Re: [PATCH 1/2] mm: krealloc: consider spare memory for __GFP_ZERO

Vlastimil Babka
2024-07-31T14:30:53Z

On 7/31/24 1:54 AM, Danilo Krummrich wrote:
> On Tue, Jul 30, 2024 at 11:14:16PM +0200, Vlastimil Babka wrote:
>> On 7/30/24 9:42 PM, Danilo Krummrich wrote:
>> > As long as krealloc() is called with __GFP_ZERO consistently, starting
>> > with the initial memory allocation, __GFP_ZERO should be fully honored.
>> >
>> > However, if for an existing allocation krealloc() is called with a
>> > decreased size, it is not ensured that the spare portion the allocation
>> > is zeroed. Thus, if krealloc() is subsequently called with a larger size
>> > again, __GFP_ZERO can't be fully honored, since we don't know the
>> > previous size, but only the bucket size.
>> >
>> > Example:
>> >
>> > buf = kzalloc(64, GFP_KERNEL);
>> > memset(buf, 0xff, 64);
>> >
>> > buf = krealloc(buf, 48, GFP_KERNEL | __GFP_ZERO);
>> >
>> > /* After this call the last 16 bytes are still 0xff. */
>> > buf = krealloc(buf, 64, GFP_KERNEL | __GFP_ZERO);
>> >
>> > Fix this, by explicitly setting spare memory to zero, when shrinking an
>> > allocation with __GFP_ZERO flag set or init_on_alloc enabled.
>> >
>> > Signed-off-by: Danilo Krummrich <dakr@kernel.org>
>> > ---
>> > mm/slab_common.c | 7 +++++++
>> > 1 file changed, 7 insertions(+)
>> >
>> > diff --git a/mm/slab_common.c b/mm/slab_common.c
>> > index 40b582a014b8..cff602cedf8e 100644
>> > --- a/mm/slab_common.c
>> > +++ b/mm/slab_common.c
>> > @@ -1273,6 +1273,13 @@ __do_krealloc(const void *p, size_t new_size, gfp_t flags)
>> >
>> > /* If the object still fits, repoison it precisely. */
>> > if (ks >= new_size) {
>> > + /* Zero out spare memory. */
>> > + if (want_init_on_alloc(flags)) {
>> > + kasan_disable_current();
>> > + memset((void *)p + new_size, 0, ks - new_size);
>> > + kasan_enable_current();
>>
>> If we do kasan_krealloc() first, shouldn't the memset then be legal
>> afterwards without the disable/enable dance?
>
> No, we always write into the poisoned area. The following tables show what we do
> in the particular case:
>
> Shrink
> ------
> new old
> 0 size size ks
> |----------|----------|----------|
> | keep | poison | <- poison
> |--------------------------------|
> | keep | zero | <- data
>
>
> Poison and zero things between old size and ks is not necessary, but we don't
> know old size, hence we have do it between new size and ks.
>
> Grow
> ----
> old new
> 0 size size ks
> |----------|----------|----------|
> | unpoison | keep | <- poison
> |--------------------------------|
> | keep | zero | <- data
>
> Zeroing between new_size and ks in not necessary in this case, since it must be
> zero already. But without knowing the old size we don't know whether we shrink
> and actually need to do something, or if we grow and don't need to do anything.
>
> Analogously, we also unpoison things between 0 and old size for the same reason.

Thanks, you're right!

>>
>> > + }
>> > +
>> > p = kasan_krealloc((void *)p, new_size, flags);
>> > return (void *)p;
>> > }
>> >
>> > base-commit: 7c3dd6d99f2df6a9d7944ee8505b195ba51c9b68
>>