Messages in this thread | | | From | Dmitry Vyukov <> | Date | Thu, 17 Sep 2015 19:57:15 +0200 | Subject | Re: [PATCH] kernel: fix data race in put_pid |
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I can update the patch description, but let me explain it here first.
Here is the essence of what happens:
// thread 1 1: pid->foo = 1; // foo is the first word of pid object // then it does put_pid 2: atomic_dec_and_test(&pid->count) // decrements count to 1 and returns false so the function returns
// thread 2 // executes put_pid 3: atomic_load(&pid->count); // returns 1, so proceed to kmem_cache_free // then kmem_cache_free does: 5: head->freelist = (void*)pid;
This can be executed as:
4: *(void**)pid = head->freelist; 1: pid->foo = 1; // foo is the first word of pid object 2: atomic_dec_and_test(&pid->count) // decrements count to 1 and returns false so the function returns 3: atomic_load(&pid->count); // returns 1, so proceed to kmem_cache_free 5: head->freelist = (void*)pid;
And we get corrupted allocator freelist.
On Thu, Sep 17, 2015 at 7:44 PM, Oleg Nesterov <oleg@redhat.com> wrote: > On 09/17, Dmitry Vyukov wrote: >> >> What happens here exactly matches what is described in CONTROL >> DEPENDENCIES section of Documentation/memory-barriers.txt. So all the >> bad things described there are possible here. > > And I still can't understand how these bad things connect to put_pid(). > Probably I should re-read memory-barriers.txt, it changes quite often. > >> I don't >> know what to add to that. > > OK, let me quote the parts of your changelog, > > For example, if store to the first word of the object to build a freelist > in kmem_cache_free() hoists above the check, stores to the first word > in other threads can corrupt the memory allocator freelist. > > I simply can't parse this. Yes, this is probably because of my bad > English, but I'll appreciate it if you can explain at least, say, > "stores to the first word in other threads". > > Did you mean that a freed pid can be reallocated by another thread, > then overwritten, and this all can happen before atomic_read(count)? > > > Hmm. or perhaps you meant that the "last" put_pid() which observes > atomic_read() == 1 can race with another thread which writes to this > pid and does put_pid()? This is another story, and if you meant this > the changelog could clearly explain your concerns. > > Or what? > > > So let me repeat. Since I can't understand you, I leave this to other > reviewers. But imho the changelog should be updated in any case. > > Oleg. >
-- Dmitry Vyukov, Software Engineer, dvyukov@google.com Google Germany GmbH, Dienerstraße 12, 80331, München Geschäftsführer: Graham Law, Christine Elizabeth Flores Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg Diese E-Mail ist vertraulich. Wenn Sie nicht der richtige Adressat sind, leiten Sie diese bitte nicht weiter, informieren Sie den Absender und löschen Sie die E-Mail und alle Anhänge. Vielen Dank. This e-mail is confidential. If you are not the right addressee please do not forward it, please inform the sender, and please erase this e-mail including any attachments. Thanks.
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