Re: [PATCH v9] selinux: sidtab: reverse lookup hash table

[Paul Moore <paul@paul-moore.com>]

On Mon, Dec 2, 2019 at 7:32 PM Paul Moore <paul@paul-moore.com> wrote:
> On Fri, Nov 22, 2019 at 4:33 AM Jeff Vander Stoep <jeffv@google.com> wrote:
> > This replaces the reverse table lookup and reverse cache with a
> > hashtable which improves cache-miss reverse-lookup times from
> > O(n) to O(1)* and maintains the same performance as a reverse
> > cache hit.
> >
> > This reduces the time needed to add a new sidtab entry from ~500us
> > to 5us on a Pixel 3 when there are ~10,000 sidtab entries.
> >
> > The implementation uses the kernel's generic hashtable API,
> > It uses the context's string represtation as the hash source,
> > and the kernels generic string hashing algorithm full_name_hash()
> > to reduce the string to a 32 bit value.
> >
> > This change also maintains the improvement introduced in
> > commit ee1a84fdfeed ("selinux: overhaul sidtab to fix bug and improve
> > performance") which removed the need to keep the current sidtab
> > locked during policy reload. It does however introduce periodic
> > locking of the target sidtab while converting the hashtable. Sidtab
> > entries are never modified or removed, so the context struct stored
> > in the sid_to_context tree can also be used for the context_to_sid
> > hashtable to reduce memory usage.
> >
> > This bug was reported by:
> > - On the selinux bug tracker.
> >   BUG: kernel softlockup due to too many SIDs/contexts #37
> >   https://github.com/SELinuxProject/selinux-kernel/issues/37
> > - Jovana Knezevic on Android's bugtracker.
> >   Bug: 140252993
> >   "During multi-user performance testing, we create and remove users
> >   many times. selinux_android_restorecon_pkgdir goes from 1ms to over
> >   20ms after about 200 user creations and removals. Accumulated over
> >   ~280 packages, that adds a significant time to user creation,
> >   making perf benchmarks unreliable."
> >
> > * Hashtable lookup is only O(1) when n < the number of buckets.
> >
> > Changes in V2:
> > -The hashtable uses sidtab_entry_leaf objects directly so these
> > objects are shared between the sid_to_context lookup tree and the
> > context_to_sid hashtable. This simplifies memory allocation and
> > was suggested by Ondrej Mosnacek.
> > -The new sidtab hash stats file in selinuxfs has been moved out of
> > the avc dir and into a new "ss" dir.
> >
> > V3:
> > -Add lock nesting notation.
> >
> > V4/V5:
> > -Moved to *_rcu variants of the various hashtable functions
> > as suggested by Will Deacon.
> > -Naming/spelling fixups.
> >
> > V6
> > -Remove nested locking. Use lock of active sidtab to gate
> > access to the new sidtab.
> > -Remove use of rcu_head/kfree_rcu(), they're unnecessary because
> > hashtable objects are never removed when read/add operations are
> > occurring. Why is this safe? Quoting Ondrej Mosnacek from the
> > selinux mailing list:
> > "It is not visible in this patch, but the sidtab (along with other
> > policy-lifetime structures) is protected by a big fat read-write lock.
> > The only places where sidtab_destroy() is called are (a) error paths
> > when initializing a new sidtab (here the sidtab isn't shared yet, so
> > no race) and (b) when freeing the old sidtab during policy reload - in
> > this case it is happening after a policy write-locked critical
> > section, which had removed the old sidtab pointer from the shared
> > structures, so at that point all sidtab readers will already be
> > accessing the new sidtab and the old one is visible only by the thread
> > doing the destruction."
> >
> > V7
> > -Change format of /sys/fs/selinux/ss/sidtab_hash_stats to match
> > /sys/fs/selinux/avc/hash_stats.
> > -Add __rcu annotation to rcu pointers.
> > -Test with CONFIG_SPARSE_RCU_POINTER and CONFIG_PROVE_RCU.
> > -Add rcu@vger.kernel.org and Paul McKenney to Cc for review of the
> > RCU logic.
> >
> > V8
> > -Removed the __rcu annotation used in V7. The annotation is
> > intended to be applied to pointers to an object, however the
> > objects referenced in the rcu hashtable are allocated in an
> > array.
> > -Fixed bug where multiple SIDs were receiving the same hash
> > due to security_get_user_sids() reusing the same context
> > struct without calling context_init() on it. This bug was
> > discovered and root-caused by Stephen Smalley.
> >
> > V9
> > -Do not compute the hash in string_to_context_struct
> > because this string representation is non-canonical.
> >
> > Signed-off-by: Jeff Vander Stoep <jeffv@google.com>
> > Reported-by: Stephen Smalley <sds@tycho.nsa.gov>
> > Reported-by: Jovana Knezevic <jovanak@google.com>
> > ---
> >  security/selinux/Kconfig            |  12 ++
> >  security/selinux/include/security.h |   1 +
> >  security/selinux/selinuxfs.c        |  65 +++++++
> >  security/selinux/ss/context.h       |  11 +-
> >  security/selinux/ss/policydb.c      |   5 +
> >  security/selinux/ss/services.c      |  96 +++++++---
> >  security/selinux/ss/services.h      |   4 +-
> >  security/selinux/ss/sidtab.c        | 263 ++++++++++++++--------------
> >  security/selinux/ss/sidtab.h        |  16 +-
> >  9 files changed, 306 insertions(+), 167 deletions(-)
>
> Thanks Jeff, as well as everyone else who contributed reviews and feedback.
>
> I've pulled this into a working branch and I'll be merging it with the
> other sidtab patches before posting it to a "next-queue" branch for
> review later this week.  When done, I'll send a note to the list, as
> well as the relevant patch authors; your help in reviewing the merge
> would be greatly appreciated.

FYI, this is now in selinux/next proper.  Thanks again everyone!

-- 
paul moore
www.paul-moore.com