On Tue, Sep 8, 2015 at 7:09 PM, Christoph Lameter <cl@xxxxxxxxx> wrote: > On Tue, 8 Sep 2015, Dmitry Vyukov wrote: > >> >> I would expect that this is illegal code. Is my understanding correct? >> > >> > This should work. It could be a problem if thread 1 is touching >> > the object. >> >> What does make it work? > > The 2nd thread gets the pointer that the first allocated and frees it. > If there is no more processing then fine. > >> There are clearly memory barriers missing when passing the object >> between threads. The typical correct pattern is: > > Why? If thread 2 gets the pointer it frees it. Thats ok. > >> // thread 1 >> smp_store_release(&p, kmalloc(8)); >> >> // thread 2 >> void *r = smp_load_acquire(&p); // or READ_ONCE_CTRL >> if (r) >> kfree(r); >> >> Otherwise stores into the object in kmalloc can reach the object when >> it is already freed, which is a use-after-free. > > Ok so there is more code executing in thread #1. That changes things. >> >> What does prevent the use-after-free? > > There is no access to p in the first thread. If there are such accesses > then they are illegal. A user of slab allocators must ensure that there > are no accesses after freeing the object. And since there is a thread > that at random checks p and frees it when not NULL then no other thread > would be allowed to touch the object. But the memory allocator itself (kmalloc/kfree) generally reads and writes the object (e.g. storing object size in header before object, writing redzone in debug mode, reading and checking redzone in debug mode, building freelist using first word of the object, etc). There is no different between user accesses and memory allocator accesses just before returning the object from kmalloc and right after accepting the object in kfree. -- Dmitry Vyukov, Software Engineer, dvyukov@xxxxxxxxxx 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href