On Sun, May 22, 2022 at 12:20:26AM +0200, Andrey Konovalov wrote: > On Fri, May 20, 2022 at 3:01 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > > This will reset the tags for all kinds of GFP_USER allocations, not > > > only for the ones intended for MAP_ANONYMOUS and RAM-based file > > > mappings, for which userspace can set tags, right? This will thus > > > weaken in-kernel MTE for pages whose tags can't even be set by > > > userspace. Is there a way to deal with this? > > > > That's correct, it will weaken some of the allocations where the user > > doesn't care about MTE. > > Well, while this is unfortunate, I don't mind the change. > > I've left some comments on the patches. Thanks. I'll update and post at -rc1. > > > > Since clearing the flags in the arch code doesn't work, try to do this > > > > at page allocation time by a new flag added to GFP_USER. > > Does this have to be GFP_USER? Can we add new flags to > GFP_HIGHUSER_MOVABLE instead? > > For instance, Peter added __GFP_SKIP_KASAN_POISON to > GFP_HIGHUSER_MOVABLE in c275c5c6d50a0. The above commit was a performance improvement. Here we need to address the correctness. However, looking through the GFP_USER cases, I don't think any of them is at risk of ending up in user space with PROT_MTE. There are places where GFP_USER is passed to kmalloc() for in-kernel objects that would never be mapped to user, though the new gfp flag won't be taken into account. I'm ok to move the new flag to the GFP_HIGHUSER_MOVABLE but probably still keep a page_kasan_tag_reset() on the set_pte_at() path together with a WARN_ON_ONCE() if we miss anything. > > > > Could we > > > > instead add __GFP_SKIP_KASAN_UNPOISON rather than a new flag? > > Adding __GFP_SKIP_KASAN_UNPOISON makes sense, but we still need to > reset the tag in page->flags. My thought was to reset the tag in page->flags based on 'unpoison' alone without any extra flags. We use this flag for vmalloc() pages but it seems we don't reset the page tags (as we do via kasan_poison_slab()). -- Catalin
