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

[Brian Gerst]

On Thu, Feb 25, 2016 at 2:39 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Thu, Feb 25, 2016 at 11:31 AM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>> 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.
>
> But maybe we can ditch that paravirt hook and just modify regs->flags
> in sys_iopl.  Xen never uses sysexit at all:
>
>     /* XEN PV guests always use IRET path */
>     ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \
>             "jmp .Lsyscall_32_done", X86_FEATURE_XENPV
>
> and if we add the missing popf, we should be good to go.

IRET won't change IOPL either at CPL != 0, so Xen still needs that hook.

--
Brian Gerst