[PATCH v5 21/27] 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...@intel.com>
---
 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 317fc59b512c..37f16269747d 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 7c3877a982f4..b91fc008f33a 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1305,6 +1305,15 @@ __do_page_fault(struct pt_regs *regs, unsigned long 
error_code,
                error_code |= X86_PF_USER;
                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 here.
+                */
+               if ((error_code & (X86_PF_USER | X86_PF_SHSTK)) ==
+                   (X86_PF_USER | X86_PF_SHSTK))
+                       error_code &= ~X86_PF_USER;
                if (regs->flags & X86_EFLAGS_IF)
                        local_irq_enable();
        }
-- 
2.17.1