On 16/05/2020 03:37, H.J. Lu wrote: > On Fri, May 15, 2020 at 5:13 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote: >> Finally seeing as the question was asked but not answered, it is >> actually quite easy to figure out whether shadow stacks are enabled in >> the current thread. >> >> mov $1, %eax >> rdsspd %eax > This is for 32-bit mode. It actually works for both, if all you need is a shstk yes/no check. Usually, you also want SSP in the yes case, so substitute rdsspq %rax as appropriate. (On a tangent - binutils mandating the D/Q suffixes is very irritating with mixed 32/64bit code because you have to #ifdef your instructions despite the register operands being totally unambiguous. Also, D is the wrong suffix for AT&T syntax, and should be L. Frankly - the Intel manuals are wrong and should not have the operand size suffix included in the opcode name, as they are consistent with all the other instructions in this regard.) > I use > > /* Check if shadow stack is in use. */ > xorl %esi, %esi > rdsspq %rsi > testq %rsi, %rsi > /* Normal return if shadow stack isn't in use. */ > je L(no_shstk) This is probably fine for user code, as I don't think it would be legitimate for shstk to be enabled, with SSP being 0. Sadly, the same is not true for kernel shadow stacks. SSP is 0 after SYSCALL, SYSENTER and CLRSSBSY, and you've got to be careful to re-establish the shadow stack before a CALL, interrupt or exception tries pushing a word onto the shadow stack at 0xfffffffffffffff8. It is a very good (lucky?) thing that frame is unmapped for other reasons, because this corner case does not protect against multiple threads/cores using the same shadow stack concurrently. ~Andrew
