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

[Vlastimil Babka]

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@xxxxxxxxxx>
>> > ---
>> >  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
>>