[PATCH v1 2/4] x86/vmware: Add the vmware_record_panic_msg sysctl

[Zack Rusin]

Full panic text is useful on ordinary guests. Encrypted guests should
export it only when an administrator opts in. Reporting a crash does not
require transferring the log.

Enable panic recording by default for ordinary VMware guests and disable
it when guest memory encryption is active. Expose the policy as the
boolean kernel.vmware_record_panic_msg sysctl and document that it
controls only the vmware.log transfer.

Register the sysctl only after the panic buffer and dumper are ready, and
only when sysctl support is built. A registration failure leaves the
internal default in force.

Signed-off-by: Zack Rusin <zack.rusin@xxxxxxxxxxxx>
---
 Documentation/admin-guide/sysctl/kernel.rst | 14 +++++++++++++
 arch/x86/kernel/cpu/vmware.c                | 23 +++++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index b6328cd0f43e..4577e935ddcf 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1690,6 +1690,20 @@ entry will default to 2 instead of 0.
 = =============================================================
 
 
+vmware_record_panic_msg
+=======================
+
+Controls whether panic kmsg data is written to the host's ``vmware.log``.
+This setting does not control the separate VMware guest-crash event.
+
+= ==============================================================
+0 Do not write panic kmsg data to ``vmware.log``. This is the
+  default for encrypted guests.
+1 Write panic kmsg data to ``vmware.log``. This is the default for
+  ordinary guests.
+= ==============================================================
+
+
 warn_limit
 ==========
 
diff --git a/arch/x86/kernel/cpu/vmware.c b/arch/x86/kernel/cpu/vmware.c
index bf59653d6e07..3848811550e9 100644
--- a/arch/x86/kernel/cpu/vmware.c
+++ b/arch/x86/kernel/cpu/vmware.c
@@ -31,6 +31,7 @@
 #include <linux/efi.h>
 #include <linux/reboot.h>
 #include <linux/static_call.h>
+#include <linux/sysctl.h>
 #include <linux/wordpart.h>
 #include <linux/sched/cputime.h>
 #include <asm/div64.h>
@@ -255,6 +256,19 @@ static int vmware_log_rpc(const char *buffer, size_t length)
 }
 
 static struct page *vmware_panic_page;
+static int vmware_record_panic_msg;
+
+static const struct ctl_table vmware_panic_sysctls[] = {
+	{
+		.procname	= "vmware_record_panic_msg",
+		.data		= &vmware_record_panic_msg,
+		.maxlen		= sizeof(vmware_record_panic_msg),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE,
+	},
+};
 
 static void vmware_panic_log_dump(struct kmsg_dumper *dumper,
 				  struct kmsg_dump_detail *detail)
@@ -263,6 +277,9 @@ static void vmware_panic_log_dump(struct kmsg_dumper *dumper,
 	char *buffer = page_address(vmware_panic_page);
 	size_t length = 0;
 
+	if (!READ_ONCE(vmware_record_panic_msg))
+		return;
+
 	memcpy(buffer, VMWARE_LOG_PREFIX, VMWARE_LOG_PREFIX_LEN);
 	kmsg_dump_rewind(&iter);
 	(void)kmsg_dump_get_buffer(&iter, true,
@@ -283,6 +300,9 @@ static int __init vmware_panic_log_init(void)
 	if (!hypervisor_is_type(X86_HYPER_VMWARE))
 		return 0;
 
+	vmware_record_panic_msg =
+		!cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT);
+
 	vmware_panic_page = alloc_page(GFP_KERNEL);
 	if (!vmware_panic_page) {
 		pr_err("failed to allocate panic log buffer\n");
@@ -295,6 +315,9 @@ static int __init vmware_panic_log_init(void)
 		__free_page(vmware_panic_page);
 		vmware_panic_page = NULL;
 	}
+	if (vmware_panic_page && IS_ENABLED(CONFIG_SYSCTL) &&
+	    !register_sysctl("kernel", vmware_panic_sysctls))
+		pr_err("failed to register panic log sysctl\n");
 
 	return 0;
 }
-- 
2.53.0