Re: [PATCH v2 26/39] x86/cet/shstk: Introduce routines modifying shstk

On 05/10/2022 23:47, Edgecombe, Rick P wrote:

On Wed, 2022-10-05 at 02:43 +0000, Andrew Cooper wrote:

quoted

On 29/09/2022 23:29, Rick Edgecombe wrote:

quoted

diff --git a/arch/x86/include/asm/special_insns.h

b/arch/x86/include/asm/special_insns.h
index 35f709f619fb..f096f52bd059 100644

--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h

@@ -223,6 +223,19 @@ static inline void clwb(volatile void *__p)
               : [pax] "a" (p));
  }
  
+#ifdef CONFIG_X86_SHADOW_STACK
+static inline int write_user_shstk_64(u64 __user *addr, u64 val)
+{
+     asm_volatile_goto("1: wrussq %[val], (%[addr])\n"
+                       _ASM_EXTABLE(1b, %l[fail])
+                       :: [addr] "r" (addr), [val] "r" (val)
+                       :: fail);

"1: wrssq %[val], %[addr]\n"
_ASM_EXTABLE(1b, %l[fail])
: [addr] "+m" (*addr)
: [val] "r" (val)
:: fail

Otherwise you've failed to tell the compiler that you wrote to *addr.

With that fixed, it's not volatile because there are no unexpressed
side
effects.

Ok, thanks!

On further consideration, it should be "=m" not "+m", which is even less
constrained, and even easier for an enterprising optimiser to try and do
something useful with.

~Andrew