lkml.org 
[lkml]   [2019]   [Jul]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Date
    SubjectRe: [5.2 REGRESSION] Generic vDSO breaks seccomp-enabled userspace on i386
    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

    \
     
     \ /
      Last update: 2019-07-22 21:18    [W:4.820 / U:0.856 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site