Subject: Re: rcu-torture: Internal error: Oops: 96000006

From: "Paul E. McKenney" <[email-masked]>
Date: Thu, 21 Jan 2021 13:43:14 -0800

On Thu, Jan 21, 2021 at 09:31:10PM +0000, Will Deacon wrote:
> On Thu, Jan 21, 2021 at 10:55:21AM -0800, Paul E. McKenney wrote:
> > On Thu, Jan 21, 2021 at 10:37:21PM +0530, Naresh Kamboju wrote:
> > > While running rcu-torture test on qemu_arm64 and arm64 Juno-r2 device
> > > the following kernel crash noticed. This started happening from Linux next
> > > next-20210111 tag to next-20210121.
> > > 
> > > metadata:
> > >   git branch: master
> > >   git repo: https://gitlab.com/Linaro/lkft/mirrors/next/linux-next
> > >   git describe: next-20210111
> > >   kernel-config: https://builds.tuxbuild.com/1muTTn7AfqcWvH5x2Alxifn7EUH/config
> > > 
> > > output log:
> > > 
> > > [  621.538050] mem_dump_obj() slab test: rcu_torture_stats =
> > > ffff0000c0a3ac40, &rhp = ffff800012debe40, rhp = ffff0000c8cba000, &z
> > > = ffff8000091ab8e0
> > > [  621.546662] mem_dump_obj(ZERO_SIZE_PTR):
> > > [  621.546696] Unable to handle kernel NULL pointer dereference at
> > > virtual address 0000000000000008
> 
> [...]
> 
> > Huh.  I am relying on virt_addr_valid() rejecting NULL pointers and
> > things like ZERO_SIZE_PTR, which is defined as ((void *)16).  It looks
> > like your configuration rejects NULL as an invalid virtual address,
> > but does not reject ZERO_SIZE_PTR.  Is this the intent, given that you
> > are not allowed to dereference a ZERO_SIZE_PTR?
> > 
> > Adding the ARM64 guys on CC for their thoughts.
> 
> Spooky timing, there was a thread _today_ about that:
> 
> https://lore.kernel.org/r/[email-masked]

Very good, then my workaround (shown below for Naresh's ease of testing)
is only a short-term workaround.  Yay!  ;-)

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/mm/slab_common.c b/mm/slab_common.c
index cefa9ae..a8375d1 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -550,7 +550,8 @@ bool kmem_valid_obj(void *object)
 {
 	struct page *page;
 
-	if (!virt_addr_valid(object))
+	/* Some arches consider ZERO_SIZE_PTR to be a valid address. */
+	if (object < (void *)PAGE_SIZE || !virt_addr_valid(object))
 		return false;
 	page = virt_to_head_page(object);
 	return PageSlab(page);