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    locking/refcounts, x86/asm: Implement fast refcount overflow protection · 7a46ec0e
    Kees Cook authored
    This implements refcount_t overflow protection on x86 without a noticeable
    performance impact, though without the fuller checking of REFCOUNT_FULL.
    
    This is done by duplicating the existing atomic_t refcount implementation
    but with normally a single instruction added to detect if the refcount
    has gone negative (e.g. wrapped past INT_MAX or below zero). When detected,
    the handler saturates the refcount_t to INT_MIN / 2. With this overflow
    protection, the erroneous reference release that would follow a wrap back
    to zero is blocked from happening, avoiding the class of refcount-overflow
    use-after-free vulnerabilities entirely.
    
    Only the overflow case of refcounting can be perfectly protected, since
    it can be detected and stopped before the reference is freed and left to
    be abused by an attacker. There isn't a way to block early decrements,
    and while REFCOUNT_FULL stops increment-from-zero cases (which would
    be the state _after_ an early decrement and stops potential double-free
    conditions), this fast implementation does not, since it would require
    the more expensive cmpxchg loops. Since the overflow case is much more
    common (e.g. missing a "put" during an error path), this protection
    provides real-world protection. For example, the two public refcount
    overflow use-after-free exploits published in 2016 would have been
    rendered unexploitable:
    
      http://perception-point.io/2016/01/14/analysis-and-exploitation-of-a-linux-kernel-vulnerability-cve-2016-0728/
    
      http://cyseclabs.com/page?n=02012016
    
    
    
    This implementation does, however, notice an unchecked decrement to zero
    (i.e. caller used refcount_dec() instead of refcount_dec_and_test() and it
    resulted in a zero). Decrements under zero are noticed (since they will
    have resulted in a negative value), though this only indicates that a
    use-after-free may have already happened. Such notifications are likely
    avoidable by an attacker that has already exploited a use-after-free
    vulnerability, but it's better to have them reported than allow such
    conditions to remain universally silent.
    
    On first overflow detection, the refcount value is reset to INT_MIN / 2
    (which serves as a saturation value) and a report and stack trace are
    produced. When operations detect only negative value results (such as
    changing an already saturated value), saturation still happens but no
    notification is performed (since the value was already saturated).
    
    On the matter of races, since the entire range beyond INT_MAX but before
    0 is negative, every operation at INT_MIN / 2 will trap, leaving no
    overflow-only race condition.
    
    As for performance, this implementation adds a single "js" instruction
    to the regular execution flow of a copy of the standard atomic_t refcount
    operations. (The non-"and_test" refcount_dec() function, which is uncommon
    in regular refcount design patterns, has an additional "jz" instruction
    to detect reaching exactly zero.) Since this is a forward jump, it is by
    default the non-predicted path, which will be reinforced by dynamic branch
    prediction. The result is this protection having virtually no measurable
    change in performance over standard atomic_t operations. The error path,
    located in .text.unlikely, saves the refcount location and then uses UD0
    to fire a refcount exception handler, which resets the refcount, handles
    reporting, and returns to regular execution. This keeps the changes to
    .text size minimal, avoiding return jumps and open-coded calls to the
    error reporting routine.
    
    Example assembly comparison:
    
    refcount_inc() before:
    
      .text:
      ffffffff81546149:       f0 ff 45 f4             lock incl -0xc(%rbp)
    
    refcount_inc() after:
    
      .text:
      ffffffff81546149:       f0 ff 45 f4             lock incl -0xc(%rbp)
      ffffffff8154614d:       0f 88 80 d5 17 00       js     ffffffff816c36d3
      ...
      .text.unlikely:
      ffffffff816c36d3:       48 8d 4d f4             lea    -0xc(%rbp),%rcx
      ffffffff816c36d7:       0f ff                   (bad)
    
    These are the cycle counts comparing a loop of refcount_inc() from 1
    to INT_MAX and back down to 0 (via refcount_dec_and_test()), between
    unprotected refcount_t (atomic_t), fully protected REFCOUNT_FULL
    (refcount_t-full), and this overflow-protected refcount (refcount_t-fast):
    
      2147483646 refcount_inc()s and 2147483647 refcount_dec_and_test()s:
    		    cycles		protections
      atomic_t           82249267387	none
      refcount_t-fast    82211446892	overflow, untested dec-to-zero
      refcount_t-full   144814735193	overflow, untested dec-to-zero, inc-from-zero
    
    This code is a modified version of the x86 PAX_REFCOUNT atomic_t
    overflow defense from the last public patch of PaX/grsecurity, based
    on my understanding of the code. Changes or omissions from the original
    code are mine and don't reflect the original grsecurity/PaX code. Thanks
    to PaX Team for various suggestions for improvement for repurposing this
    code to be a refcount-only protection.
    
    Signed-off-by: default avatarKees Cook <keescook@chromium.org>
    Reviewed-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
    Cc: Alexey Dobriyan <adobriyan@gmail.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Arnd Bergmann <arnd@arndb.de>
    Cc: Christoph Hellwig <hch@infradead.org>
    Cc: David S. Miller <davem@davemloft.net>
    Cc: Davidlohr Bueso <dave@stgolabs.net>
    Cc: Elena Reshetova <elena.reshetova@intel.com>
    Cc: Eric Biggers <ebiggers3@gmail.com>
    Cc: Eric W. Biederman <ebiederm@xmission.com>
    Cc: Greg KH <gregkh@linuxfoundation.org>
    Cc: Hans Liljestrand <ishkamiel@gmail.com>
    Cc: James Bottomley <James.Bottomley@hansenpartnership.com>
    Cc: Jann Horn <jannh@google.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Manfred Spraul <manfred@colorfullife.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Serge E. Hallyn <serge@hallyn.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: arozansk@redhat.com
    Cc: axboe@kernel.dk
    Cc: kernel-hardening@lists.openwall.com
    Cc: linux-arch <linux-arch@vger.kernel.org>
    Link: http://lkml.kernel.org/r/20170815161924.GA133115@beast
    
    
    Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
    7a46ec0e