+ 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? :)
I see the original code did this, but obviously was under an mmap lock.
I guess given you check that the file is the same below this.... should be
fine? Assuming nothing can come in and invalidate the security_mmap_file()
check in the mean time somehow?
Jann any thoughts?
> + if (ret) {
> + fput(file);
> + return ret;
> + }
> +
> if (mmap_write_lock_killable(mm))
> return -EINTR;
>
> @@ -1654,6 +1674,9 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
> 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 +1711,11 @@ 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))
> --
> Kiryl Shutsemau / Kirill A. Shutemov
