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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: Kees Cook <keescook@chromium.org> Reviewed-by: Josh 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: Ingo Molnar <mingo@kernel.org>
102 lines
2.6 KiB
C
102 lines
2.6 KiB
C
#ifndef _LINUX_REFCOUNT_H
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#define _LINUX_REFCOUNT_H
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#include <linux/atomic.h>
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#include <linux/mutex.h>
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#include <linux/spinlock.h>
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#include <linux/kernel.h>
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/**
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* refcount_t - variant of atomic_t specialized for reference counts
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* @refs: atomic_t counter field
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*
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* The counter saturates at UINT_MAX and will not move once
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* there. This avoids wrapping the counter and causing 'spurious'
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* use-after-free bugs.
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*/
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typedef struct refcount_struct {
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atomic_t refs;
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} refcount_t;
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#define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), }
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/**
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* refcount_set - set a refcount's value
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* @r: the refcount
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* @n: value to which the refcount will be set
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*/
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static inline void refcount_set(refcount_t *r, unsigned int n)
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{
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atomic_set(&r->refs, n);
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}
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/**
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* refcount_read - get a refcount's value
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* @r: the refcount
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*
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* Return: the refcount's value
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*/
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static inline unsigned int refcount_read(const refcount_t *r)
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{
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return atomic_read(&r->refs);
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}
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#ifdef CONFIG_REFCOUNT_FULL
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extern __must_check bool refcount_add_not_zero(unsigned int i, refcount_t *r);
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extern void refcount_add(unsigned int i, refcount_t *r);
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extern __must_check bool refcount_inc_not_zero(refcount_t *r);
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extern void refcount_inc(refcount_t *r);
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extern __must_check bool refcount_sub_and_test(unsigned int i, refcount_t *r);
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extern __must_check bool refcount_dec_and_test(refcount_t *r);
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extern void refcount_dec(refcount_t *r);
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#else
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# ifdef CONFIG_ARCH_HAS_REFCOUNT
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# include <asm/refcount.h>
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# else
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static inline __must_check bool refcount_add_not_zero(unsigned int i, refcount_t *r)
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{
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return atomic_add_unless(&r->refs, i, 0);
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}
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static inline void refcount_add(unsigned int i, refcount_t *r)
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{
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atomic_add(i, &r->refs);
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}
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static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
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{
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return atomic_add_unless(&r->refs, 1, 0);
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}
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static inline void refcount_inc(refcount_t *r)
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{
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atomic_inc(&r->refs);
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}
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static inline __must_check bool refcount_sub_and_test(unsigned int i, refcount_t *r)
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{
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return atomic_sub_and_test(i, &r->refs);
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}
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static inline __must_check bool refcount_dec_and_test(refcount_t *r)
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{
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return atomic_dec_and_test(&r->refs);
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}
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static inline void refcount_dec(refcount_t *r)
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{
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atomic_dec(&r->refs);
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}
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# endif /* !CONFIG_ARCH_HAS_REFCOUNT */
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#endif /* CONFIG_REFCOUNT_FULL */
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extern __must_check bool refcount_dec_if_one(refcount_t *r);
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extern __must_check bool refcount_dec_not_one(refcount_t *r);
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extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock);
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extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock);
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#endif /* _LINUX_REFCOUNT_H */
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