Re: [PATCH v2] Fixes: null pointer dereference in pfnmap_lockdep_assert

[Manas]

On 04.10.2024 09:21, Peter Xu wrote:
> On Fri, Oct 04, 2024 at 06:35:53PM +0530, Manas via B4 Relay wrote:
> > From: Manas <manas18244@xxxxxxxxxxx>
> >
> > syzbot has pointed to a possible null pointer dereference in
> > pfnmap_lockdep_assert. vm_file member of vm_area_struct is being
> > dereferenced without any checks.
> >
> > This fix returns if vm_file member in vm_area_struct is NULL.
> >
> > Reported-by: syzbot+093d096417e7038a689b@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b
> > ---
> > This bug[1] triggers a general protection fault in follow_pfnmap_start
> > function. An assertion pfnmap_lockdep_assert inside this function
> > dereferences vm_file member of vm_area_struct. And panic gets triggered
> > when vm_file is NULL.
> >
> > This patch returns from the assertion pfnmap_lockdep_assert if vm_file
> > is found to be NULL.
> >
> > [1] https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b
> >
> > Signed-off-by: Manas <manas18244@xxxxxxxxxxx>
>
> Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
>
> One nitpick:
>
> > ---
> > Changes in v2:
> > - v2: use ternary operator according to feedback
> > - Link to v1: https://lore.kernel.org/r/20241003-fix-null-deref-v1-1-0a45df9d016a@xxxxxxxxxxx
> > ---
> >  mm/memory.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 2366578015ad..5ed109a8f02e 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -6346,7 +6346,7 @@ static inline void pfnmap_args_setup(struct follow_pfnmap_args *args,
> >  static inline void pfnmap_lockdep_assert(struct vm_area_struct *vma)
> >  {
> >  #ifdef CONFIG_LOCKDEP
> > -	struct address_space *mapping = vma->vm_file->f_mapping;
> > +	struct address_space *mapping = vma->vm_file ? vma->vm_file->f_mapping : NULL;
> >
> >  	if (mapping)
> >  		lockdep_assert(lockdep_is_held(&vma->vm_file->f_mapping->i_mmap_rwsem) ||
>
> This can use "mapping" directly, as I mentioned in previous email (but
> probably got overlooked..).
>
> Thanks!
Oh no! I missed it. Sending v3...

> > ---
> > base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
> > change-id: 20241003-fix-null-deref-6bfa0337efc3
> >
> > Best regards,
> > --
> > Manas <manas18244@xxxxxxxxxxx>
>
> --
> Peter Xu

--
Manas