[PATCH v3 3/4] x86/vmware: Add the vmware_record_panic_msg sysctl

[Zack Rusin]

Allow administrators to disable host log transfer on ordinary guests or
opt in on encrypted guests. Expose the boolean recording policy through
kernel.vmware_record_panic_msg using proc_dobool(), as Joel suggested.
Zero disables recording, nonzero integers enable it, and reads return
0 or 1.

Register the sysctl only after the logger is ready. A registration failure
leaves the internal default in force. Document that the log is sent
before kdump, and that disable_panic_notifiers_platform_list also skips
it.

Link: https://lore.kernel.org/r/4e73yd2ofpdzl6rptzvd74no3hnyfblknoq7wzxlw7f3zjbz7c@srfabx6lsusj
Signed-off-by: Zack Rusin <zack.rusin@xxxxxxxxxxxx>
---
v3: Document the platform list ordering and the disable switch, and fix
the short section-title underline reported by Sashiko. No code change
from v2.

 Documentation/admin-guide/sysctl/kernel.rst | 20 ++++++++++++++++++++
 arch/x86/kernel/cpu/vmware.c                | 15 +++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 608055d7ccc4..be2d5aa52362 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1680,6 +1680,26 @@ entry will default to 2 instead of 0.
 = =============================================================
 
 
+vmware_record_panic_msg
+=======================
+
+Controls whether kmsg data is written to the host's ``vmware.log`` on
+panic. This setting does not control the separate VMware guest-crash
+event. Writing zero disables recording; writing a nonzero integer
+enables it. Reads return 0 or 1.
+
+= ==============================================================
+0 Do not write kmsg data to ``vmware.log``. This is the default
+  for encrypted guests.
+1 Write kmsg data to ``vmware.log``. This is the default for
+  ordinary guests.
+= ==============================================================
+
+The data is written by a platform panic notifier: early in panic,
+before kdump, and also when a fatal oops enters kdump without a panic.
+Setting ``disable_panic_notifiers_platform_list`` skips it as well.
+
+
 warn_limit
 ==========
 
diff --git a/arch/x86/kernel/cpu/vmware.c b/arch/x86/kernel/cpu/vmware.c
index 0c6f7248a665..8c8b6dd7c3e8 100644
--- a/arch/x86/kernel/cpu/vmware.c
+++ b/arch/x86/kernel/cpu/vmware.c
@@ -33,6 +33,7 @@
 #include <linux/reboot.h>
 #include <linux/sizes.h>
 #include <linux/static_call.h>
+#include <linux/sysctl.h>
 #include <linux/wordpart.h>
 #include <linux/sched/cputime.h>
 #include <asm/div64.h>
@@ -260,6 +261,16 @@ static int vmware_log_rpc(const char *buffer, size_t length)
 static struct page *vmware_panic_page;
 static bool 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_dobool,
+	},
+};
+
 static int vmware_panic_log_notify(struct notifier_block *nb,
 				   unsigned long action, void *data)
 {
@@ -311,6 +322,10 @@ static int __init vmware_panic_log_init(void)
 		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;
 }
 early_initcall(vmware_panic_log_init);