Re: [tip:x86/urgent] x86/entry/32: Add an ASM_CLAC to entry_SYSENTER_32

[Brian Gerst]

On Thu, Feb 25, 2016 at 1:30 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Feb 25, 2016 at 10:20 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>
>> Ideally we'd fix this up and restore flags on sysexit.  At least
>> failing to restore arithmetic flags isn't an info leak because the
>> exit code clobbers them with entirely predictable data.  I doubt
>> anyone cares all that much if we clobber AC.
>
> As long as the "clobber AC" is purely about clearing it, it's probably fine.
>
> Although there may be programs that set AC in order to actually get
> notified about alignment issues (perhaps for portability reasons,
> perhaps for small performance reasons). Clearing it will make those
> programs still work, but they lose the checking.
>
>> I wrote a test for NT and the test fails for a different reason: our
>> TF handling appears broken as well.  (Our sysenter TF handling is
>> *crap*, but it seems to work on 64-bit kernels at least.)
>
> TF should be entirely immaterial for system calls. Why would we care?
> We need it for correct handling of real traps, but not for the system
> call case afaik. Returning with TF clear is the right thing, since
> we're not returning *to* the system call instruction, but the
> instruction after.
>
>> My personal preference would be to add the missing popf.
>
> I don't mind adding the popf, but it won't help for iopl. Only iret
> restores iopl, if I recall correctly (but maybe I don't, and I'm too
> lazy to take the 30 seconds to look it up).
>
>                Linus

According to the SDM, popf will change IOPL only at CPL0, which is why
Xen (which runs at CPL1 on 32-bit) has a paravirt hook for it.

--
Brian Gerst