On Thu, Apr 24, 2025 at 11:11:47AM +0200, Danilo Krummrich wrote:
> On Wed, Apr 23, 2025 at 07:31:23PM -0700, Kees Cook wrote:
> > Introduce struct vm_struct::requested_size so that the requested
> > (re)allocation size is retained separately from the allocated area
> > size. This means that KASAN will correctly poison the correct spans
> > of requested bytes. This also means we can support growing the usable
> > portion of an allocation that can already be supported by the existing
> > area's existing allocation.
> >
> > Reported-by: Erhard Furtner <[email protected]>
> > Closes: https://lore.kernel.org/all/[email protected]/
> > Fixes: 3ddc2fefe6f3 ("mm: vmalloc: implement vrealloc()")
> > Signed-off-by: Kees Cook <[email protected]>
>
> Good catch!
>
> One question below, otherwise
>
> Reviewed-by: Danilo Krummrich <[email protected]>
>
> > @@ -4088,14 +4093,27 @@ void *vrealloc_noprof(const void *p, size_t size,
> > gfp_t flags)
> > * would be a good heuristic for when to shrink the vm_area?
> > */
> > if (size <= old_size) {
> > - /* Zero out spare memory. */
> > - if (want_init_on_alloc(flags))
> > + /* Zero out "freed" memory. */
> > + if (want_init_on_free())
> > memset((void *)p + size, 0, old_size - size);
> > + vm->requested_size = size;
> > kasan_poison_vmalloc(p + size, old_size - size);
> > kasan_unpoison_vmalloc(p, size, KASAN_VMALLOC_PROT_NORMAL);
> > return (void *)p;
> > }
> >
> > + /*
> > + * We already have the bytes available in the allocation; use them.
> > + */
> > + if (size <= alloced_size) {
> > + kasan_unpoison_vmalloc(p, size, KASAN_VMALLOC_PROT_NORMAL);
> > + /* Zero out "alloced" memory. */
> > + if (want_init_on_alloc(flags))
> > + memset((void *)p + old_size, 0, size - old_size);
> > + vm->requested_size = size;
> > + kasan_poison_vmalloc(p + size, alloced_size - size);
>
> Do we need this? We know that old_size < size <= alloced_size. And since
> previously [p + old_size, p + alloced_size) must have been poisoned,
> [p + size, p + alloced_size) must be poisoned already?
>
> Maybe there was a reason, since in the above (size <= old_size) case
> kasan_unpoison_vmalloc() seems unnecessary too.
Honestly I was just copying the logic from the prior case. But yeah, it
should be possible (in both cases) to just apply the changed span. For
the "size <= old_size" case, it would just be:
kasan_poison_vmalloc(p + size, old_size - size);
(i.e. the kasan_unpoison_vmalloc() call isn't needed at all, as you say.)
And in the "size <= alloced_size" case, it would just be:
kasan_unpoison_vmalloc(p + old_size, size - old_size,
KASAN_VMALLOC_PROT_NORMAL);
and no kasan_poison_vmalloc() should be needed.
Do the KASAN folks on CC have any opinion on best practices here?
Thanks for looking it over!
-Kees
--
Kees Cook
