[PATCH v2] x86/shstk: Provide kernel command line knob to disable

[Mathias Krause]

Provide a kernel command line option 'shstk=off' to disable CET shadow
stacks, much like 'ibt=off' can be used to disable CET IBT.

With both set to off, it avoids setting CR4.CET on capable hardware to
allow debugging related issues during early boot which I happened to
have done way too many times in the recent past.

Document it along with its sibling option 'ibt' in kernel-parameters.txt
to allow others to find it more easily.

Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
---
v2:
- pick up Ack's
- document the new option as well as ibt=
- tweak changelog accordingly

 Documentation/admin-guide/kernel-parameters.txt | 14 ++++++++++++++
 arch/x86/kernel/shstk.c                         |  9 +++++++++
 2 files changed, 23 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 03a550630644..43bdf72f6495 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2248,6 +2248,16 @@ Kernel parameters
 			syscalls, essentially overriding IA32_EMULATION_DEFAULT_DISABLED at
 			boot time. When false, unconditionally disables IA32 emulation.
 
+	ibt=		[X86-64]
+			Format: ibt=warn, ibt=off
+			Changes the handling of CET IBT violations in the kernel.
+
+			The 'warn' setting makes CET IBT violations emit a
+			warning only instead of being fatal while the 'off'
+			setting completely disables CET IBT for the kernel.
+
+			To fully disable CET, use 'ibt=off shstk=off'.
+
 	icn=		[HW,ISDN]
 			Format: <io>[,<membase>[,<icn_id>[,<icn_id2>]]]
 
@@ -6924,6 +6934,10 @@ Kernel parameters
 			Specify the MCLK divider for Intel SoundWire buses in
 			case the BIOS does not provide the clock rate properly.
 
+	shstk=off	[X86-64] Disable CET userspace shadow stack support.
+
+			To fully disable CET, use 'ibt=off shstk=off'.
+
 	skew_tick=	[KNL,EARLY] Offset the periodic timer tick per cpu to mitigate
 			xtime_lock contention on larger systems, and/or RCU lock
 			contention on all systems with CONFIG_MAXSMP set.
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 978232b6d48d..68b46bf1540b 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -542,6 +542,15 @@ static int shstk_disable(void)
 	return 0;
 }
 
+static int __init shstk_configure(char *str)
+{
+	if (!strcmp(str, "off"))
+		setup_clear_cpu_cap(X86_FEATURE_SHSTK);
+
+	return 1;
+}
+__setup("shstk=", shstk_configure);
+
 SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags)
 {
 	bool set_tok = flags & SHADOW_STACK_SET_TOKEN;
-- 
2.47.3