lkml.org 
[lkml]   [2019]   [Feb]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v2] x86/asm: Pin sensitive CR4 bits
    On Wed, Feb 20, 2019 at 01:20:58PM -0800, Kees Cook wrote:
    > On Wed, Feb 20, 2019 at 10:49 AM Solar Designer <solar@openwall.com> wrote:
    > >
    > > On Wed, Feb 20, 2019 at 10:09:34AM -0800, Kees Cook wrote:
    > > > + if (WARN_ONCE((val & cr4_pin) != cr4_pin, "cr4 bypass attempt?!\n"))
    > > > + goto again;
    > >
    > > I think "goto again" is too mild a response given that it occurs after a
    > > successful write of a non-pinned value to CR4. I think it'd allow some
    > > exploits to eventually win the race: make their desired use of whatever
    > > functionality SMEP, etc. would have prevented - which may be just a few
    > > instructions they need to run - before the CR4 value is reverted after
    > > "goto again". I think it's one of those cases where a kernel panic
    > > would be more appropriate.
    >
    > It will not land upstream with a BUG() or panic(). Linus has
    > explicitly stated that none of this work can do that until it has
    > "baked" in the kernel for a couple years.

    OK.

    > In his defense, anyone sufficiently paranoid can already raise the
    > priority of a WARN() into a panic via sysctl kernel.panic_on_warn (and
    > kernel.panic_on_oops).

    I think there are too many uses of WARN() for anyone sane to enable
    that in production, whereas it'd have made sense to enable it for the
    few security-related uses.

    > > Also, WARN_ONCE possibly introduces a delay sufficient to realistically
    > > win this race on the first try. If we choose to warn, we should do it
    > > after having reverted the CR4 value, not before.
    >
    > Isn't cr4 CPU-local though?

    Good point. I don't know. If CR4 is per hardware thread, then the race
    would require an interrupt and would be much harder to win.

    > Couldn't we turn off interrupts to stop the race?

    This won't help. An attack would skip the code that disables interrupts
    and land right on the MOV instruction.

    Alexander

    \
     
     \ /
      Last update: 2019-02-21 14:07    [W:2.604 / U:0.004 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site