diff options
| author | Jann Horn <jannh@google.com> | 2020-03-03 11:54:27 +0100 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2020-04-08 12:05:07 +0200 |
| commit | a13f58a0cafa7b0416a2898bc3b0defbb305d108 (patch) | |
| tree | 9833ebfb6c8dc617e4782d023c0b6355acb3d374 /include/linux | |
| parent | d22cc7f67d55ebf2d5be865453971c783e9fb21a (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
Diffstat (limited to 'include/linux')
| -rw-r--r-- | include/linux/refcount.h | 23 |
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 * =============== |