[tip: x86/shstk] x86/shstk: Support WRSS for userspace

[tip-bot2 for Rick Edgecombe]

The following commit has been merged into the x86/shstk branch of tip:

Commit-ID:     5dbdc4f10b8315d30f351ea6fdab1964374f8ca5
Gitweb:        https://git.kernel.org/tip/5dbdc4f10b8315d30f351ea6fdab1964374f8ca5
Author:        Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
AuthorDate:    Mon, 12 Jun 2023 17:11:01 -07:00
Committer:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Thu, 15 Jun 2023 16:31:34 -07:00

x86/shstk: Support WRSS for userspace

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 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.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Acked-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://lore.kernel.org/all/20230613001108.3040476-36-rick.p.edgecombe%40intel.com
---
 arch/x86/include/uapi/asm/prctl.h |  1 +-
 arch/x86/kernel/shstk.c           | 43 +++++++++++++++++++++++++++++-
 2 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index 6a8e0e1..eedfde3 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -36,5 +36,6 @@
 
 /* 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 04c37b3..ea0bf11 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -390,6 +390,47 @@ void shstk_free(struct task_struct *tsk)
 	unmap_shadow_stack(shstk->base, shstk->size);
 }
 
+static int wrss_control(bool enable)
+{
+	u64 msrval;
+
+	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();
+	rdmsrl(MSR_IA32_U_CET, msrval);
+
+	if (enable) {
+		features_set(ARCH_SHSTK_WRSS);
+		msrval |= CET_WRSS_EN;
+	} else {
+		features_clr(ARCH_SHSTK_WRSS);
+		if (!(msrval & CET_WRSS_EN))
+			goto unlock;
+
+		msrval &= ~CET_WRSS_EN;
+	}
+
+	wrmsrl(MSR_IA32_U_CET, msrval);
+
+unlock:
+	fpregs_unlock();
+
+	return 0;
+}
+
 static int shstk_disable(void)
 {
 	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
@@ -406,7 +447,7 @@ 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;
 }