linux-kernel - Re: [PATCH v3] x86/sev: Do not touch VMSA pages during kdump of SNP guest memory

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Message-ID: <20250430124728.GDaBIb4JTMxW2zO9gB@fat_crate.local>
Date: Wed, 30 Apr 2025 14:47:28 +0200
From: Borislav Petkov <bp@...en8.de>
To: Ashish Kalra <Ashish.Kalra@....com>
Cc: tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
	x86@...nel.org, thomas.lendacky@....com, hpa@...or.com,
	kees@...nel.org, michael.roth@....com, nikunj@....com,
	seanjc@...gle.com, ardb@...nel.org, gustavoars@...nel.org,
	sgarzare@...hat.com, stable@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
	kexec@...ts.infradead.org, linux-coco@...ts.linux.dev
Subject: Re: [PATCH v3] x86/sev: Do not touch VMSA pages during kdump of SNP
 guest memory

On Mon, Apr 28, 2025 at 09:41:51PM +0000, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@....com>
> 
> When kdump is running makedumpfile to generate vmcore and dumping SNP
> guest memory it touches the VMSA page of the vCPU executing kdump which
> then results in unrecoverable #NPF/RMP faults as the VMSA page is
> marked busy/in-use when the vCPU is running and subsequently causes
> guest softlockup/hang.
> 
> Additionally other APs may be halted in guest mode and their VMSA pages
> are marked busy and touching these VMSA pages during guest memory dump
> will also cause #NPF.
> 
> Issue AP_DESTROY GHCB calls on other APs to ensure they are kicked out
> of guest mode and then clear the VMSA bit on their VMSA pages.
> 
> If the vCPU running kdump is an AP, mark it's VMSA page as offline to
> ensure that makedumpfile excludes that page while dumping guest memory.
> 
> Cc: stable@...r.kernel.org
> Fixes: 3074152e56c9 ("x86/sev: Convert shared memory back to private on kexec")
> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
> ---
>  arch/x86/coco/sev/core.c | 241 +++++++++++++++++++++++++--------------
>  1 file changed, 155 insertions(+), 86 deletions(-)

Some minor cleanups ontop:

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index b031cabb2ccf..9ac902d022bf 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -961,6 +961,7 @@ void snp_accept_memory(phys_addr_t start, phys_addr_t end)
 
 static int vmgexit_ap_control(u64 event, struct sev_es_save_area *vmsa, u32 apic_id)
 {
+	bool create = event == SVM_VMGEXIT_AP_CREATE;
 	struct ghcb_state state;
 	unsigned long flags;
 	struct ghcb *ghcb;
@@ -971,8 +972,10 @@ static int vmgexit_ap_control(u64 event, struct sev_es_save_area *vmsa, u32 apic
 	ghcb = __sev_get_ghcb(&state);
 
 	vc_ghcb_invalidate(ghcb);
-	if (event == SVM_VMGEXIT_AP_CREATE)
+
+	if (create)
 		ghcb_set_rax(ghcb, vmsa->sev_features);
+
 	ghcb_set_sw_exit_code(ghcb, SVM_VMGEXIT_AP_CREATION);
 	ghcb_set_sw_exit_info_1(ghcb,
 				((u64)apic_id << 32)	|
@@ -985,7 +988,7 @@ static int vmgexit_ap_control(u64 event, struct sev_es_save_area *vmsa, u32 apic
 
 	if (!ghcb_sw_exit_info_1_is_valid(ghcb) ||
 	    lower_32_bits(ghcb->save.sw_exit_info_1)) {
-		pr_err("SNP AP %s error\n", (event == SVM_VMGEXIT_AP_CREATE ? "CREATE" : "DESTROY"));
+		pr_err("SNP AP %s error\n", (create ? "CREATE" : "DESTROY"));
 		ret = -EINVAL;
 	}
 
@@ -1168,8 +1171,8 @@ static void shutdown_all_aps(void)
 		vmsa = per_cpu(sev_vmsa, cpu);
 
 		/*
-		 * BSP does not have guest allocated VMSA and there is no need
-		 * to clear the VMSA tag for this page.
+		 * The BSP or offlined APs do not have guest allocated VMSA
+		 * and there is no need  to clear the VMSA tag for this page.
 		 */
 		if (!vmsa)
 			continue;

-- 
Regards/Gruss,
    Boris.

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