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

[Andy Lutomirski]

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.

--Andy