On Sun, Oct 30, 2016 at 09:29:10AM -0700, Andy Lutomirski wrote: > On Sun, Oct 30, 2016 at 8:44 AM, Jann Horn <j...@thejh.net> wrote: > > free_pardevice() is called by parport_unregister_device() and already frees > > pp->pdev->name, don't try to do it again. > > > > This bug causes kernel crashes. > > > > I found and verified this with KASAN and some added pr_emerg()s: > > > > [ 60.316568] pp_release: pp->pdev->name == ffff88039cb264c0 > > [ 60.316692] free_pardevice: freeing par_dev->name at ffff88039cb264c0 > > [ 60.316706] pp_release: kfree(ffff88039cb264c0) > > [ 60.316714] ========================================================== > > [ 60.316722] BUG: Double free or freeing an invalid pointer > > [ 60.316731] Unexpected shadow byte: 0xFB > > [ 60.316801] Object at ffff88039cb264c0, in cache kmalloc-32 size: 32 > > [ 60.316813] Allocated: > > [ 60.316824] PID = 1695 > > [ 60.316869] Freed: > > [ 60.316880] PID = 1695 > > [ 60.316935] ========================================================== > > > > CCing Andy Lutomirski because I think this is what broke vmapped stacks > > for me - after applying this patch, vmapped stacks worked for me. > > Previously, I got oopses (and lockups) caused by area->pages[0] being > > 0x400000000 in __vunmap(), with area->pages being allocated in the kmalloc > > area. > > That's an odd symptom. I assume that what's happening is that the > pages array is being freed early by the extra kfree in here and then > getting corrupted.
Well, as far as I can tell, there are two ways to reach that. Obvious first way, but a pretty tight race: Task A: free(name) Task B: allocate area->pages in same place Task A: second free(name), releasing area->pages Second way (if the SLAB allocator, which I'm using, is used): Task A: free(name), appends the object to ac->entry in ___cache_free() Task A: second free(name), appends the object to ac->entry again Task B: ____cache_alloc() returns object from array cache Task C: ____cache_alloc() returns same object again So then the same memory would be used by two separate objects?
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