On Thu, Aug 30, 2018 at 4:44 PM Yu-cheng Yu [off-list ref] wrote:

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
fixup.

Signed-off-by: Yu-cheng Yu <redacted>

[...]

+static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
+{
+       int err = 0;
+
+       asm volatile("1: wrussq %1, (%0)\n"
+                    "2:\n"
+                    _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_wruss)
+                    :
+                    : "r" (addr), "r" (val));
+
+       return err;
+}

What's up with "err"? You set it to zero, and then you return it, but
nothing can ever set it to non-zero, right?

+__visible bool ex_handler_wruss(const struct exception_table_entry *fixup,
+                               struct pt_regs *regs, int trapnr)
+{
+       regs->ip = ex_fixup_addr(fixup);
+       regs->ax = -1;
+       return true;
+}

And here you just write into regs->ax, but your "asm volatile" doesn't
reserve that register. This looks wrong to me.

I think you probably want to add something like an explicit
`"+&a"(err)` output to the asm statements.

Nits: typo ("instrcution"), weird grammar ("and but writes")