On Tue, Jan 23, 2018 at 02:50:26PM -0800, Andrew Morton wrote: > On Mon, 22 Jan 2018 12:32:58 +0530 Balasubramani Vivekanandan <balasubramani_vivekanandan@xxxxxxxxxx> wrote: > > > From: Balasubramani Vivekanandan <balasubramani_vivekanandan@xxxxxxxxxx> > > > > Start address calculated for slab padding restoration was wrong. > > Wrong address would point to some section before padding and > > could cause corruption > > > > ... > > > > --- a/mm/slub.c > > +++ b/mm/slub.c > > @@ -838,6 +838,7 @@ static int slab_pad_check(struct kmem_cache *s, struct page *page) > > u8 *start; > > u8 *fault; > > u8 *end; > > + u8 *pad; > > int length; > > int remainder; > > > > @@ -851,8 +852,9 @@ static int slab_pad_check(struct kmem_cache *s, struct page *page) > > if (!remainder) > > return 1; > > > > + pad = end - remainder; > > metadata_access_enable(); > > - fault = memchr_inv(end - remainder, POISON_INUSE, remainder); > > + fault = memchr_inv(pad, POISON_INUSE, remainder); > > metadata_access_disable(); > > if (!fault) > > return 1; > > @@ -860,9 +862,9 @@ static int slab_pad_check(struct kmem_cache *s, struct page *page) > > end--; > > > > slab_err(s, page, "Padding overwritten. 0x%p-0x%p", fault, end - 1); > > - print_section(KERN_ERR, "Padding ", end - remainder, remainder); > > + print_section(KERN_ERR, "Padding ", pad, remainder); > > > > - restore_bytes(s, "slab padding", POISON_INUSE, end - remainder, end); > > + restore_bytes(s, "slab padding", POISON_INUSE, fault, end); > > return 0; > > } > > I don't see why it matters? The current code will overwrite > POISON_INUSE bytes with POISON_INUSE, won't it? > > That's a bit strange but not incorrect? Not really. The bug will overwrite into the object area with POISON_INUSE. The end pointer initially points to end of the padding area. Then in the loop, end is decremented till it points to the end of the fault area. while (end > fault && end[-1] == POISON_INUSE) end--; Now using end - remainder, will not point to the begining of the padding area but will sneak into the object area. So restore_bytes will overwrite the object area -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
