x86/sev: Fix kernel crash due to late update to read-only ghcb_version

JIRA: https://issues.redhat.com/browse/RHEL-39439

commit 27d2534
Author: Ashwin Dayanand Kamat <ashwin.kamat@broadcom.com>
Date:   Wed Nov 29 16:10:29 2023 +0530

    x86/sev: Fix kernel crash due to late update to read-only ghcb_version

    A write-access violation page fault kernel crash was observed while running
    cpuhotplug LTP testcases on SEV-ES enabled systems. The crash was
    observed during hotplug, after the CPU was offlined and the process
    was migrated to different CPU. setup_ghcb() is called again which
    tries to update ghcb_version in sev_es_negotiate_protocol(). Ideally this
    is a read_only variable which is initialised during booting.

    Trying to write it results in a pagefault:

      BUG: unable to handle page fault for address: ffffffffba556e70
      #PF: supervisor write access in kernel mode
      #PF: error_code(0x0003) - permissions violation
      [ ...]
      Call Trace:
       <TASK>
       ? __die_body.cold+0x1a/0x1f
       ? __die+0x2a/0x35
       ? page_fault_oops+0x10c/0x270
       ? setup_ghcb+0x71/0x100
       ? __x86_return_thunk+0x5/0x6
       ? search_exception_tables+0x60/0x70
       ? __x86_return_thunk+0x5/0x6
       ? fixup_exception+0x27/0x320
       ? kernelmode_fixup_or_oops+0xa2/0x120
       ? __bad_area_nosemaphore+0x16a/0x1b0
       ? kernel_exc_vmm_communication+0x60/0xb0
       ? bad_area_nosemaphore+0x16/0x20
       ? do_kern_addr_fault+0x7a/0x90
       ? exc_page_fault+0xbd/0x160
       ? asm_exc_page_fault+0x27/0x30
       ? setup_ghcb+0x71/0x100
       ? setup_ghcb+0xe/0x100
       cpu_init_exception_handling+0x1b9/0x1f0

    The fix is to call sev_es_negotiate_protocol() only in the BSP boot phase,
    and it only needs to be done once in any case.

    [ mingo: Refined the changelog. ]

    Fixes: 95d33bf ("x86/sev: Register GHCB memory when SEV-SNP is active")
    Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
    Co-developed-by: Bo Gan <bo.gan@broadcom.com>
    Signed-off-by: Bo Gan <bo.gan@broadcom.com>
    Signed-off-by: Ashwin Dayanand Kamat <ashwin.kamat@broadcom.com>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
    Link: https://lore.kernel.org/r/1701254429-18250-1-git-send-email-kashwindayan@vmware.com

Signed-off-by: Bandan Das <bsd@redhat.com>

Author: whitebrandy

arch/x86/kernel/sev.c

@@ -1253,10 +1253,6 @@ void setup_ghcb(void)
 	if (!cc_platform_has(CC_ATTR_GUEST_STATE_ENCRYPT))
 		return;
 
-	/* First make sure the hypervisor talks a supported protocol. */
-	if (!sev_es_negotiate_protocol())
-		sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SEV_ES_GEN_REQ);
-
 	/*
 	 * Check whether the runtime #VC exception handler is active. It uses
 	 * the per-CPU GHCB page which is set up by sev_es_init_vc_handling().
@@ -1273,6 +1269,13 @@ void setup_ghcb(void)
 		return;
 	}
 
+	/*
+	 * Make sure the hypervisor talks a supported protocol.
+	 * This gets called only in the BSP boot phase.
+	 */
+	if (!sev_es_negotiate_protocol())
+		sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SEV_ES_GEN_REQ);
+
 	/*
 	 * Clear the boot_ghcb. The first exception comes in before the bss
 	 * section is cleared.