[tip: x86/seves] x86/boot/compressed/64: Check SEV encryption in 64-bit boot-path
From: tip-bot2 for Joerg Roedel
Date: Thu Oct 29 2020 - 15:18:46 EST
The following commit has been merged into the x86/seves branch of tip:
Commit-ID: 86ce43f7dde81562f58b24b426cef068bd9f7595
Gitweb: https://git.kernel.org/tip/86ce43f7dde81562f58b24b426cef068bd9f7595
Author: Joerg Roedel <jroedel@xxxxxxx>
AuthorDate: Wed, 28 Oct 2020 17:46:57 +01:00
Committer: Borislav Petkov <bp@xxxxxxx>
CommitterDate: Thu, 29 Oct 2020 18:06:52 +01:00
x86/boot/compressed/64: Check SEV encryption in 64-bit boot-path
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 a separate file:
arch/x86/kernel/sev_verify_cbit.S
so that it can be re-used in the running kernel image.
[ bp: Massage. ]
Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
Link: https://lkml.kernel.org/r/20201028164659.27002-4-joro@xxxxxxxxxx
---
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 | 89 ++++++++++++++++++++++++-
4 files changed, 96 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 a5e5db6..39b2ede 100644
--- a/arch/x86/boot/compressed/ident_map_64.c
+++ b/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);
}
diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S
index 3092ae1..aa56179 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
@@ -111,4 +114,5 @@ 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 6d31f1b..d9a631c 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);
+unsigned long 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 0000000..ee04941
--- /dev/null
+++ b/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@xxxxxxx)
+ *
+ * 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)