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; 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
