1/* SPDX-License-Identifier: GPL-2.0-only */
2/*
3 *	sev_verify_cbit.S - Code for verification of the C-bit position reported
4 *			    by the Hypervisor when running with SEV enabled.
5 *
6 *	Copyright (c) 2020  Joerg Roedel (jroedel@suse.de)
7 *
8 * sev_verify_cbit() is called before switching to a new long-mode page-table
9 * at boot.
10 *
11 * Verify that the C-bit position is correct by writing a random value to
12 * an encrypted memory location while on the current page-table. Then it
13 * switches to the new page-table to verify the memory content is still the
14 * same. After that it switches back to the current page-table and when the
15 * check succeeded it returns. If the check failed the code invalidates the
16 * stack pointer and goes into a hlt loop. The stack-pointer is invalidated to
17 * make sure no interrupt or exception can get the CPU out of the hlt loop.
18 *
19 * New page-table pointer is expected in %rdi (first parameter)
20 *
21 */
22SYM_FUNC_START(sev_verify_cbit)
23#ifdef CONFIG_AMD_MEM_ENCRYPT
24	/* First check if a C-bit was detected */
25	movq	sme_me_mask(%rip), %rsi
26	testq	%rsi, %rsi
27	jz	3f
28
29	/* sme_me_mask != 0 could mean SME or SEV - Check also for SEV */
30	movq	sev_status(%rip), %rsi
31	testq	%rsi, %rsi
32	jz	3f
33
34	/* Save CR4 in %rsi */
35	movq	%cr4, %rsi
36
37	/* Disable Global Pages */
38	movq	%rsi, %rdx
39	andq	$(~X86_CR4_PGE), %rdx
40	movq	%rdx, %cr4
41
42	/*
43	 * Verified that running under SEV - now get a random value using
44	 * RDRAND. This instruction is mandatory when running as an SEV guest.
45	 *
46	 * Don't bail out of the loop if RDRAND returns errors. It is better to
47	 * prevent forward progress than to work with a non-random value here.
48	 */
491:	rdrand	%rdx
50	jnc	1b
51
52	/* Store value to memory and keep it in %rdx */
53	movq	%rdx, sev_check_data(%rip)
54
55	/* Backup current %cr3 value to restore it later */
56	movq	%cr3, %rcx
57
58	/* Switch to new %cr3 - This might unmap the stack */
59	movq	%rdi, %cr3
60
61	/*
62	 * Compare value in %rdx with memory location. If C-bit is incorrect
63	 * this would read the encrypted data and make the check fail.
64	 */
65	cmpq	%rdx, sev_check_data(%rip)
66
67	/* Restore old %cr3 */
68	movq	%rcx, %cr3
69
70	/* Restore previous CR4 */
71	movq	%rsi, %cr4
72
73	/* Check CMPQ result */
74	je	3f
75
76	/*
77	 * The check failed, prevent any forward progress to prevent ROP
78	 * attacks, invalidate the stack and go into a hlt loop.
79	 */
80	xorq	%rsp, %rsp
81	subq	$0x1000, %rsp
822:	hlt
83	jmp 2b
843:
85#endif
86	/* Return page-table pointer */
87	movq	%rdi, %rax
88	ret
89SYM_FUNC_END(sev_verify_cbit)
90