On 7/15/19 6:06 AM, Walter Wu wrote: > On Fri, 2019-07-12 at 13:52 +0300, Andrey Ryabinin wrote: >> >> On 7/11/19 1:06 PM, Walter Wu wrote: >>> On Wed, 2019-07-10 at 21:24 +0300, Andrey Ryabinin wrote: >>>> >>>> On 7/9/19 5:53 AM, Walter Wu wrote: >>>>> On Mon, 2019-07-08 at 19:33 +0300, Andrey Ryabinin wrote: >>>>>> >>>>>> On 7/5/19 4:34 PM, Dmitry Vyukov wrote: >>>>>>> On Mon, Jul 1, 2019 at 11:56 AM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote: >>>> >>>>>>> >>>>>>> Sorry for delays. I am overwhelm by some urgent work. I afraid to >>>>>>> promise any dates because the next week I am on a conference, then >>>>>>> again a backlog and an intern starting... >>>>>>> >>>>>>> Andrey, do you still have concerns re this patch? This change allows >>>>>>> to print the free stack. >>>>>> >>>>>> I 'm not sure that quarantine is a best way to do that. Quarantine is made to delay freeing, but we don't that here. >>>>>> If we want to remember more free stacks wouldn't be easier simply to remember more stacks in object itself? >>>>>> Same for previously used tags for better use-after-free identification. >>>>>> >>>>> >>>>> Hi Andrey, >>>>> >>>>> We ever tried to use object itself to determine use-after-free >>>>> identification, but tag-based KASAN immediately released the pointer >>>>> after call kfree(), the original object will be used by another >>>>> pointer, if we use object itself to determine use-after-free issue, then >>>>> it has many false negative cases. so we create a lite quarantine(ring >>>>> buffers) to record recent free stacks in order to avoid those false >>>>> negative situations. >>>> >>>> I'm telling that *more* than one free stack and also tags per object can be stored. >>>> If object reused we would still have information about n-last usages of the object. >>>> It seems like much easier and more efficient solution than patch you proposing. >>>> >>> To make the object reused, we must ensure that no other pointers uses it >>> after kfree() release the pointer. >>> Scenario: >>> 1). The object reused information is valid when no another pointer uses >>> it. >>> 2). The object reused information is invalid when another pointer uses >>> it. >>> Do you mean that the object reused is scenario 1) ? >>> If yes, maybe we can change the calling quarantine_put() location. It >>> will be fully use that quarantine, but at scenario 2) it looks like to >>> need this patch. >>> If no, maybe i miss your meaning, would you tell me how to use invalid >>> object information? or? >>> >> >> >> KASAN keeps information about object with the object, right after payload in the kasan_alloc_meta struct. >> This information is always valid as long as slab page allocated. Currently it keeps only one last free stacktrace. >> It could be extended to record more free stacktraces and also record previously used tags which will allow you >> to identify use-after-free and extract right free stacktrace. > > Thanks for your explanation. > > For extend slub object, if one record is 9B (sizeof(u8)+ sizeof(struct > kasan_track)) and add five records into slub object, every slub object > may add 45B usage after the system runs longer. > Slub object number is easy more than 1,000,000(maybe it may be more > bigger), then the extending object memory usage should be 45MB, and > unfortunately it is no limit. The memory usage is more bigger than our > patch. No, it's not necessarily more. And there are other aspects to consider such as performance, how simple reliable the code is. > > We hope tag-based KASAN advantage is smaller memory usage. If it’s > possible, we should spend less memory in order to identify > use-after-free. Would you accept our patch after fine tune it? Sure, if you manage to fix issues and demonstrate that performance penalty of your patch is close to zero.
