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

From: Andy Lutomirski
Date: Thu Feb 25 2016 - 13:41:26 EST

On Thu, Feb 25, 2016 at 10:30 AM, 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.

TF is very material to SYSENTER because Intel completely fucked up.

SYSENTER with TF set causes SYSENTER to trap in the sense that an
interrupt is delivered after SYSENTER, *in kernel mode*, *from CPL 0*,
with whatever probably-bullshit stack pointer we have set up.

We have overcomplicated code to fix up the resulting mess, but it
doesn't seem to work right on 32-bit. So I'm thinking of rewriting it
from scratch to make sense.

>> 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).

OK, I'll make sure to check this.