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

[Russell King - ARM Linux <linux@armlinux.org.uk>]

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.

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