On Fri, Oct 04, 2024 at 03:43:12PM +0530, Manas wrote: > 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; Sorry, that was a pretty stupid mistake of mine, just to show what I meant without any compilation tests. Yes this one. Thanks, > > 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 > -- Peter Xu
