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

[Andy Lutomirski <luto@amacapital.net>]

> On Jan 9, 2021, at 12:17 PM, ebiederm@xmission.com wrote:
> 
> Andy Lutomirski <luto@kernel.org> writes:
> 
>> 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.
> 
> In another age this was used by dosemu.  Have you looked at dosemu to
> see if it still uses this support (on 32bit where dosemu can use vm86)?
> 
> It may still be a valid removal target I just wanted to point out what
> the original user was.

I’m pretty sure that dosemu2 does not use this support.  I think the original dosemu doesn’t either, but I’m also not convinced it has any users at all.

I meant to cc Stas, and I will for v2.

> 
> Eric
> 
>> 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;
>> }