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Date: Tue, 20 Oct 2020 10:12:59 -0400
From: Arvind Sankar <nivedita@...m.mit.edu>
To: Joerg Roedel <joro@...tes.org>
Cc: x86@...nel.org, Joerg Roedel <jroedel@...e.de>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Kees Cook <keescook@...omium.org>,
Arvind Sankar <nivedita@...m.mit.edu>,
Martin Radev <martin.b.radev@...il.com>,
Tom Lendacky <thomas.lendacky@....com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/5] x86/boot/compressed/64: Check SEV encryption in
64-bit boot-path
On Tue, Oct 20, 2020 at 02:18:54PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
>
> Check whether the hypervisor reported the correct C-bit when running as
> an SEV guest. Using a wrong C-bit position could be used to leak
> sensitive data from the guest to the hypervisor.
>
> The check function is in arch/x86/kernel/sev_verify_cbit.S so that it
> can be re-used in the running kernel image.
>
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
> arch/x86/boot/compressed/ident_map_64.c | 1 +
> arch/x86/boot/compressed/mem_encrypt.S | 4 ++
> arch/x86/boot/compressed/misc.h | 2 +
> arch/x86/kernel/sev_verify_cbit.S | 91 +++++++++++++++++++++++++
> 4 files changed, 98 insertions(+)
> create mode 100644 arch/x86/kernel/sev_verify_cbit.S
>
> diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
> index 063a60edcf99..73abba3312a7 100644
> --- a/arch/x86/boot/compressed/ident_map_64.c
> +++ b/arch/x86/boot/compressed/ident_map_64.c
> @@ -153,6 +153,7 @@ void initialize_identity_maps(void)
> * into cr3.
> */
> add_identity_map((unsigned long)_head, (unsigned long)_end);
> + sev_verify_cbit(top_level_pgt);
> write_cr3(top_level_pgt);
> }
>
> diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S
> index 2192b3bd78d8..7409f2343d38 100644
> --- a/arch/x86/boot/compressed/mem_encrypt.S
> +++ b/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
> @@ -105,4 +108,5 @@ SYM_FUNC_END(set_sev_encryption_mask)
> .balign 8
> SYM_DATA(sme_me_mask, .quad 0)
> SYM_DATA(sev_status, .quad 0)
> +SYM_DATA(sev_check_data, .quad 0)
> #endif
> diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
> index 6d31f1b4c4d1..53f4848ad392 100644
> --- a/arch/x86/boot/compressed/misc.h
> +++ b/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);
>
> +void sev_verify_cbit(unsigned long cr3);
> +
> #endif /* BOOT_COMPRESSED_MISC_H */
> diff --git a/arch/x86/kernel/sev_verify_cbit.S b/arch/x86/kernel/sev_verify_cbit.S
> new file mode 100644
> index 000000000000..3f7153607956
> --- /dev/null
> +++ b/arch/x86/kernel/sev_verify_cbit.S
> @@ -0,0 +1,91 @@
> +/* 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@...e.de)
> + *
> + * Implements sev_verify_cbit() which is called before switching to a new
> + * long-mode page-table at boot.
> + *
> + * It verifies 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), %r10
> + testq %r10, %r10
> + jz 3f
> +
> + /* sme_me_mask != 0 could mean SME or SEV - Check also for SEV */
> + movq sev_status(%rip), %r10
> + testq %r10, %r10
> + jz 3f
> +
> + /* Save CR4 in %r12 */
> + pushq %r12
> + movq %cr4, %r12
> +
> + /* Disable Global Pages */
> + pushq %r12
> + andq $(~X86_CR4_PGE), %r12
> + movq %r12, %cr4
> + popq %r12
> +
> + /*
> + * 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 %r10
> + jnc 1b
> +
> + /* Store value to memory and keep it in %r10 */
> + movq %r10, sev_check_data(%rip)
> +
> + /* Backup current %cr3 value to restore it later */
> + movq %cr3, %r11
> +
> + /* Switch to new %cr3 - This might unmap the stack */
> + movq %rdi, %cr3
> +
> + /*
> + * Compare value in %r10 with memory location - If C-Bit is incorrect
> + * this would read the encrypted data and make the check fail.
> + */
> + cmpq %r10, sev_check_data(%rip)
> +
> + /* Restore old %cr3 */
> + movq %r11, %cr3
> +
> + /* Restore previous CR4 and %r12 */
> + movq %r12, %cr4
> + popq %r12
> +
> + /* 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
> + ret
> +SYM_FUNC_END(sev_verify_cbit)
> +
> --
> 2.28.0
>
Why use r10-r12 rather than the caller-save registers? Even for the head
code where you need to perserve the cr3 value you can just return it in
rax?
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