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 xorl %esp, %esp
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