Re: [PATCH v3 27/37] x86/shstk: Introduce routines modifying shstk

On Tue, 2022-11-15 at 15:18 +0100, Peter Zijlstra wrote:

On Fri, Nov 04, 2022 at 03:35:54PM -0700, Rick Edgecombe wrote:

quoted

+#ifdef CONFIG_X86_USER_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);
+     return 0;
+fail:
+     return -EFAULT;
+}
+#endif /* CONFIG_X86_USER_SHADOW_STACK */

Why isn't this modelled after put_user() ?

You mean as far as supporting multiple sizes? It just isn't really
needed yet. We are only writing single frames. I suppose it might make
more sense with the alt shadow stack support, but that is dropped for
now.

The other difference here is that WRUSS is a weird instruction that is
treated as a user access even if it comes from the kernel mode. So it's
doesn't need to stac/clac.

Should you write a 64bit value even if the task receiving a signal is
32bit ?

32 bit support was also dropped.