Re: [PATCH] x86/smap: Fix the smap_save() asm

[Bill Wendling]

On Tue, Sep 15, 2020 at 4:40 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 16/09/2020 00:11, Andy Lutomirski wrote:
> >> On Sep 15, 2020, at 2:24 PM, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
> >>
> >> On Tue, Sep 15, 2020 at 1:56 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >>> The old smap_save() code was:
> >>>
> >>>  pushf
> >>>  pop %0
> >>>
> >>> with %0 defined by an "=rm" constraint.  This is fine if the
> >>> compiler picked the register option, but it was incorrect with an
> >>> %rsp-relative memory operand.
> >> It is incorrect because ... (I think mentioning the point about the
> >> red zone would be good, unless there were additional concerns?)
> > This isn’t a red zone issue — it’s a just-plain-wrong issue.  The popf is storing the result in the wrong place in memory — it’s RSP-relative, but RSP is whatever the compiler thinks it should be minus 8, because the compiler doesn’t know that pushfq changed RSP.
>
> It's worse than that.  Even when stating that %rsp is modified in the
> asm, the generated code sequence is still buggy, for recent Clang and GCC.
>
> https://godbolt.org/z/ccz9v7
>
> It's clearly not safe to ever use memory operands with pushf/popf asm
> fragments.
>
Would this apply to native_save_fl() and native_restore_fl in
arch/x86/include/asm/irqflags.h? It was like that two revisions ago,
but it was changed (back) to "=rm" with a comment about it being safe.

> >> This is something we should fix.  Bill, James, and I are discussing
> >> this internally.  Thank you for filing a bug; I owe you a beer just
> >> for that.
> > I’m looking forward to the day that beers can be exchanged in person again :)
>
> +1 to that.
>
+100

-bw