigned long address);
+extern int set_page_non_present(unsigned long address);
extern unsigned char _pgtable[];
#endif

@@ -117,8 +118,13 @@ void set_sev_encryption_mask(void);

#ifdef CONFIG_AMD_MEM_ENCRYPT
void sev_es_shutdown_ghcb(void);
+extern bool sev_es_check_ghcb_fault(unsigned long address);
#else
static inline void sev_es_shutdown_ghcb(void) { }
+static inline bool sev_es_check_ghcb_fault(unsigned long address)
+{
+ return false;
+}
#endif

/* acpi.c */
diff --git a/arch/x86/boot/compressed/sev-es.c b/arch/x86/boot/compressed/sev-es.c
index 7e2cec170026..e3abf8737015 100644
--- a/arch/x86/boot/compressed/sev-es.c
+++ b/arch/x86/boot/compressed/sev-es.c
@@ -121,6 +121,20 @@ void sev_es_shutdown_ghcb(void)
*/
if (set_page_encrypted((unsigned long)&boot_ghcb_page))
error("Can't map GHCB page encrypted");
+
+ /*
+ * GHCB page is mapped encrypted again and flushed from the cache.
+ * Mark it non-present now to catch bugs when #VC exceptions trigger
+ * after this point.
+ */
+ if (set_page_non_present((unsigned long)&boot_ghcb_page))
+ error("Can't unmap GHCB page");
+}
+
+bool sev_es_check_ghcb_fault(unsigned long address)
+{
+ /* Check whether the fault was on the GHCB page */
+ return ((address & PAGE_MASK) == (unsigned long)&boot_ghcb_page);
}

void do_boot_stage2_vc(struct pt_regs *regs, unsigned long exit_code)
--
2.28.0