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