Messages in this thread | | | Subject | Re: [PATCH v4 12/12] mm: SLUB hardened usercopy support | From | Laura Abbott <> | Date | Mon, 25 Jul 2016 16:29:51 -0700 |
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On 07/25/2016 02:42 PM, Rik van Riel wrote: > On Mon, 2016-07-25 at 12:16 -0700, Laura Abbott wrote: >> On 07/20/2016 01:27 PM, Kees Cook wrote: >>> Under CONFIG_HARDENED_USERCOPY, this adds object size checking to >>> the >>> SLUB allocator to catch any copies that may span objects. Includes >>> a >>> redzone handling fix discovered by Michael Ellerman. >>> >>> Based on code from PaX and grsecurity. >>> >>> Signed-off-by: Kees Cook <keescook@chromium.org> >>> Tested-by: Michael Ellerman <mpe@ellerman.id.au> >>> --- >>> init/Kconfig | 1 + >>> mm/slub.c | 36 ++++++++++++++++++++++++++++++++++++ >>> 2 files changed, 37 insertions(+) >>> >>> diff --git a/init/Kconfig b/init/Kconfig >>> index 798c2020ee7c..1c4711819dfd 100644 >>> --- a/init/Kconfig >>> +++ b/init/Kconfig >>> @@ -1765,6 +1765,7 @@ config SLAB >>> >>> config SLUB >>> bool "SLUB (Unqueued Allocator)" >>> + select HAVE_HARDENED_USERCOPY_ALLOCATOR >>> help >>> SLUB is a slab allocator that minimizes cache line >>> usage >>> instead of managing queues of cached objects (SLAB >>> approach). >>> diff --git a/mm/slub.c b/mm/slub.c >>> index 825ff4505336..7dee3d9a5843 100644 >>> --- a/mm/slub.c >>> +++ b/mm/slub.c >>> @@ -3614,6 +3614,42 @@ void *__kmalloc_node(size_t size, gfp_t >>> flags, int node) >>> EXPORT_SYMBOL(__kmalloc_node); >>> #endif >>> >>> +#ifdef CONFIG_HARDENED_USERCOPY >>> +/* >>> + * Rejects objects that are incorrectly sized. >>> + * >>> + * Returns NULL if check passes, otherwise const char * to name of >>> cache >>> + * to indicate an error. >>> + */ >>> +const char *__check_heap_object(const void *ptr, unsigned long n, >>> + struct page *page) >>> +{ >>> + struct kmem_cache *s; >>> + unsigned long offset; >>> + size_t object_size; >>> + >>> + /* Find object and usable object size. */ >>> + s = page->slab_cache; >>> + object_size = slab_ksize(s); >>> + >>> + /* Find offset within object. */ >>> + offset = (ptr - page_address(page)) % s->size; >>> + >>> + /* Adjust for redzone and reject if within the redzone. */ >>> + if (kmem_cache_debug(s) && s->flags & SLAB_RED_ZONE) { >>> + if (offset < s->red_left_pad) >>> + return s->name; >>> + offset -= s->red_left_pad; >>> + } >>> + >>> + /* Allow address range falling entirely within object >>> size. */ >>> + if (offset <= object_size && n <= object_size - offset) >>> + return NULL; >>> + >>> + return s->name; >>> +} >>> +#endif /* CONFIG_HARDENED_USERCOPY */ >>> + >> >> I compared this against what check_valid_pointer does for SLUB_DEBUG >> checking. I was hoping we could utilize that function to avoid >> duplication but a) __check_heap_object needs to allow accesses >> anywhere >> in the object, not just the beginning b) accessing page->objects >> is racy without the addition of locking in SLUB_DEBUG. >> >> Still, the ptr < page_address(page) check from __check_heap_object >> would >> be good to add to avoid generating garbage large offsets and trying >> to >> infer C math. >> >> diff --git a/mm/slub.c b/mm/slub.c >> index 7dee3d9..5370e4f 100644 >> --- a/mm/slub.c >> +++ b/mm/slub.c >> @@ -3632,6 +3632,9 @@ const char *__check_heap_object(const void >> *ptr, unsigned long n, >> s = page->slab_cache; >> object_size = slab_ksize(s); >> >> + if (ptr < page_address(page)) >> + return s->name; >> + >> /* Find offset within object. */ >> offset = (ptr - page_address(page)) % s->size; >> > > I don't get it, isn't that already guaranteed because we > look for the page that ptr is in, before __check_heap_object > is called? > > Specifically, in patch 3/12: > > + page = virt_to_head_page(ptr); > + > + /* Check slab allocator for flags and size. */ > + if (PageSlab(page)) > + return __check_heap_object(ptr, n, page); > > How can that generate a ptr that is not inside the page? > > What am I overlooking? And, should it be in the changelog or > a comment? :) >
I ran into the subtraction issue when the vmalloc detection wasn't working on ARM64, somehow virt_to_head_page turned into a page that happened to have PageSlab set. I agree if everything is working properly this is redundant but given the type of feature this is, a little bit of redundancy against a system running off into the weeds or bad patches might be warranted.
I'm not super attached to the check if other maintainers think it is redundant. Updating the __check_heap_object header comment with a note of what we are assuming could work
Thanks, Laura
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