[PATCH] proc: prevent a task from writing on its own /proc/*/mem

[Jann Horn]

On Sat, May 26, 2018 at 4:50 PM, Salvatore Mesoraca
<s.mesoraca16 at gmail.com> wrote:
> Prevent a task from opening, in "write" mode, any /proc/*/mem
> file that operates on the task's mm.
> /proc/*/mem is mainly a debugging means and, as such, it shouldn't
> be used by the inspected process itself.
> Current implementation always allow a task to access its own
> /proc/*/mem file.
> A process can use it to overwrite read-only memory, making
> pointless the use of security_file_mprotect() or other ways to
> enforce RO memory.
>
> Signed-off-by: Salvatore Mesoraca <s.mesoraca16 at gmail.com>
> ---
>  fs/proc/base.c       | 25 ++++++++++++++++++-------
>  fs/proc/internal.h   |  3 ++-
>  fs/proc/task_mmu.c   |  4 ++--
>  fs/proc/task_nommu.c |  2 +-
>  4 files changed, 23 insertions(+), 11 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 1a76d75..01ecfec 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -762,8 +762,9 @@ static int proc_single_open(struct inode *inode, struct file *filp)
>         .release        = single_release,
>  };
>
> -
> -struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)
> +struct mm_struct *proc_mem_open(struct inode *inode,
> +                               unsigned int mode,
> +                               fmode_t f_mode)
>  {
>         struct task_struct *task = get_proc_task(inode);
>         struct mm_struct *mm = ERR_PTR(-ESRCH);
> @@ -773,10 +774,20 @@ struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)
>                 put_task_struct(task);
>
>                 if (!IS_ERR_OR_NULL(mm)) {
> -                       /* ensure this mm_struct can't be freed */
> -                       mmgrab(mm);
> -                       /* but do not pin its memory */
> -                       mmput(mm);
> +                       /*
> +                        * Prevent this interface from being used as a mean
> +                        * to bypass memory restrictions, including those
> +                        * imposed by LSMs.
> +                        */
> +                       if (mm == current->mm &&
> +                           f_mode & FMODE_WRITE)
> +                               mm = ERR_PTR(-EACCES);
> +                       else {
> +                               /* ensure this mm_struct can't be freed */
> +                               mmgrab(mm);
> +                               /* but do not pin its memory */
> +                               mmput(mm);
> +                       }
>                 }
>         }

I don't have an opinion on the overall patch, but this part looks
buggy: In the error path, you set `mm` to an error pointer, but you
still own the reference that mm_access() took on the old `mm`. The
error path needs to call `mmput(mm)`.
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