linux-kernel - [PATCH v2] x86/sev: Update ghcb

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Date:   Wed, 29 Nov 2023 16:10:29 +0530
From:   Ashwin Dayanand Kamat <kashwindayan@...are.com>
To:     linux-kernel@...r.kernel.org, thomas.lendacky@....com,
        bp@...en8.de, brijesh.singh@....com
Cc:     kashwindayan@...are.com, tglx@...utronix.de, mingo@...hat.com,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        jroedel@...e.de, stable@...r.kernel.org, ganb@...are.com,
        tkundu@...are.com, vsirnapalli@...are.com, akaher@...are.com,
        amakhalov@...are.com, namit@...are.com
Subject: [PATCH v2] x86/sev: Update ghcb_version only once

From: Ashwin Dayanand Kamat <ashwin.kamat@...adcom.com>

kernel crash was observed because of page fault, 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.
This results in pagefault.

>From logs,
[  256.447466] BUG: unable to handle page fault for address: ffffffffba556e70
[  256.447476] #PF: supervisor write access in kernel mode
[  256.447478] #PF: error_code(0x0003) - permissions violation
[  256.447479] PGD 8000667c0f067 P4D 8000667c0f067 PUD 8000667c10063 PMD 80080006674001e1
[  256.447483] Oops: 0003 [#1] PREEMPT SMP NOPTI
[  256.447487] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 6.1.45-8.ph5 #1-photon
.
.
.
.
.
[  256.447511] CR2: ffffffffba556e70 CR3: 0008000667c0a004 CR4: 0000000000770ee0
[  256.447514] PKRU: 55555554
[  256.447515] Call Trace:
[  256.447516]  <TASK>
[  256.447519]  ? __die_body.cold+0x1a/0x1f
[  256.447526]  ? __die+0x2a/0x35
[  256.447528]  ? page_fault_oops+0x10c/0x270
[  256.447531]  ? setup_ghcb+0x71/0x100
[  256.447533]  ? __x86_return_thunk+0x5/0x6
[  256.447537]  ? search_exception_tables+0x60/0x70
[  256.447541]  ? __x86_return_thunk+0x5/0x6
[  256.447543]  ? fixup_exception+0x27/0x320
[  256.447546]  ? kernelmode_fixup_or_oops+0xa2/0x120
[  256.447549]  ? __bad_area_nosemaphore+0x16a/0x1b0
[  256.447551]  ? kernel_exc_vmm_communication+0x60/0xb0
[  256.447556]  ? bad_area_nosemaphore+0x16/0x20
[  256.447558]  ? do_kern_addr_fault+0x7a/0x90
[  256.447560]  ? exc_page_fault+0xbd/0x160
[  256.447563]  ? asm_exc_page_fault+0x27/0x30
[  256.447570]  ? setup_ghcb+0x71/0x100
[  256.447572]  ? setup_ghcb+0xe/0x100
[  256.447574]  cpu_init_exception_handling+0x1b9/0x1f0

Fix is to call sev_es_negotiate_protocol() only in the BSP boot phase (and
it only needs to be done once)

Fixes: 95d33bfaa3e1 ("x86/sev: Register GHCB memory when SEV-SNP is active")
Co-developed-by: Bo Gan <bo.gan@...adcom.com>
Signed-off-by: Bo Gan <bo.gan@...adcom.com>
Signed-off-by: Ashwin Dayanand Kamat <ashwin.kamat@...adcom.com>
---
v2:
As per the review comments given by Tom Lendacky,  did below changes in v2,
 - Moved sev_es_negotiate_protocol() after initial_vc_handler if-check in setup_ghcb()
 - Added Signed-off of Co-developer
---
 arch/x86/kernel/sev.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 70472eebe719..c67285824e82 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -1234,10 +1234,6 @@ 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().
@@ -1254,6 +1250,13 @@ 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.
-- 
2.39.0