[PATCH v4 31/39] x86/shstk: Support wrss for userspace

[Rick Edgecombe]

For the current shadow stack implementation, shadow stacks contents can't
easily be provisioned with arbitrary data. This property helps apps
protect themselves better, but also restricts any potential apps that may
want to do exotic things at the expense of a little security.

The x86 shadow stack feature introduces a new instruction, wrss, which
can be enabled to write directly to shadow stack permissioned memory from
userspace. Allow it to get enabled via the prctl interface.

Only enable the userspace wrss instruction, which allows writes to
userspace shadow stacks from userspace. Do not allow it to be enabled
independently of shadow stack, as HW does not support using WRSS when
shadow stack is disabled.

>From a fault handler perspective, WRSS will behave very similar to WRUSS,
which is treated like a user access from a #PF err code perspective.

Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
---

v3:
 - Make wrss_control() static
 - Fix verbiage in commit log (Kees)

v2:
 - Add some commit log verbiage from (Dave Hansen)

v1:
 - New patch.

 arch/x86/include/uapi/asm/prctl.h |  1 +
 arch/x86/kernel/shstk.c           | 31 ++++++++++++++++++++++++++++++-
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index fc97ca7c4884..f13751c6bae4 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -33,5 +33,6 @@
 
 /* ARCH_SHSTK_ features bits */
 #define ARCH_SHSTK_SHSTK		(1ULL <<  0)
+#define ARCH_SHSTK_WRSS			(1ULL <<  1)
 
 #endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 8f329c22728a..e59544fec96d 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -364,6 +364,35 @@ void shstk_free(struct task_struct *tsk)
 	unmap_shadow_stack(shstk->base, shstk->size);
 }
 
+static int wrss_control(bool enable)
+{
+	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
+		return -EOPNOTSUPP;
+
+	/*
+	 * Only enable wrss if shadow stack is enabled. If shadow stack is not
+	 * enabled, wrss will already be disabled, so don't bother clearing it
+	 * when disabling.
+	 */
+	if (!features_enabled(ARCH_SHSTK_SHSTK))
+		return -EPERM;
+
+	/* Already enabled/disabled? */
+	if (features_enabled(ARCH_SHSTK_WRSS) == enable)
+		return 0;
+
+	fpregs_lock_and_load();
+	if (enable) {
+		set_clr_bits_msrl(MSR_IA32_U_CET, CET_WRSS_EN, 0);
+		features_set(ARCH_SHSTK_WRSS);
+	} else {
+		set_clr_bits_msrl(MSR_IA32_U_CET, 0, CET_WRSS_EN);
+		features_clr(ARCH_SHSTK_WRSS);
+	}
+	fpregs_unlock();
+
+	return 0;
+}
 
 static int shstk_disable(void)
 {
@@ -381,7 +410,7 @@ static int shstk_disable(void)
 	fpregs_unlock();
 
 	shstk_free(current);
-	features_clr(ARCH_SHSTK_SHSTK);
+	features_clr(ARCH_SHSTK_SHSTK | ARCH_SHSTK_WRSS);
 
 	return 0;
 }
-- 
2.17.1