On Thu, Aug 30, 2018 at 4:44 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> 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 <yu-cheng.yu@xxxxxxxxx>
[...]
> +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.
> @@ -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 instrcution and but writes
Nits: typo ("instrcution"), weird grammar ("and but writes")
