[RFC PATCH v6 20/26] x86/cet/shstk: Introduce WRUSS instruction

[Yu-cheng Yu]

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@xxxxxxxxx>
---
 arch/x86/include/asm/special_insns.h | 32 ++++++++++++++++++++++++++++
 arch/x86/mm/fault.c                  |  9 ++++++++
 2 files changed, 41 insertions(+)

diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 43c029cdc3fe..43957f197a9a 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -237,6 +237,38 @@ 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")
 
 
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index c3368fed706c..7b5de629748e 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1318,6 +1318,15 @@ void do_user_addr_fault(struct pt_regs *regs,
 		}
 		flags |= FAULT_FLAG_USER;
 	} else {
+		/*
+		 * WRUSS is a kernel instruction and but writes
+		 * to user shadow stack.  When a fault occurs,
+		 * both X86_PF_USER and X86_PF_SHSTK are set.
+		 * Clear X86_PF_USER from sw_error_code.
+		 */
+		if ((hw_error_code & (X86_PF_USER | X86_PF_SHSTK)) ==
+		    (X86_PF_USER | X86_PF_SHSTK))
+			sw_error_code &= ~X86_PF_USER;
 		if (regs->flags & X86_EFLAGS_IF)
 			local_irq_enable();
 	}
-- 
2.17.1