Re: rseq: How to test for compat task at signal delivery

[Andy Lutomirski]

On Tue, Jun 26, 2018 at 12:50 PM Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
> ----- On Jun 26, 2018, at 3:32 PM, Andy Lutomirski luto@xxxxxxxxxxxxxx wrote:
>
> > On Tue, Jun 26, 2018 at 11:45 AM Mathieu Desnoyers
> > <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> >>
> >> ----- On Jun 26, 2018, at 1:38 PM, Mathieu Desnoyers
> >> mathieu.desnoyers@xxxxxxxxxxxx wrote:
> >>
> >> > Hi Andy,
> >> >
> >> > I would like to make the behavior rseq on compat tasks more robust
> >> > by ensuring that kernel/rseq.c:rseq_get_rseq_cs() clears the high
> >> > bits of rseq_cs->abort_ip, rseq_cs->start_ip and
> >> > rseq_cs->post_commit_offset when a 32-bit binary is run on a 64-bit
> >> > kernel.
> >> >
> >> > The intent here is that if user-space has garbage rather than zeroes
> >> > in its struct rseq_cs fields padding, the behavior will be the same
> >> > whether the binary is run on 32-bit or 64 kernels.
> >> >
> >> > I know that internally, the kernel is making a transition from
> >> > is_compat_task() to in_compat_syscall().
> >> >
> >> > I'm fine with using in_compat_syscall() when rseq_get_rseq_cs() is
> >> > invoked from a system call, but is it OK to call it when it is
> >> > invoked from signal delivery ? AFAIU, signals can be delivered
> >> > upon return from interrupt as well.
> >> >
> >> > If not, what strategy do you recommend for arch-agnostic code ?
> >>
> >> I think what we're missing here is a new "is_compat_frame(struct ksignal *ksig)"
> >> which I could use in the rseq code. I'll prepare a patch and we can discuss
> >> from there.
> >>
> >
> > That sounds about right.
> >
> > I'm confused, though.  Wouldn't it be more consistent to just segfault
> > if the high 32 bits are not clear when rseq transitions to a 32-bit
> > context?  If there's garbage in 64-bit mode, the program will crash.
> > Why should 32-bit mode be any different?
>
> Currently, if a 32-bit binary puts garbage in the high bits of
> start_ip, post_commit_offset, and abort_ip in
>
> include/uapi/linux/rseq.h:
>
> struct rseq_cs {
>         /* Version of this structure. */
>         __u32 version;
>         /* enum rseq_cs_flags */
>         __u32 flags;
>         LINUX_FIELD_u32_u64(start_ip);
>         /* Offset from start_ip. */
>         LINUX_FIELD_u32_u64(post_commit_offset);
>         LINUX_FIELD_u32_u64(abort_ip);
> } __attribute__((aligned(4 * sizeof(__u64))));

This ABI isn't real ABI until a stable kernel happens, right?  So how
about just making all those fields be u64?

>
> A 32-bit kernel just never reads the padding, thus in reality acting
> as if those were zeroes. However, a 64-bit kernel dealing with this
> 32-bit compat task will read that padding, handling those as very
> large values.

Sounds like a design error.  Have all kernels read the fields no
matter what.  A 32-bit kernel will send SIGSEGV if the high bits are
set.  A 64-bit kernel running compat userspace should make sure that a
32-bit task dies if the high bits are set.

>
> We need to improve that by introducing a consistent behavior across
> native 32-bit kernels and 32-bit compat mode on 64-bit kernels.
>
> There are two ways to achieve this: either the 32-bit kernel validates
> the padding by killing the process if padding is non-zero, or the
> 64-bit kernel treats compat mode by zeroing the high bits of padding.
>
> If we look at system call interfaces in general, I think the usual
> approach is to clear the top bits whenever a value read from a
> compat task ends up being used as a pointer. This is why I am tempted
> to go for the "clear high bits" approach rather than killing the task.

I think the modern preference is to use fields of fixed size rather
than long when UABI is involved.

In any event, I think the test you want is user_64bit_mode().

>
> Also, validating that the top 32-bit is zeroes from a native 32-bit
> kernel requires extra loads, whereas not caring about their content
> is free, which makes me slightly prefer an approach where 32-bit
> compat mode on 64-bit kernel just clears the top bits.
>

But performance is totally irrelvant here, right?  This only affects
the abort path, unless I'm rather confused.

--Andy