[PATCH v8 20/27] x86/cet/shstk: Introduce WRUSS instruction

Yu-cheng Yu
2019-08-13T20:52:18Z

WRUSS is a new kernel-mode instruction but writes directly to user
shadow stack memory. This is used to construct a return address on
the shadow stack for the signal handler.

This instruction can fault if the user shadow stack is invalid shadow
stack memory. In that case, the kernel does a fixup.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
arch/x86/include/asm/special_insns.h | 32 ++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)

diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 219be88a59d2..10f821d6b469 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -246,6 +246,38 @@ static inline void clwb(volatile void *__p)
: [pax] "a" (p));
}

+#ifdef CONFIG_X86_INTEL_CET
+#if defined(CONFIG_IA32_EMULATION) || defined(CONFIG_X86_X32)
+static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
+{
+ asm_volatile_goto("1: wrussd %1, (%0)\n"
+ _ASM_EXTABLE(1b, %l[fail])
+ :: "r" (addr), "r" (val)
+ :: fail);
+ return 0;
+fail:
+ return -EPERM;
+}
+#else
+static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
+{
+ WARN_ONCE(1, "%s used but not supported.\n", __func__);
+ return -EFAULT;
+}
+#endif
+
+static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
+{
+ asm_volatile_goto("1: wrussq %1, (%0)\n"
+ _ASM_EXTABLE(1b, %l[fail])
+ :: "r" (addr), "r" (val)
+ :: fail);
+ return 0;
+fail:
+ return -EPERM;
+}
+#endif /* CONFIG_X86_INTEL_CET */
+
#define nop() asm volatile ("nop")

--
2.17.1