[PATCH] x86/umip: Add a umip= cmdline switch

[Borislav Petkov]

From: Borislav Petkov <bp@xxxxxxx>

And add the first control option

  umip=warnings_off

which disables warnings resulting from emulating UMIP-enabled insns.

The actual use case is for users playing games in wine, games like
Overwatch, for example, which go nuts with SIDT:

  [Di Sep  7 00:24:05 2021] umip_printk: 1345 callbacks suppressed
  [Di Sep  7 00:24:05 2021] umip: someapp.exe[29231] ip:14064cdba sp:11b7c0: SIDT instruction cannot be used by applications.
  [Di Sep  7 00:24:05 2021] umip: someapp.exe[29231] ip:14064cdba sp:11b7c0: For now, expensive software emulation returns the result.
  ...
  [Di Sep  7 00:26:06 2021] umip_printk: 2227 callbacks suppressed
  [Di Sep  7 00:26:06 2021] umip: someapp.exe[29231] ip:14064cdba sp:11b940: SIDT instruction cannot be used by applications.

filling up the kernel log unnecessarily with the same info over and over
again which doesn't mean a whit to the users - they just wanna play.

So add a boot-time control switch for those warning messages.

Reported-by: Marcus Rückert <mrueckert@xxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
---
 .../admin-guide/kernel-parameters.txt         |  5 +++
 arch/x86/kernel/umip.c                        | 33 +++++++++++++++++--
 2 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 828d11441ebf..815d022c3e87 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5887,6 +5887,11 @@
 	unknown_nmi_panic
 			[X86] Cause panic on unknown NMI.
 
+	umip=warnings_off
+			[X86]
+			* warnings_off - do not issue warnings when emulating
+			  UMIP-enabled instructions.
+
 	usbcore.authorized_default=
 			[USB] Default USB device authorization:
 			(default -1 = authorized except for wireless USB,
diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
index 576b47e7523d..1d37dc626011 100644
--- a/arch/x86/kernel/umip.c
+++ b/arch/x86/kernel/umip.c
@@ -90,10 +90,19 @@ static const char * const umip_insns[5] = {
 	[UMIP_INST_STR] = "STR",
 };
 
-#define umip_pr_err(regs, fmt, ...) \
+static struct umip_config {
+	__u64 warnings_off	: 1,
+	      __reserved	: 63;
+} umip_cfg;
+
+#define umip_pr_err(regs, fmt, ...)				\
 	umip_printk(regs, KERN_ERR, fmt, ##__VA_ARGS__)
-#define umip_pr_warn(regs, fmt, ...) \
-	umip_printk(regs, KERN_WARNING, fmt,  ##__VA_ARGS__)
+
+#define umip_pr_warn(regs, fmt, ...)					\
+({									\
+	if (!umip_cfg.warnings_off)					\
+		umip_printk(regs, KERN_WARNING, fmt,  ##__VA_ARGS__);	\
+})
 
 /**
  * umip_printk() - Print a rate-limited message
@@ -407,5 +416,23 @@ bool fixup_umip_exception(struct pt_regs *regs)
 
 	/* increase IP to let the program keep going */
 	regs->ip += insn.length;
+
 	return true;
 }
+
+static int __init parse_umip_param(char *str)
+{
+	if (!str)
+		return 0;
+
+	if (*str == '=')
+		str++;
+
+	if (!strcmp(str, "warnings_off"))
+		umip_cfg.warnings_off = 1;
+	else
+		return 0;
+
+	return 1;
+}
+__setup("umip", parse_umip_param);
-- 
2.29.2