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Date:   Thu, 21 May 2020 14:17:18 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: [RFC PATCH 3/5] selftest/x86: Fix sigreturn_64 test.

When shadow stack is enabled, selftests/x86/sigreturn_64 triggers a fault
when doing sigreturn to 32-bit context but the task's shadow stack pointer
is above 32-bit address range.  Fix it by:

- Allocate a small shadow stack below 32-bit address,
- Switch to the new shadow stack,
- Run tests,
- Switch back to the original 64-bit shadow stack.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
---
 tools/testing/selftests/x86/sigreturn.c | 28 +++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/tools/testing/selftests/x86/sigreturn.c b/tools/testing/selftests/x86/sigreturn.c
index 57c4f67f16ef..5bcd74d416ff 100644
--- a/tools/testing/selftests/x86/sigreturn.c
+++ b/tools/testing/selftests/x86/sigreturn.c
@@ -45,6 +45,14 @@
 #include <stdbool.h>
 #include <sys/ptrace.h>
 #include <sys/user.h>
+#include <x86intrin.h>
+#include <asm/prctl.h>
+#include <sys/prctl.h>
+
+#ifdef __x86_64__
+int arch_prctl(int code, unsigned long *addr);
+#define ARCH_CET_ALLOC_SHSTK 0x3004
+#endif
 
 /* Pull in AR_xyz defines. */
 typedef unsigned int u32;
@@ -766,6 +774,20 @@ int main()
 	int total_nerrs = 0;
 	unsigned short my_cs, my_ss;
 
+#ifdef __x86_64__
+	/* Alloc a shadow stack within 32-bit address range */
+	unsigned long arg, ssp_64, ssp_32;
+	ssp_64 = _get_ssp();
+
+	if (ssp_64 != 0) {
+		arg = 0x1001;
+		arch_prctl(ARCH_CET_ALLOC_SHSTK, &arg);
+		ssp_32 = arg + 0x1000 - 8;
+		asm volatile("RSTORSSP (%0)\n":: "r" (ssp_32));
+		asm volatile("SAVEPREVSSP");
+	}
+#endif
+
 	asm volatile ("mov %%cs,%0" : "=r" (my_cs));
 	asm volatile ("mov %%ss,%0" : "=r" (my_ss));
 	setup_ldt();
@@ -870,6 +892,12 @@ int main()
 
 #ifdef __x86_64__
 	total_nerrs += test_nonstrict_ss();
+
+	if (ssp_64 != 0) {
+		ssp_64 -= 8;
+		asm volatile("RSTORSSP (%0)\n":: "r" (ssp_64));
+		asm volatile("SAVEPREVSSP");
+	}
 #endif
 
 	return total_nerrs ? 1 : 0;
-- 
2.21.0

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