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Date:   Tue,  3 Mar 2020 11:54:27 +0100
Message-Id: <20200303105427.260620-1-jannh@google.com>
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Subject: [PATCH v2] lib/refcount: Document interaction with PID_MAX_LIMIT
From:   Jann Horn <jannh@google.com>
To:     Will Deacon <will@kernel.org>, Kees Cook <keescook@chromium.org>,
        Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     kernel list <linux-kernel@vger.kernel.org>,
        Elena Reshetova <elena.reshetova@intel.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Hanjun Guo <guohanjun@huawei.com>,
        Jan Glauber <jglauber@marvell.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>
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Sender: linux-kernel-owner@vger.kernel.org
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Document the circumstances under which refcount_t's saturation mechanism
works deterministically.

Signed-off-by: Jann Horn <jannh@google.com>
---

Notes:
    v2:
     - write down the math (Kees)

 include/linux/refcount.h | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/include/linux/refcount.h b/include/linux/refcount.h
index 0ac50cf62d062..0e3ee25eb156a 100644
--- a/include/linux/refcount.h
+++ b/include/linux/refcount.h
@@ -38,11 +38,24 @@
  * atomic operations, then the count will continue to edge closer to 0. If it
  * reaches a value of 1 before /any/ of the threads reset it to the saturated
  * value, then a concurrent refcount_dec_and_test() may erroneously free the
- * underlying object. Given the precise timing details involved with the
- * round-robin scheduling of each thread manipulating the refcount and the need
- * to hit the race multiple times in succession, there doesn't appear to be a
- * practical avenue of attack even if using refcount_add() operations with
- * larger increments.
+ * underlying object.
+ * Linux limits the maximum number of tasks to PID_MAX_LIMIT, which is currently
+ * 0x400000 (and can't easily be raised in the future beyond FUTEX_TID_MASK).
+ * With the current PID limit, if no batched refcounting operations are used and
+ * the attacker can't repeatedly trigger kernel oopses in the middle of refcount
+ * operations, this makes it impossible for a saturated refcount to leave the
+ * saturation range, even if it is possible for multiple uses of the same
+ * refcount to nest in the context of a single task:
+ *
+ *     (UINT_MAX+1-REFCOUNT_SATURATED) / PID_MAX_LIMIT =
+ *     0x40000000 / 0x400000 = 0x100 = 256
+ *
+ * If hundreds of references are added/removed with a single refcounting
+ * operation, it may potentially be possible to leave the saturation range; but
+ * given the precise timing details involved with the round-robin scheduling of
+ * each thread manipulating the refcount and the need to hit the race multiple
+ * times in succession, there doesn't appear to be a practical avenue of attack
+ * even if using refcount_add() operations with larger increments.
  *
  * Memory ordering
  * ===============

base-commit: 98d54f81e36ba3bf92172791eba5ca5bd813989b
-- 
2.25.0.265.gbab2e86ba0-goog