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From:   Rick Edgecombe <rick.p.edgecombe@intel.com>
To:     x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
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        dethoma@microsoft.com, akpm@linux-foundation.org
Cc:     rick.p.edgecombe@intel.com
Subject: [PATCH v3 30/37] x86/shstk: Support wrss for userspace
Date:   Fri,  4 Nov 2022 15:35:57 -0700
Message-Id: <20221104223604.29615-31-rick.p.edgecombe@intel.com>
In-Reply-To: <20221104223604.29615-1-rick.p.edgecombe@intel.com>
References: <20221104223604.29615-1-rick.p.edgecombe@intel.com>
Precedence: bulk

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@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

---

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           | 33 +++++++++++++++++++++++++++++--
 2 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index dad5288bf086..5f1d3181e4a1 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -28,5 +28,6 @@
 
 /* ARCH_CET_ features bits */
 #define CET_SHSTK			(1ULL <<  0)
+#define CET_WRSS			(1ULL <<  1)
 
 #endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 9a025eea520f..cbd0970b26d7 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(CET_SHSTK))
+		return -EPERM;
+
+	/* Already enabled/disabled? */
+	if (features_enabled(CET_WRSS) == enable)
+		return 0;
+
+	fpregs_lock_and_load();
+	if (enable) {
+		set_clr_bits_msrl(MSR_IA32_U_CET, CET_WRSS_EN, 0);
+		features_set(CET_WRSS);
+	} else {
+		set_clr_bits_msrl(MSR_IA32_U_CET, 0, CET_WRSS_EN);
+		features_clr(CET_WRSS);
+	}
+	fpregs_unlock();
+
+	return 0;
+}
 
 static int shstk_disable(void)
 {
@@ -376,12 +405,12 @@ static int shstk_disable(void)
 
 	fpregs_lock_and_load();
 	/* Disable WRSS too when disabling shadow stack */
-	set_clr_bits_msrl(MSR_IA32_U_CET, 0, CET_SHSTK_EN);
+	set_clr_bits_msrl(MSR_IA32_U_CET, 0, CET_SHSTK_EN | CET_WRSS_EN);
 	wrmsrl(MSR_IA32_PL3_SSP, 0);
 	fpregs_unlock();
 
 	shstk_free(current);
-	features_clr(CET_SHSTK);
+	features_clr(CET_SHSTK | CET_WRSS);
 
 	return 0;
 }
-- 
2.17.1