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authorJann Horn <jannh@google.com>2020-03-03 11:54:27 +0100
committerIngo Molnar <mingo@kernel.org>2020-04-08 12:05:07 +0200
commita13f58a0cafa7b0416a2898bc3b0defbb305d108 (patch)
tree9833ebfb6c8dc617e4782d023c0b6355acb3d374
parentd22cc7f67d55ebf2d5be865453971c783e9fb21a (diff)
locking/refcount: Document interaction with PID_MAX_LIMIT
Document the circumstances under which refcount_t's saturation mechanism works deterministically. Acked-by: Kees Cook <keescook@chromium.org> Acked-by: Will Deacon <will@kernel.org> Signed-off-by: Jann Horn <jannh@google.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Ingo Molnar <mingo@kernel.org> Link: https://lkml.kernel.org/r/20200303105427.260620-1-jannh@google.com
-rw-r--r--include/linux/refcount.h23
1 files changed, 18 insertions, 5 deletions
diff --git a/include/linux/refcount.h b/include/linux/refcount.h
index 0ac50cf62d06..0e3ee25eb156 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
* ===============