Re: [PATCH v2 07/10] x86/entry: Vastly simplify SYSENTER TF handling

[Brian Gerst]

On Mon, Mar 7, 2016 at 1:03 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Mon, Mar 7, 2016 at 9:17 AM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>> On Sun, Mar 6, 2016 at 12:52 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>> Due to a blatant design error, SYSENTER doesn't clear TF.  As a result,
>>> if a user does SYSENTER with TF set, we will single-step through the
>>> kernel until something clears TF.  There is absolutely nothing we can
>>> do to prevent this short of turning off SYSENTER [1].
>>>
>>> Simplify the handling considerably with two changes:
>>>
>>> 1. We already sanitize EFLAGS in SYSENTER to clear NT and AC.  We can
>>>    add TF to that list of flags to sanitize with no overhead whatsoever.
>>>
>>> 2. Teach do_debug to ignore single-step traps in the SYSENTER prologue.
>>
>> What is wrong with the current method of clearing TF and setting
>> TIF_SINGLESTEP on the first debug trap?  This patch actually increases
>> complexity because it has to check for a range of addresses rather
>> than just the first instruction, plus it has to singlestep all the way
>> through the SYSENTER prologue.
>>
>> Unless there is an actual issue with TIF_SINGLESTEP, I don't think
>> this patch is an improvement.
>
> TIF_SINGLESTEP has bizarrely overloaded semantics.
>
> There's this:
>
>     /*
>      * If we stepped into a sysenter/syscall insn, it trapped in
>      * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP.
>      * If user-mode had set TF itself, then it's still clear from
>      * do_debug() and we need to set it again to restore the user
>      * state so we don't wrongly set TIF_FORCED_TF below.
>      * If enable_single_step() was used last and that is what
>      * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are
>      * already set and our bookkeeping is fine.
>      */
>     if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP)))
>         regs->flags |= X86_EFLAGS_TF;
>
> but TIF_SINGLESTEP is also used for other things.  (And I need to
> follow up with a patch to remove that comment.)  This results in
> incomprehensible behavior: if a user program sets TF and does
> SYSENTER, then TIF_SINGLESTEP gets set (and stays set!).  This does
> not happen if a user sets TF and does INT80 or SYSCALL.  There was at
> least one bug in here that Oleg fixed a while back, and I wouldn't be
> at all surprised if there were others.
>
> I don't know what would happen if TF were set and SYSENTER were used
> to do a syscall where the __get_user to load the syscall nr failed.
> That happens before the TIF_SINGLESTEP fixup.
>
> Basically, the overloaded use of TIF_SINGLESTEP was complicated and
> hard to understand, and the new behavior is straightforward and
> consistent with other entries, even if it's a bit slower.
>
> We could introduce a new TIF_SYSENTER_TF and use it directly, or we
> could accelerate the TF fixup in regs->flags by switching to a
> different entry path (I had a draft patch to do that), but I tend to
> favor simplicity for things that aren't performance-critical.

The alternate entry path wouldn't be very big, just 5 instructions
with the OR being the only difference.

Another option is to use a per-cpu var instead of a TIF flag.

--
Brian Gerst