linux-kernel - Re: [PATCH] kasan: improve double-free detection

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Date:	Thu, 5 May 2016 08:55:01 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	"Luruo, Kuthonuzo" <kuthonuzo.luruo@....com>
Cc:	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	Alexander Potapenko <glider@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	kasan-dev <kasan-dev@...glegroups.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kasan: improve double-free detection

On Thu, May 5, 2016 at 8:23 AM, Luruo, Kuthonuzo
<kuthonuzo.luruo@....com> wrote:
>> >> >> I missed that Alexander already landed patches that reduce header size
>> >> >> to 16 bytes.
>> >> >> It is not OK to increase them again. Please leave state as bitfield
>> >> >> and update it with CAS (if we introduce helper functions for state
>> >> >> manipulation, they will hide the CAS loop, which is nice).
>> >> >>
>> >> >
>> >> > Available CAS primitives/compiler do not support CAS with bitfield. I
>> propose
>> >> > to change kasan_alloc_meta to:
>> >> >
>> >> > struct kasan_alloc_meta {
>> >> >         struct kasan_track track;
>> >> >         u16 size_delta;         /* object_size - alloc size */
>> >> >         u8 state;                    /* enum kasan_state */
>> >> >         u8 reserved1;
>> >> >         u32 reserved2;
>> >> > }
>> >> >
>> >> > This shrinks _used_ meta object by 1 byte wrt the original. (btw, patch v1
>> does
>> >> > not increase overall alloc meta object size). "Alloc size", where needed, is
>> >> > easily calculated as a delta from cache->object_size.
>> >>
>> >>
>> >> What is the maximum size that slab can allocate?
>> >> I remember seeing slabs as large as 4MB some time ago (or did I
>> >> confuse it with something else?). If there are such large objects,
>> >> that 2 bytes won't be able to hold even delta.
>> >> However, now on my desktop I don't see slabs larger than 16KB in
>> >> /proc/slabinfo.
>> >
>> > max size for SLAB's slab is 32MB; default is 4MB. I must have gotten confused
>> by
>> > SLUB's 8KB limit. Anyway, new kasan_alloc_meta in patch V2:
>> >
>> > struct kasan_alloc_meta {
>> >         struct kasan_track track;
>> >         union {
>> >                 u8 lock;
>> >                 struct {
>> >                         u32 dummy : 8;
>> >                         u32 size_delta : 24;    /* object_size - alloc size */
>> >                 };
>> >         };
>> >         u32 state : 2;                          /* enum kasan_alloc_state */
>> >         u32 unused : 30;
>> > };
>> >
>> > This uses 2 more bits than current, but given the constraints I think this is
>> > close to optimal.
>>
>>
>> We plan to use the unused part for another depot_stack_handle_t (u32)
>> to memorize stack of the last call_rcu on the object (this will
>> greatly simplify debugging of use-after-free for objects freed by
>> rcu). So we need that unused part.
>>
>> I would would simply put all these fields into a single u32:
>>
>> struct kasan_alloc_meta {
>>         struct kasan_track track;
>>         u32 status;  // contains lock, state and size
>>         u32 unused;  // reserved for call_rcu stack handle
>> };
>>
>> And then separately a helper type to pack/unpack status:
>>
>> union kasan_alloc_status {
>>         u32 raw;
>>         struct {
>>                    u32 lock : 1;
>>                    u32 state : 2;
>>                    u32 unused : 5;
>>                    u32 size : 24;
>>         };
>> };
>>
>>
>> Then, when we need to read/update the header we do something like:
>>
>> kasan_alloc_status status, new_status;
>>
>> for (;;) {
>>     status.raw = READ_ONCE(header->status);
>>     // read status, form new_status, for example:
>>     if (status.lock)
>>           continue;
>>     new_status.raw = status.raw;
>>     new_status.lock = 1;
>>     if (cas(&header->status, status.raw, new_status.raw))
>>              break;
>> }
>>
>>
>> This will probably make state manipulation functions few lines longer,
>> but since there are like 3 such functions I don't afraid that. And we
>> still can use bitfield magic to extract fields and leave whole 5 bits
>> unused bits for future.
>
> The difficulty is that the lock managed by CAS needs 1 byte, mininum; TAS bit
> is even 'worse': address must be that of an unsigned long.

cmpxchg function can operate on bytes, words, double words and quad words:
http://lxr.free-electrons.com/source/arch/x86/include/asm/cmpxchg.h#L146


> Might it be possible for you to employ the 'kasan_free_meta' header for your
> RCU stack handle instead since KASAN does not currently store state for RCU
> slabs on free?

Free meta is overlapped with user object. The object is not freed yet
when call_rcu is invoked, so free meta cannot be used yet (user still
holds own data there). Free meta can only be used after kfree is
invoked on the object.