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

Yu-cheng Yu
2018-11-19T21:56:36Z

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 ++++++++++++++++++++++++++++
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