On Fri, 2024-10-18 at 14:05 +0300, Kirill A. Shutemov wrote:
> On Fri, Oct 18, 2024 at 12:00:22PM +0100, Lorenzo Stoakes wrote:
> > + Liam, Jann
> >
> > On Fri, Oct 18, 2024 at 01:49:06PM +0300, Kirill A. Shutemov wrote:
> > > On Fri, Oct 18, 2024 at 11:24:06AM +0200, Roberto Sassu wrote:
> > > > Probably it is hard, @Kirill would there be any way to safely move
> > > > security_mmap_file() out of the mmap_lock lock?
> > >
> > > What about something like this (untested):
> > >
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index dd4b35a25aeb..03473e77d356 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -1646,6 +1646,26 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
> > > if (pgoff + (size >> PAGE_SHIFT) < pgoff)
> > > return ret;
> > >
> > > + if (mmap_read_lock_killable(mm))
> > > + return -EINTR;
> > > +
> > > + vma = vma_lookup(mm, start);
> > > +
> > > + if (!vma || !(vma->vm_flags & VM_SHARED)) {
> > > + mmap_read_unlock(mm);
> > > + return -EINVAL;
> > > + }
> > > +
> > > + file = get_file(vma->vm_file);
> > > +
> > > + mmap_read_unlock(mm);
> > > +
> > > + ret = security_mmap_file(vma->vm_file, prot, flags);
> >
> > Accessing VMA fields without any kind of lock is... very much not advised.
> >
> > I'm guessing you meant to say:
> >
> > ret = security_mmap_file(file, prot, flags);
> >
> > Here? :)
>
> Sure. My bad.
>
> Patch with all fixups:
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index dd4b35a25aeb..541787d526b6 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1646,14 +1646,41 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
> if (pgoff + (size >> PAGE_SHIFT) < pgoff)
> return ret;
>
> - if (mmap_write_lock_killable(mm))
> + if (mmap_read_lock_killable(mm))
> return -EINTR;
>
> vma = vma_lookup(mm, start);
>
> + if (!vma || !(vma->vm_flags & VM_SHARED)) {
> + mmap_read_unlock(mm);
> + return -EINVAL;
> + }
> +
> + file = get_file(vma->vm_file);
> +
> + mmap_read_unlock(mm);
> +
> + ret = security_mmap_file(file, prot, flags);
Uhm, I have to calculate prot and flags before. I can check if what I
used here changed in the next lock, and refuse.
Roberto
> + if (ret) {
> + fput(file);
> + return ret;
> + }
> +
> + ret = -EINVAL;
> +
> + if (mmap_write_lock_killable(mm)) {
> + fput(file);
> + return -EINTR;
> + }
> +
> + vma = vma_lookup(mm, start);
> +
> if (!vma || !(vma->vm_flags & VM_SHARED))
> goto out;
>
> + if (vma->vm_file != file)
> + goto out;
> +
> if (start + size > vma->vm_end) {
> VMA_ITERATOR(vmi, mm, vma->vm_end);
> struct vm_area_struct *next, *prev = vma;
> @@ -1688,16 +1715,11 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
> if (vma->vm_flags & VM_LOCKED)
> flags |= MAP_LOCKED;
>
> - file = get_file(vma->vm_file);
> - ret = security_mmap_file(vma->vm_file, prot, flags);
> - if (ret)
> - goto out_fput;
> ret = do_mmap(vma->vm_file, start, size,
> prot, flags, 0, pgoff, &populate, NULL);
> -out_fput:
> - fput(file);
> out:
> mmap_write_unlock(mm);
> + fput(file);
> if (populate)
> mm_populate(ret, populate);
> if (!IS_ERR_VALUE(ret))
