On 3/4/22 14:11, Marco Elver wrote: > On Fri, 4 Mar 2022 at 13:02, Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote: >> >> On Fri, Mar 04, 2022 at 12:50:21PM +0100, Marco Elver wrote: >> > On Fri, 4 Mar 2022 at 07:34, Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote: >> > > >> > > Changes from v1: >> > > Now SLAB passes requests larger than order-1 page >> > > to page allocator. >> > > >> > > Adjusted comments from Matthew, Vlastimil, Rientjes. >> > > Thank you for feedback! >> > > >> > > BTW, I have no idea what __ksize() should return when an object that >> > > is not allocated from slab is passed. both 0 and folio_size() >> > > seems wrong to me. >> > >> > Didn't we say 0 would be the safer of the two options? >> > https://lkml.kernel.org/r/0e02416f-ef43-dc8a-9e8e-50ff63dd3c61@xxxxxxx >> > >> >> Oh sorry, I didn't understand why 0 was safer when I was reading it. >> >> Reading again, 0 is safer because kasan does not unpoison for >> wrongly passed object, right? > > Not quite. KASAN can tell if something is wrong, i.e. invalid object. > Similarly, if you are able to tell if the passed pointer is not a > valid object some other way, you can do something better - namely, > return 0. Hmm, but how paranoid do we have to be? Patch 1 converts SLAB to use kmalloc_large(). So it's now legitimate to have objects allocated by SLAB's kmalloc() that don't have a slab folio flag set, and their size is folio_size(). It would be more common than getting a bogus pointer, so should we return 0 just because a bogus pointer is possible? If we do that, then KASAN will fail to unpoison legitimate kmalloc_large() objects, no? What I suggested earlier is we could make the checks more precise - if folio_size() is smaller or equal order-1 page, then it's bogus because we only do kmalloc_large() for >order-1. If the object pointer is not to the beginning of the folio, then it's bogus, because kmalloc_large() returns the beginning of the folio. Then in these case we return 0, but otherwise we should return folio_size()? > The intuition here is that the caller has a pointer to an > invalid object, and wants to use ksize() to determine its size, and > most likely access all those bytes. Arguably, at that point the kernel > is already in a degrading state. But we can try to not let things get > worse by having ksize() return 0, in the hopes that it will stop > corrupting more memory. It won't work in all cases, but should avoid > things like "s = ksize(obj); touch_all_bytes(obj, s)" where the size > bounds the memory accessed corrupting random memory. > > The other reason is that a caller could actually check the size, and > if 0, do something else. Few callers will do so, because nobody > expects that their code has a bug. :-)
