x86/head/64: Check SEV encryption before switching to kernel page-table

When SEV is enabled, the kernel requests the C-bit position again from
the hypervisor to build its own page-table. Since the hypervisor is an
untrusted source, the C-bit position needs to be verified before the
kernel page-table is used.

Call sev_verify_cbit() before writing the CR3.

 [ bp: Massage. ]

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lkml.kernel.org/r/20201028164659.27002-5-joro@8bytes.org

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 7eb2a1c879695dcf1712964eb546e77b26d5513a..3c417734790f0390089beaa9da01e612c750c54a 100644 (file)
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -161,6 +161,21 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 
        /* Setup early boot stage 4-/5-level pagetables. */
        addq    phys_base(%rip), %rax
+
+       /*
+        * For SEV guests: Verify that the C-bit is correct. A malicious
+        * hypervisor could lie about the C-bit position to perform a ROP
+        * attack on the guest by writing to the unencrypted stack and wait for
+        * the next RET instruction.
+        * %rsi carries pointer to realmode data and is callee-clobbered. Save
+        * and restore it.
+        */
+       pushq   %rsi
+       movq    %rax, %rdi
+       call    sev_verify_cbit
+       popq    %rsi
+
+       /* Switch to new page-table */
        movq    %rax, %cr3
 
        /* Ensure I am executing from virtual addresses */
@@ -279,6 +294,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 SYM_CODE_END(secondary_startup_64)
 
 #include "verify_cpu.S"
+#include "sev_verify_cbit.S"
 
 #ifdef CONFIG_HOTPLUG_CPU
 /*

diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
index efbb3de472df4613a32308b000cf9ee1300bd154..bc0833713be955444359d83ad013125f295f1dfd 100644 (file)
--- a/arch/x86/mm/mem_encrypt.c
+++ b/arch/x86/mm/mem_encrypt.c
@@ -39,6 +39,7 @@
  */
 u64 sme_me_mask __section(".data") = 0;
 u64 sev_status __section(".data") = 0;
+u64 sev_check_data __section(".data") = 0;
 EXPORT_SYMBOL(sme_me_mask);
 DEFINE_STATIC_KEY_FALSE(sev_enable_key);
 EXPORT_SYMBOL_GPL(sev_enable_key);