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authorLinus Torvalds <torvalds@linux-foundation.org>2020-04-12 19:47:10 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2020-04-12 19:47:10 +0300
commit652fa53caa01dbfdef1b8c5997767e284ac21a5c (patch)
treed4a0998c9501a6f525ac269169865613a123be16 /include
parent4119bf9f1d093b495f5fe3fcb32bde3156d2ba6e (diff)
parent9a019db0b6bebc84d6b64636faf73ed6d64cd4bb (diff)
downloadlinux-652fa53caa01dbfdef1b8c5997767e284ac21a5c.tar.xz
Merge tag 'locking-urgent-2020-04-12' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull locking fixes from Thomas Gleixner: "Three small fixes/updates for the locking core code: - Plug a task struct reference leak in the percpu rswem implementation. - Document the refcount interaction with PID_MAX_LIMIT - Improve the 'invalid wait context' data dump in lockdep so it contains all information which is required to decode the problem" * tag 'locking-urgent-2020-04-12' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: locking/lockdep: Improve 'invalid wait context' splat locking/refcount: Document interaction with PID_MAX_LIMIT locking/percpu-rwsem: Fix a task_struct refcount
Diffstat (limited to 'include')
-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
* ===============