Re: [PATCH v7 0/3] mm/slub: extend redzone check for kmalloc objects

[Feng Tang]

On Fri, Nov 11, 2022 at 04:16:32PM +0800, Vlastimil Babka wrote:
> On 10/21/22 05:24, Feng Tang wrote:
> > kmalloc's API family is critical for mm, and one of its nature is that
> > it will round up the request size to a fixed one (mostly power of 2).
> > When user requests memory for '2^n + 1' bytes, actually 2^(n+1) bytes
> > could be allocated, so there is an extra space than what is originally
> > requested.
> > 
> > This patchset tries to extend the redzone sanity check to the extra
> > kmalloced buffer than requested, to better detect un-legitimate access
> > to it. (dependson SLAB_STORE_USER & SLAB_RED_ZONE)
> > 
> > The redzone part has been tested with code below:
> > 
> > 	for (shift = 3; shift <= 12; shift++) {
> > 		size = 1 << shift;
> > 		buf = kmalloc(size + 4, GFP_KERNEL);
> > 		/* We have 96, 196 kmalloc size, which is not power of 2 */
> > 		if (size == 64 || size == 128)
> > 			oob_size = 16;
> > 		else
> > 			oob_size = size - 4;
> > 		memset(buf + size + 4, 0xee, oob_size);
> > 		kfree(buf);
> > 	}
> 
> Sounds like a new slub_kunit test would be useful? :) doesn't need to be
> that exhaustive wrt all sizes, we could just pick one and check that a write
> beyond requested kmalloc size is detected?

Just git-grepped out slub_kunit.c :), will try to add a case to it.
I'll also check if the case will also be caught by other sanitizer
tools like kasan/kfence etc.

Thanks,
Feng


> Thanks!
>