Re: [PATCH] lkdtm: Add a tests for NULL pointer dereference

[Christophe Leroy]

Le 09/01/2019 Ã 02:14, Kees Cook a ÃcritÂ:
> On Fri, Dec 14, 2018 at 7:26 AM Christophe Leroy
> <christophe.leroy@xxxxxx> wrote:
> > Introduce lkdtm tests for NULL pointer dereference: check
> > access or exec at NULL address.
>
> Why is this not already covered by the existing tests? (Is there
> something special about NULL that is being missed?) I'd expect SMAP
> and SMEP to cover NULL as well.

Most arches print a different message whether the faulty address is 
above or under PAGE_SIZE. Below is exemple from x86:

	pr_alert("BUG: unable to handle kernel %s at %px\n",
		 address < PAGE_SIZE ? "NULL pointer dereference" : "paging request",
		 (void *)address);


Until recently, the powerpc arch didn't do it. When I implemented it 
(https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=49a502ea23bf9dec47f8f3c3960909ff409cd1bb), 
I needed a way to test it and couldn't find an existing one, hence this 
new LKDTM test.

But maybe I missed something ?

Christophe

> -Kees
>
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
> > ---
> >   drivers/misc/lkdtm/core.c  |  2 ++
> >   drivers/misc/lkdtm/lkdtm.h |  2 ++
> >   drivers/misc/lkdtm/perms.c | 18 ++++++++++++++++++
> >   3 files changed, 22 insertions(+)
> >
> > diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> > index bc76756b7eda..36910e1d5c09 100644
> > --- a/drivers/misc/lkdtm/core.c
> > +++ b/drivers/misc/lkdtm/core.c
> > @@ -157,7 +157,9 @@ static const struct crashtype crashtypes[] = {
> >          CRASHTYPE(EXEC_VMALLOC),
> >          CRASHTYPE(EXEC_RODATA),
> >          CRASHTYPE(EXEC_USERSPACE),
> > +       CRASHTYPE(EXEC_NULL),
> >          CRASHTYPE(ACCESS_USERSPACE),
> > +       CRASHTYPE(ACCESS_NULL),
> >          CRASHTYPE(WRITE_RO),
> >          CRASHTYPE(WRITE_RO_AFTER_INIT),
> >          CRASHTYPE(WRITE_KERN),
> > diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
> > index 3c6fd327e166..b69ee004a3f7 100644
> > --- a/drivers/misc/lkdtm/lkdtm.h
> > +++ b/drivers/misc/lkdtm/lkdtm.h
> > @@ -45,7 +45,9 @@ void lkdtm_EXEC_KMALLOC(void);
> >   void lkdtm_EXEC_VMALLOC(void);
> >   void lkdtm_EXEC_RODATA(void);
> >   void lkdtm_EXEC_USERSPACE(void);
> > +void lkdtm_EXEC_NULL(void);
> >   void lkdtm_ACCESS_USERSPACE(void);
> > +void lkdtm_ACCESS_NULL(void);
> >
> >   /* lkdtm_refcount.c */
> >   void lkdtm_REFCOUNT_INC_OVERFLOW(void);
> > diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> > index fa54add6375a..62f76d506f04 100644
> > --- a/drivers/misc/lkdtm/perms.c
> > +++ b/drivers/misc/lkdtm/perms.c
> > @@ -164,6 +164,11 @@ void lkdtm_EXEC_USERSPACE(void)
> >          vm_munmap(user_addr, PAGE_SIZE);
> >   }
> >
> > +void lkdtm_EXEC_NULL(void)
> > +{
> > +       execute_location(NULL, CODE_AS_IS);
> > +}
> > +
> >   void lkdtm_ACCESS_USERSPACE(void)
> >   {
> >          unsigned long user_addr, tmp = 0;
> > @@ -195,6 +200,19 @@ void lkdtm_ACCESS_USERSPACE(void)
> >          vm_munmap(user_addr, PAGE_SIZE);
> >   }
> >
> > +void lkdtm_ACCESS_NULL(void)
> > +{
> > +       unsigned long tmp;
> > +       unsigned long *ptr = (unsigned long *)NULL;
> > +
> > +       pr_info("attempting bad read at %px\n", ptr);
> > +       tmp = *ptr;
> > +       tmp += 0xc0dec0de;
> > +
> > +       pr_info("attempting bad write at %px\n", ptr);
> > +       *ptr = tmp;
> > +}
> > +
> >   void __init lkdtm_perms_init(void)
> >   {
> >          /* Make sure we can write to __ro_after_init values during __init */
> > --
> > 2.13.3