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

[Manas]

Hi Peter, thanks for reviewing.

On 03.10.2024 12:41, Peter Xu wrote:
> On Thu, Oct 03, 2024 at 09:31:06PM +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>
>
> Thanks for the patch!
>
> > ---
> >  mm/memory.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 2366578015ad..b152a95e543f 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -6346,6 +6346,9 @@ 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
> > +	if (!vma->vm_file)
> > +		return;
> > +
>
> Hmm I guess I wasn't careful enough here as I was mostly only thinking
> about file mappings, but I just notice we have other pfnmaps like the vvar
> mappings.. the mapping var can also already be reused later when available.
>
> Logically even if !vm_file we can still check against mmap write lock.  So
> would it be better to do this instead:
>
>        struct address_space *mapping = vma->vm_file && vma->vm_file->f_mapping;
This will lead to `-Wint-conversion` error in the assignment. We can either do a
cast like the following:

        struct address_space *mapping = (struct address_space *)(vma->vm_file && vma->vm_file->f_mapping);

But I am not sure if it is the canonical way of doing it. It will also lead to
warning about pointer from integer casting.

Or will a conditional like this work here?

        struct address_space *mapping = vma->vm_file ? vma->vm_file->f_mapping : NULL;
      
>        if (mapping)
>                lockdep_assert(lockdep_is_held(&mapping->i_mmap_rwsem) ||
>                               lockdep_is_held(&vma->vm_mm->mmap_lock));
>        else
>                lockdep_assert(lockdep_is_held(&vma->vm_mm->mmap_lock));
>
> ?

> >  	struct address_space *mapping = vma->vm_file->f_mapping;
> >
> >  	if (mapping)
> >
> > ---
> > base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
> > change-id: 20241003-fix-null-deref-6bfa0337efc3
> >
> > Best regards,
> > --
> > Manas <manas18244@xxxxxxxxxxx>
>
> --
> Peter Xu

--
Manas