Re: [PATCH 5/5] coredump: Use the vma snapshot in fill_files_note

[Jann Horn]

On Mon, Jan 31, 2022 at 7:47 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> Matthew Wilcox reported that there is a missing mmap_lock in
> file_files_note that could possibly lead to a user after free.
>
> Solve this by using the existing vma snapshot for consistency
> and to avoid the need to take the mmap_lock anywhere in the
> coredump code except for dump_vma_snapshot.
>
> Update the dump_vma_snapshot to capture vm_pgoff and vm_file
> that are neeeded by fill_files_note.
>
> Add free_vma_snapshot to free the captured values of vm_file.
>
> Reported-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Link: https://lkml.kernel.org/r/20220131153740.2396974-1-willy@xxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: a07279c9a8cd ("binfmt_elf, binfmt_elf_fdpic: use a VMA list snapshot")
> Fixes: 2aa362c49c31 ("coredump: extend core dump note section to contain file names of mapped files")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
[...]
> +static int fill_files_note(struct memelfnote *note, struct coredump_params *cprm)
>  {
>         struct mm_struct *mm = current->mm;
> -       struct vm_area_struct *vma;
>         unsigned count, size, names_ofs, remaining, n;
>         user_long_t *data;
>         user_long_t *start_end_ofs;
>         char *name_base, *name_curpos;
> +       int i;
>
>         /* *Estimated* file count and total data size needed */
>         count = mm->map_count;

This function is still looking at mm->map_count in two spots, please
change those spots to also look at cprm->vma_count. In particular the
second one looks like it can cause memory corruption if the map_count
changed since we created the snapshot.

[...]
> +static void free_vma_snapshot(struct coredump_params *cprm)
> +{
> +       if (cprm->vma_meta) {
> +               int i;
> +               for (i = 0; i < cprm->vma_count; i++) {
> +                       struct file *file = cprm->vma_meta[i].file;
> +                       if (file)
> +                               fput(file);
> +               }
> +               kvfree(cprm->vma_meta);
> +               cprm->vma_meta = NULL;

(this NULL write is superfluous, but it also doesn't hurt)

> +       }
> +}