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

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Date:   Mon,  6 Nov 2023 12:02:41 +0530
From:   Ashwin Dayanand Kamat <kashwindayan@...are.com>
To:     linux-kernel@...r.kernel.org
Cc:     kashwindayan@...are.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, thomas.lendacky@....com, jroedel@...e.de,
        brijesh.singh@....com, ganb@...are.com, tkundu@...are.com,
        vsirnapalli@...are.com, akaher@...are.com, amakhalov@...are.com
Subject: [PATCH] x86/sev: Update ghcb_version only once

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 avoid updating the variable after it has been initialised during booting.

Fixes: 95d33bfaa3e1 ("x86/sev: Register GHCB memory when SEV-SNP is active")
Signed-off-by: Ashwin Dayanand Kamat <kashwindayan@...are.com>
Co-developed-by: Bo Gan <ganb@...are.com>
---
 arch/x86/kernel/sev-shared.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index ccb0915e84e1..a447908f2b4d 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -144,6 +144,9 @@ static bool sev_es_negotiate_protocol(void)
 {
 	u64 val;
 
+	if (ghcb_version)
+		return true;
+
 	/* Do the GHCB protocol version negotiation */
 	sev_es_wr_ghcb_msr(GHCB_MSR_SEV_INFO_REQ);
 	VMGEXIT();
-- 
2.39.0