Re: KASAN vs realloc

[Kees Cook <kees@kernel.org>]

On Tue, Jan 06, 2026 at 01:42:45PM +0100, Maciej Żenczykowski wrote:
> We've got internal reports (b/467571011 - from CC'ed Samsung
> developer) that kasan realloc is broken for sizes that are not a
> multiple of the granule.  This appears to be triggered during Android
> bootup by some ebpf program loading operations (a struct is 88 bytes
> in size, which is a multiple of 8, but not 16, which is the granule
> size).
> 
> (this is on 6.18 with
> https://lore.kernel.org/all/38dece0a4074c43e48150d1e242f8242c73bf1a5.1764874575.git.m.wieczorretman@pm.me/
> already included)
> 
> joonki.min@samsung-slsi.corp-partner.google.com summarized it as
> "When newly requested size is not bigger than allocated size and old
> size was not 16 byte aligned, it failed to unpoison extended area."
> 
> and *very* rough comment:
> 
> Right. "size - old_size" is not guaranteed 16-byte alignment in this case.
> 
> I think we may unpoison 16-byte alignment size, but it allowed more
> than requested :(
> 
> I'm not sure that's right approach.
> 
> if (size <= alloced_size) {
> - kasan_unpoison_vmalloc(p + old_size, size - old_size,
> +               kasan_unpoison_vmalloc(p + old_size, round_up(size -
> old_size, KASAN_GRANULE_SIZE),
>       KASAN_VMALLOC_PROT_NORMAL |
>       KASAN_VMALLOC_VM_ALLOC |
>       KASAN_VMALLOC_KEEP_TAG);
> /*
> * No need to zero memory here, as unused memory will have
> * already been zeroed at initial allocation time or during
> * realloc shrink time.
> */
> - vm->requested_size = size;
> +               vm->requested_size = round_up(size, KASAN_GRANULE_SIZE);
> 
> my personal guess is that
> 
> But just above the code you quoted in mm/vmalloc.c I see:
>         if (size <= old_size) {
> ...
>                 kasan_poison_vmalloc(p + size, old_size - size);
> 
> is also likely wrong?? Considering:
> 
> mm/kasan/shadow.c
> 
> void __kasan_poison_vmalloc(const void *start, unsigned long size)
> {
>         if (!is_vmalloc_or_module_addr(start))
>                 return;
> 
>         size = round_up(size, KASAN_GRANULE_SIZE);
>         kasan_poison(start, size, KASAN_VMALLOC_INVALID, false);
> }
> 
> This doesn't look right - if start isn't a multiple of the granule.

I don't think we can ever have the start not be a granule multiple, can
we?

I'm not sure how any of this is supposed to be handled by KASAN, though.
It does seem like a round_up() is missing, though?

-Kees

-- 
Kees Cook