[v3,0/3] Actually fix freelist pointer vs redzoning

Message

Kees Cook Oct. 15, 2020, 3:37 a.m. UTC

v3:
- fix commit messages to properly reflect the direction of the overwrite
- justify the less-than-word-size patch better
- add Acks
- move some Fixes up into the commit log as just references
v2: https://lore.kernel.org/lkml/[e-mail]
v1: https://lore.kernel.org/lkml/[e-mail]

This fixes redzoning vs the freelist pointer (both for middle-position
and very small caches). Both are "theoretical" fixes, in that I see no
evidence of such small-sized caches actually be used in the kernel, but
that's no reason to let the bugs continue to exist. :)

Note on patch 2: Christopher NAKed it, but I actually think this is a
reasonable thing to add -- the "too small" check is only made when built
with CONFIG_DEBUG_VM, so it *is* actually possible for someone to trip
over this directly, even if it would never make it into a released
kernel. I see no reason to just leave this foot-gun in place, though, so
we might as well just fix it too. (Which seems to be what Longman was
similarly supporting, IIUC.)

Anyway, if patch 2 stays NAKed, that's fine. It's entirely separable,
and the other 2 can land. :)

Thanks!

-Kees


Kees Cook (3):
  mm/slub: Clarify verification reporting
  mm/slub: Fix redzoning for small allocations
  mm/slub: Actually fix freelist pointer vs redzoning

 Documentation/vm/slub.rst | 10 +++++-----
 mm/slub.c                 | 36 +++++++++++++++---------------------
 2 files changed, 20 insertions(+), 26 deletions(-)

Comments

Christoph Lameter (Ampere) Oct. 15, 2020, 8:23 a.m. UTC | #1

On Wed, 14 Oct 2020, Kees Cook wrote:

> Note on patch 2: Christopher NAKed it, but I actually think this is a
> reasonable thing to add -- the "too small" check is only made when built
> with CONFIG_DEBUG_VM, so it *is* actually possible for someone to trip
> over this directly, even if it would never make it into a released
> kernel. I see no reason to just leave this foot-gun in place, though, so
> we might as well just fix it too. (Which seems to be what Longman was
> similarly supporting, IIUC.)

Well then remove the duplication of checks. The NAK was there because it
seems that you were not aware of the existing checks.

> Anyway, if patch 2 stays NAKed, that's fine. It's entirely separable,
> and the other 2 can land. :)

Just deal with the old checks too and it will be fine.

Vlastimil Babka Oct. 15, 2020, 9:44 a.m. UTC | #2

On 10/15/20 10:23 AM, Christopher Lameter wrote:
> On Wed, 14 Oct 2020, Kees Cook wrote:
> 
>> Note on patch 2: Christopher NAKed it, but I actually think this is a
>> reasonable thing to add -- the "too small" check is only made when built
>> with CONFIG_DEBUG_VM, so it *is* actually possible for someone to trip
>> over this directly, even if it would never make it into a released
>> kernel. I see no reason to just leave this foot-gun in place, though, so
>> we might as well just fix it too. (Which seems to be what Longman was
>> similarly supporting, IIUC.)
> 
> Well then remove the duplication of checks. The NAK was there because it
> seems that you were not aware of the existing checks.
> 
>> Anyway, if patch 2 stays NAKed, that's fine. It's entirely separable,
>> and the other 2 can land. :)
> 
> Just deal with the old checks too and it will be fine.

Yeah, the existing check is under CONFIG_DEBUG_VM, which means it's not active 
on some configurations. Creating a cache is not exactly fast path operation, so 
I would remove this guard.
As for the minimum size check, I would probably remove it (but watch out if 
SLAB/SLOB can handle it). It's not effective to use slab cache for 4-byte 
objects, but why make it an error.

Kees Cook Oct. 15, 2020, 10:03 p.m. UTC | #3

On Thu, Oct 15, 2020 at 11:44:15AM +0200, Vlastimil Babka wrote:
> On 10/15/20 10:23 AM, Christopher Lameter wrote:
> > On Wed, 14 Oct 2020, Kees Cook wrote:
> > 
> > > Note on patch 2: Christopher NAKed it, but I actually think this is a
> > > reasonable thing to add -- the "too small" check is only made when built
> > > with CONFIG_DEBUG_VM, so it *is* actually possible for someone to trip
> > > over this directly, even if it would never make it into a released
> > > kernel. I see no reason to just leave this foot-gun in place, though, so
> > > we might as well just fix it too. (Which seems to be what Longman was
> > > similarly supporting, IIUC.)
> > 
> > Well then remove the duplication of checks. The NAK was there because it
> > seems that you were not aware of the existing checks.
> > 
> > > Anyway, if patch 2 stays NAKed, that's fine. It's entirely separable,
> > > and the other 2 can land. :)
> > 
> > Just deal with the old checks too and it will be fine.
> 
> Yeah, the existing check is under CONFIG_DEBUG_VM, which means it's not
> active on some configurations. Creating a cache is not exactly fast path
> operation, so I would remove this guard.
> As for the minimum size check, I would probably remove it (but watch out if
> SLAB/SLOB can handle it). It's not effective to use slab cache for 4-byte
> objects, but why make it an error.

Err, why did the check exist to begin with? If the check isn't wanted,
that's one thing, but I was just trying to fix what I saw in the redzone
handling. What is preferred here?

1) drop patch 2
2) keep patch 2, but also:
	a) validate slab/slob can handle < word-sized allocations
	b) remove check in kmem_cache_sanity_check

option 2a seems like it could be fragile if I miss something. I think
I'd rather just take option 1.