Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 22 Jul 2019 12:17:16 -0700 | Subject | Re: [5.2 REGRESSION] Generic vDSO breaks seccomp-enabled userspace on i386 |
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On Mon, Jul 22, 2019 at 11:39 AM Kees Cook <keescook@chromium.org> wrote: > > On Mon, Jul 22, 2019 at 08:31:32PM +0200, Thomas Gleixner wrote: > > On Mon, 22 Jul 2019, Kees Cook wrote: > > > Just so I'm understanding: the vDSO change introduced code to make an > > > actual syscall on i386, which for most seccomp filters would be rejected? > > > > No. The old x86 specific VDSO implementation had a fallback syscall as > > well, i.e. clock_gettime(). On 32bit clock_gettime() uses the y2038 > > endangered timespec. > > > > So when the VDSO was made generic we changed the internal data structures > > to be 2038 safe right away. As a consequence the fallback syscall is not > > clock_gettime(), it's clock_gettime64(). which seems to surprise seccomp. > > Okay, it's didn't add a syscall, it just changed it. Results are the > same: conservative filters suddenly start breaking due to the different > call. (And now I see why Andy's alias suggestion would help...) > > I'm not sure which direction to do with this. It seems like an alias > list is a large hammer for this case, and a "seccomp-bypass when calling > from vDSO" solution seems too fragile? >
I don't like the seccomp bypass at all. If someone uses seccomp to disallow all clock_gettime() variants, there shouldn't be a back door to learn the time.
Here's the restart_syscall() logic that makes me want aliases: we have different syscall numbers for restart_syscall() on 32-bit and 64-bit. The logic to decide which one to use is dubious at best. I'd like to introduce a restart_syscall2() that is identical to restart_syscall() except that it has the same number on both variants.
--Andy
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