On Tue, Jun 02, 2020 at 04:15:17PM +0200, Vlastimil Babka wrote: > SLUB allows runtime changing of page allocation order by writing into the > /sys/kernel/slab/<cache>/order file. Jann has reported [1] that this interface > allows the order to be set too small, leading to crashes. > > While it's possible to fix the immediate issue, closer inspection reveals > potential races. Storing the new order calls calculate_sizes() which > non-atomically updates a lot of kmem_cache fields while the cache is still in > use. Unexpected behavior might occur even if the fields are set to the same > value as they were. > > This could be fixed by splitting out the part of calculate_sizes() that depends > on forced_order, so that we only update kmem_cache.oo field. This could still > race with init_cache_random_seq(), shuffle_freelist(), allocate_slab(). Perhaps > it's possible to audit and e.g. add some READ_ONCE/WRITE_ONCE accesses, it > might be easier just to remove the runtime order changes, which is what this > patch does. If there are valid usecases for per-cache order setting, we could > e.g. extend the boot parameters to do that. > > [1] https://lore.kernel.org/r/CAG48ez31PP--h6_FzVyfJ4H86QYczAFPdxtJHUEEan+7VJETAQ@xxxxxxxxxxxxxx > > Reported-by: Jann Horn <jannh@xxxxxxxxxx> > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> Acked-by: Roman Gushchin <guro@xxxxxx> Thanks! > --- > mm/slub.c | 19 +------------------ > 1 file changed, 1 insertion(+), 18 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index ac198202dbb0..58c1e9e7b3b3 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -5111,28 +5111,11 @@ static ssize_t objs_per_slab_show(struct kmem_cache *s, char *buf) > } > SLAB_ATTR_RO(objs_per_slab); > > -static ssize_t order_store(struct kmem_cache *s, > - const char *buf, size_t length) > -{ > - unsigned int order; > - int err; > - > - err = kstrtouint(buf, 10, &order); > - if (err) > - return err; > - > - if (order > slub_max_order || order < slub_min_order) > - return -EINVAL; > - > - calculate_sizes(s, order); > - return length; > -} > - > static ssize_t order_show(struct kmem_cache *s, char *buf) > { > return sprintf(buf, "%u\n", oo_order(s->oo)); > } > -SLAB_ATTR(order); > +SLAB_ATTR_RO(order); > > static ssize_t min_partial_show(struct kmem_cache *s, char *buf) > { > -- > 2.26.2 > >
