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>
Subject: [PATCH v2] lib/refcount: Document interaction with PID_MAX_LIMIT
Date: Tue, 3 Mar 2020 11:54:27 +0100 [thread overview]
Message-ID: <20200303105427.260620-1-jannh@google.com> (raw)
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
next reply other threads:[~2020-03-03 10:54 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-03 10:54 Jann Horn [this message]
2020-03-03 13:07 ` [PATCH v2] lib/refcount: Document interaction with PID_MAX_LIMIT Ard Biesheuvel
2020-03-03 13:57 ` Jann Horn
2020-03-03 21:09 ` Kees Cook
2020-03-17 22:27 ` Will Deacon
2020-03-24 15:29 ` Peter Zijlstra
2020-04-08 12:20 ` [tip: locking/urgent] locking/refcount: " tip-bot2 for Jann Horn
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