Re: [kernel-hardening] Re: [PATCH v10 2/3] arm/syscalls: Check address limit on user-mode return

[Thomas Garnier <thgarnie@google.com>]

On Wed, Jul 19, 2017 at 11:35 AM, Russell King - ARM Linux
<linux@armlinux.org.uk> wrote:
> On Wed, Jul 19, 2017 at 10:20:35AM -0700, Thomas Garnier wrote:
>> On Wed, Jul 19, 2017 at 10:06 AM, Russell King - ARM Linux
>> <linux@armlinux.org.uk> wrote:
>> > On Wed, Jul 19, 2017 at 05:58:20PM +0300, Leonard Crestez wrote:
>> > Probably best to revert.  I stopped looking at these patches during
>> > the discussion, as the discussion seemed to be mainly around other
>> > architectures, and I thought we had ARM settled.
>> >
>> > Looking at this patch now, there's several things I'm not happy with.
>> >
>> > The effect of adding a the new TIF flag for FSCHECK amongst the other
>> > flags is that we end up overflowing the 8-bit constant, and have to
>> > split the tests, meaning more instructions in the return path.  Eg:
>> >
>> > -       tst     r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
>> > +       tst     r1, #_TIF_SYSCALL_WORK
>> > +       bne     fast_work_pending
>> > +       tst     r1, #_TIF_WORK_MASK
>> >         bne     fast_work_pending
>> >
>> > should be written:
>> >
>> >         tst     r1, #_TIF_SYSCALL_WORK
>> >         tsteq   r1, #_TIF_WORK_MASK
>> >         bne     fast_work_pending
>> >
>> > and:
>> >
>> > -       tst     r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
>> > +       tst     r1, #_TIF_SYSCALL_WORK
>> > +       bne     fast_work_pending
>> > +       tst     r1, #_TIF_WORK_MASK
>> >
>> > should be:
>> >
>> >         tst     r1, #_TIF_SYSCALL_WORK
>> >         tsteq   r1, #_TIF_WORK_MASK
>> >
>> > There's no need for extra branches.
>> >
>> > Now, the next issue is that I don't think this TIF-flag approach is
>> > good for ARM - alignment faults can happen any time due to misaligned
>> > packets in the networking code, and we really don't want to be doing
>> > this check in a place that we can loop.
>> >
>> > My original suggestion for ARM was to do the address limit check after
>> > all work had been processed, with interrupts disabled (so no
>> > possibility of this kind of loop happening.)  However, that seems to
>> > have been replaced with this TIF approach, which is going to cause
>> > loops - I suspect if the probes code is enabled, this will suffer
>> > the same problem.  Remember, the various probes stuff can walk
>> > userspace stacks, which means they'll be using set_fs().
>> >
>> > I don't see why we've ended up with this (imho) sub-standard TIF-flag
>> > approach, and I think it's going to be very problematical.
>> >
>> > Can we please go back to the approach I suggested back in March for
>> > ARM that doesn't suffer from this problem?
>>
>> During the extensive thread discussion, Linus asked to move away from
>> architecture specific changes to this work flag system. I am glad to
>> fix the assembly as you asked on a separate patch.
>
> Well, for the record, I don't think you've got to the bottom of the
> "infinite loop" potential of Linus' approach.
>
> Eg, perf will likely trigger this same issue.  Eg, perf record -a -g
> will attempt to record the callchain both in kernel space and userspace
> each time a perf interrupt happens.  If the perf interrupt frequency is
> sufficiently high that we have multiple interrupts during the execution
> of do_work_pending() and its called functions, then that will turn this
> into an infinite loop yet again.

Do you think it applies to the patch I just sent? The other approach
is to check at the entrance, ignore _TIF_FSCHECK on the loop and clear
it on exit.

>
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-- 
Thomas