The following commit has been merged into the locking/urgent branch of tip: Commit-ID: a13f58a0cafa7b0416a2898bc3b0defbb305d108 Gitweb: https://git.kernel.org/tip/a13f58a0cafa7b0416a2898bc3b0defbb305d108 Author: Jann Horn <jannh@xxxxxxxxxx> AuthorDate: Tue, 03 Mar 2020 11:54:27 +01:00 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitterDate: Wed, 08 Apr 2020 12:05:07 +02:00 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@xxxxxxxxxxxx> Acked-by: Will Deacon <will@xxxxxxxxxx> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Link: https://lkml.kernel.org/r/20200303105427.260620-1-jannh@xxxxxxxxxx --- 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 0ac50cf..0e3ee25 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 * ===============
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