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

From: Linus Torvalds
Date: Thu Feb 25 2016 - 13:30:58 EST

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).