A couple of changes to the SEV-ES code to perform more stringent

hypervisor checks before enabling encryption. (Joerg Roedel)
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Merge tag 'x86_seves_for_v5.10_rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 SEV-ES fixes from Borislav Petkov:
 "A couple of changes to the SEV-ES code to perform more stringent
  hypervisor checks before enabling encryption (Joerg Roedel)"

* tag 'x86_seves_for_v5.10_rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/sev-es: Do not support MMIO to/from encrypted memory
  x86/head/64: Check SEV encryption before switching to kernel page-table
  x86/boot/compressed/64: Check SEV encryption in 64-bit boot-path
  x86/boot/compressed/64: Sanity-check CPUID results in the early #VC handler
  x86/boot/compressed/64: Introduce sev_status

arch/x86/boot/compressed/ident_map_64.c

@@ -164,6 +164,7 @@ void initialize_identity_maps(void *rmode)
 	add_identity_map(cmdline, cmdline + COMMAND_LINE_SIZE);
 
 	/* Load the new page-table. */
+	sev_verify_cbit(top_level_pgt);
 	write_cr3(top_level_pgt);
 }
 

arch/x86/boot/compressed/mem_encrypt.S

@@ -68,6 +68,9 @@ SYM_FUNC_START(get_sev_encryption_bit)
 SYM_FUNC_END(get_sev_encryption_bit)
 
 	.code64
+
+#include "../../kernel/sev_verify_cbit.S"
+
 SYM_FUNC_START(set_sev_encryption_mask)
 #ifdef CONFIG_AMD_MEM_ENCRYPT
 	push	%rbp
@@ -81,6 +84,19 @@ SYM_FUNC_START(set_sev_encryption_mask)
 
 	bts	%rax, sme_me_mask(%rip)	/* Create the encryption mask */
 
+	/*
+	 * Read MSR_AMD64_SEV again and store it to sev_status. Can't do this in
+	 * get_sev_encryption_bit() because this function is 32-bit code and
+	 * shared between 64-bit and 32-bit boot path.
+	 */
+	movl	$MSR_AMD64_SEV, %ecx	/* Read the SEV MSR */
+	rdmsr
+
+	/* Store MSR value in sev_status */
+	shlq	$32, %rdx
+	orq	%rdx, %rax
+	movq	%rax, sev_status(%rip)
+
 .Lno_sev_mask:
 	movq	%rbp, %rsp		/* Restore original stack pointer */
 
@@ -96,5 +112,7 @@ SYM_FUNC_END(set_sev_encryption_mask)
 
 #ifdef CONFIG_AMD_MEM_ENCRYPT
 	.balign	8
-SYM_DATA(sme_me_mask, .quad 0)
+SYM_DATA(sme_me_mask,		.quad 0)
+SYM_DATA(sev_status,		.quad 0)
+SYM_DATA(sev_check_data,	.quad 0)
 #endif

arch/x86/boot/compressed/misc.h

@@ -159,4 +159,6 @@ void boot_page_fault(void);
 void boot_stage1_vc(void);
 void boot_stage2_vc(void);
 
+unsigned long sev_verify_cbit(unsigned long cr3);
+
 #endif /* BOOT_COMPRESSED_MISC_H */

arch/x86/kernel/head_64.S

@@ -161,6 +161,21 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 
 	/* Setup early boot stage 4-/5-level pagetables. */
 	addq	phys_base(%rip), %rax
+
+	/*
+	 * For SEV guests: Verify that the C-bit is correct. A malicious
+	 * hypervisor could lie about the C-bit position to perform a ROP
+	 * attack on the guest by writing to the unencrypted stack and wait for
+	 * the next RET instruction.
+	 * %rsi carries pointer to realmode data and is callee-clobbered. Save
+	 * and restore it.
+	 */
+	pushq	%rsi
+	movq	%rax, %rdi
+	call	sev_verify_cbit
+	popq	%rsi
+
+	/* Switch to new page-table */
 	movq	%rax, %cr3
 
 	/* Ensure I am executing from virtual addresses */
@@ -279,6 +294,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 SYM_CODE_END(secondary_startup_64)
 
 #include "verify_cpu.S"
+#include "sev_verify_cbit.S"
 
 #ifdef CONFIG_HOTPLUG_CPU
 /*

arch/x86/kernel/sev-es-shared.c

@@ -178,6 +178,32 @@ void __init do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
 		goto fail;
 	regs->dx = val >> 32;
 
+	/*
+	 * This is a VC handler and the #VC is only raised when SEV-ES is
+	 * active, which means SEV must be active too. Do sanity checks on the
+	 * CPUID results to make sure the hypervisor does not trick the kernel
+	 * into the no-sev path. This could map sensitive data unencrypted and
+	 * make it accessible to the hypervisor.
+	 *
+	 * In particular, check for:
+	 *	- Hypervisor CPUID bit
+	 *	- Availability of CPUID leaf 0x8000001f
+	 *	- SEV CPUID bit.
+	 *
+	 * The hypervisor might still report the wrong C-bit position, but this
+	 * can't be checked here.
+	 */
+
+	if ((fn == 1 && !(regs->cx & BIT(31))))
+		/* Hypervisor bit */
+		goto fail;
+	else if (fn == 0x80000000 && (regs->ax < 0x8000001f))
+		/* SEV leaf check */
+		goto fail;
+	else if ((fn == 0x8000001f && !(regs->ax & BIT(1))))
+		/* SEV bit */
+		goto fail;
+
 	/* Skip over the CPUID two-byte opcode */
 	regs->ip += 2;
 

arch/x86/kernel/sev-es.c

@@ -374,8 +374,8 @@ fault:
 	return ES_EXCEPTION;
 }
 
-static bool vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
-				 unsigned long vaddr, phys_addr_t *paddr)
+static enum es_result vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
+					   unsigned long vaddr, phys_addr_t *paddr)
 {
 	unsigned long va = (unsigned long)vaddr;
 	unsigned int level;
@@ -394,15 +394,19 @@ static bool vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
 		if (user_mode(ctxt->regs))
 			ctxt->fi.error_code |= X86_PF_USER;
 
-		return false;
+		return ES_EXCEPTION;
 	}
 
+	if (WARN_ON_ONCE(pte_val(*pte) & _PAGE_ENC))
+		/* Emulated MMIO to/from encrypted memory not supported */
+		return ES_UNSUPPORTED;
+
 	pa = (phys_addr_t)pte_pfn(*pte) << PAGE_SHIFT;
 	pa |= va & ~page_level_mask(level);
 
 	*paddr = pa;
 
-	return true;
+	return ES_OK;
 }
 
 /* Include code shared with pre-decompression boot stage */
@@ -731,6 +735,7 @@ static enum es_result vc_do_mmio(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
 {
 	u64 exit_code, exit_info_1, exit_info_2;
 	unsigned long ghcb_pa = __pa(ghcb);
+	enum es_result res;
 	phys_addr_t paddr;
 	void __user *ref;
 
@@ -740,11 +745,12 @@ static enum es_result vc_do_mmio(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
 
 	exit_code = read ? SVM_VMGEXIT_MMIO_READ : SVM_VMGEXIT_MMIO_WRITE;
 
-	if (!vc_slow_virt_to_phys(ghcb, ctxt, (unsigned long)ref, &paddr)) {
-		if (!read)
+	res = vc_slow_virt_to_phys(ghcb, ctxt, (unsigned long)ref, &paddr);
+	if (res != ES_OK) {
+		if (res == ES_EXCEPTION && !read)
 			ctxt->fi.error_code |= X86_PF_WRITE;
 
-		return ES_EXCEPTION;
+		return res;
 	}
 
 	exit_info_1 = paddr;

arch/x86/kernel/sev_verify_cbit.S

@@ -0,0 +1,89 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ *	sev_verify_cbit.S - Code for verification of the C-bit position reported
+ *			    by the Hypervisor when running with SEV enabled.
+ *
+ *	Copyright (c) 2020  Joerg Roedel (jroedel@suse.de)
+ *
+ * sev_verify_cbit() is called before switching to a new long-mode page-table
+ * at boot.
+ *
+ * Verify that the C-bit position is correct by writing a random value to
+ * an encrypted memory location while on the current page-table. Then it
+ * switches to the new page-table to verify the memory content is still the
+ * same. After that it switches back to the current page-table and when the
+ * check succeeded it returns. If the check failed the code invalidates the
+ * stack pointer and goes into a hlt loop. The stack-pointer is invalidated to
+ * make sure no interrupt or exception can get the CPU out of the hlt loop.
+ *
+ * New page-table pointer is expected in %rdi (first parameter)
+ *
+ */
+SYM_FUNC_START(sev_verify_cbit)
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+	/* First check if a C-bit was detected */
+	movq	sme_me_mask(%rip), %rsi
+	testq	%rsi, %rsi
+	jz	3f
+
+	/* sme_me_mask != 0 could mean SME or SEV - Check also for SEV */
+	movq	sev_status(%rip), %rsi
+	testq	%rsi, %rsi
+	jz	3f
+
+	/* Save CR4 in %rsi */
+	movq	%cr4, %rsi
+
+	/* Disable Global Pages */
+	movq	%rsi, %rdx
+	andq	$(~X86_CR4_PGE), %rdx
+	movq	%rdx, %cr4
+
+	/*
+	 * Verified that running under SEV - now get a random value using
+	 * RDRAND. This instruction is mandatory when running as an SEV guest.
+	 *
+	 * Don't bail out of the loop if RDRAND returns errors. It is better to
+	 * prevent forward progress than to work with a non-random value here.
+	 */
+1:	rdrand	%rdx
+	jnc	1b
+
+	/* Store value to memory and keep it in %rdx */
+	movq	%rdx, sev_check_data(%rip)
+
+	/* Backup current %cr3 value to restore it later */
+	movq	%cr3, %rcx
+
+	/* Switch to new %cr3 - This might unmap the stack */
+	movq	%rdi, %cr3
+
+	/*
+	 * Compare value in %rdx with memory location. If C-bit is incorrect
+	 * this would read the encrypted data and make the check fail.
+	 */
+	cmpq	%rdx, sev_check_data(%rip)
+
+	/* Restore old %cr3 */
+	movq	%rcx, %cr3
+
+	/* Restore previous CR4 */
+	movq	%rsi, %cr4
+
+	/* Check CMPQ result */
+	je	3f
+
+	/*
+	 * The check failed, prevent any forward progress to prevent ROP
+	 * attacks, invalidate the stack and go into a hlt loop.
+	 */
+	xorq	%rsp, %rsp
+	subq	$0x1000, %rsp
+2:	hlt
+	jmp 2b
+3:
+#endif
+	/* Return page-table pointer */
+	movq	%rdi, %rax
+	ret
+SYM_FUNC_END(sev_verify_cbit)

arch/x86/mm/mem_encrypt.c

@@ -39,6 +39,7 @@
  */
 u64 sme_me_mask __section(".data") = 0;
 u64 sev_status __section(".data") = 0;
+u64 sev_check_data __section(".data") = 0;
 EXPORT_SYMBOL(sme_me_mask);
 DEFINE_STATIC_KEY_FALSE(sev_enable_key);
 EXPORT_SYMBOL_GPL(sev_enable_key);