On Fri, 4 Mar 2022 at 17:42, Vlastimil Babka <vbabka@xxxxxxx> wrote: > > 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? No of course not, which is why I asked in the earlier email if it's a "definitive failure case". > 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. :-) >
