Re: [PATCH] x86/vm86/32: Remove VM86_SCREEN_BITMAP support

[Brian Gerst <brgerst@gmail.com>]

On Fri, Jan 8, 2021 at 1:59 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> The implementation was rather buggy.  It unconditionally marked PTEs
> read-only, even for VM_SHARED mappings.  I'm not sure whether this is
> actually a problem, but it certainly seems unwise.  More importantly, it
> released the mmap lock before flushing the TLB, which could allow a racing
> CoW operation to falsely believe that the underlying memory was not
> writable.
>
> I can't find any users at all of this mechanism, so just remove it.
>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Linux-MM <linux-mm@kvack.org>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: x86@kernel.org
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Jann Horn <jannh@google.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Yu Zhao <yuzhao@google.com>
> Cc: Peter Xu <peterx@redhat.com>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>  arch/x86/include/uapi/asm/vm86.h |  2 +-
>  arch/x86/kernel/vm86_32.c        | 55 ++++++--------------------------
>  2 files changed, 10 insertions(+), 47 deletions(-)
>
> diff --git a/arch/x86/include/uapi/asm/vm86.h b/arch/x86/include/uapi/asm/vm86.h
> index d2ee4e307ef8..50004fb4590d 100644
> --- a/arch/x86/include/uapi/asm/vm86.h
> +++ b/arch/x86/include/uapi/asm/vm86.h
> @@ -106,7 +106,7 @@ struct vm86_struct {
>  /*
>   * flags masks
>   */
> -#define VM86_SCREEN_BITMAP     0x0001
> +#define VM86_SCREEN_BITMAP     0x0001        /* no longer supported */
>
>  struct vm86plus_info_struct {
>         unsigned long force_return_for_pic:1;
> diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
> index 764573de3996..28b9e8d511e1 100644
> --- a/arch/x86/kernel/vm86_32.c
> +++ b/arch/x86/kernel/vm86_32.c
> @@ -160,49 +160,6 @@ void save_v86_state(struct kernel_vm86_regs *regs, int retval)
>         do_exit(SIGSEGV);
>  }
>
> -static void mark_screen_rdonly(struct mm_struct *mm)
> -{
> -       struct vm_area_struct *vma;
> -       spinlock_t *ptl;
> -       pgd_t *pgd;
> -       p4d_t *p4d;
> -       pud_t *pud;
> -       pmd_t *pmd;
> -       pte_t *pte;
> -       int i;
> -
> -       mmap_write_lock(mm);
> -       pgd = pgd_offset(mm, 0xA0000);
> -       if (pgd_none_or_clear_bad(pgd))
> -               goto out;
> -       p4d = p4d_offset(pgd, 0xA0000);
> -       if (p4d_none_or_clear_bad(p4d))
> -               goto out;
> -       pud = pud_offset(p4d, 0xA0000);
> -       if (pud_none_or_clear_bad(pud))
> -               goto out;
> -       pmd = pmd_offset(pud, 0xA0000);
> -
> -       if (pmd_trans_huge(*pmd)) {
> -               vma = find_vma(mm, 0xA0000);
> -               split_huge_pmd(vma, pmd, 0xA0000);
> -       }
> -       if (pmd_none_or_clear_bad(pmd))
> -               goto out;
> -       pte = pte_offset_map_lock(mm, pmd, 0xA0000, &ptl);
> -       for (i = 0; i < 32; i++) {
> -               if (pte_present(*pte))
> -                       set_pte(pte, pte_wrprotect(*pte));
> -               pte++;
> -       }
> -       pte_unmap_unlock(pte, ptl);
> -out:
> -       mmap_write_unlock(mm);
> -       flush_tlb_mm_range(mm, 0xA0000, 0xA0000 + 32*PAGE_SIZE, PAGE_SHIFT, false);
> -}
> -
> -
> -
>  static int do_vm86_irq_handling(int subfunction, int irqnumber);
>  static long do_sys_vm86(struct vm86plus_struct __user *user_vm86, bool plus);
>
> @@ -282,6 +239,15 @@ static long do_sys_vm86(struct vm86plus_struct __user *user_vm86, bool plus)
>                         offsetof(struct vm86_struct, int_revectored)))
>                 return -EFAULT;
>
> +
> +       /* VM86_SCREEN_BITMAP had numerous bugs and appears to have no users. */
> +       if (v.flags & VM86_SCREEN_BITMAP) {
> +               char comm[TASK_COMM_LEN];
> +
> +               pr_info_once("vm86: '%s' uses VM86_SCREEN_BITMAP, which is no longer supported\n", get_task_comm(comm, current);
> +               return -EINVAL;
> +       }
> +
>         memset(&vm86regs, 0, sizeof(vm86regs));
>
>         vm86regs.pt.bx = v.regs.ebx;
> @@ -370,9 +336,6 @@ static long do_sys_vm86(struct vm86plus_struct __user *user_vm86, bool plus)
>         update_task_stack(tsk);
>         preempt_enable();
>
> -       if (vm86->flags & VM86_SCREEN_BITMAP)
> -               mark_screen_rdonly(tsk->mm);
> -
>         memcpy((struct kernel_vm86_regs *)regs, &vm86regs, sizeof(vm86regs));
>         return regs->ax;
>  }

You can also remove screen_bitmap from struct vm86 (the kernel
internal structure), and the check_v8086_mode() function.

--
Brian Gerst