linux-kernel - [PATCH v5 23/75] x86/boot/compressed/64: Unmap GHCB page before booting the kernel

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Message-Id: <20200724160336.5435-24-joro@8bytes.org>
Date:   Fri, 24 Jul 2020 18:02:44 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     x86@...nel.org
Cc:     Joerg Roedel <joro@...tes.org>, Joerg Roedel <jroedel@...e.de>,
        hpa@...or.com, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Martin Radev <martin.b.radev@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: [PATCH v5 23/75] x86/boot/compressed/64: Unmap GHCB page before booting the kernel

From: Joerg Roedel <jroedel@...e.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@...e.de>
---
 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 4d37a28370ed..2e5f82acc122 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)
-- 
2.27.0