Andrew Morton <[email protected]> wrote: > On Wed, 16 Sep 2020 05:07:33 -0400 Miaohe Lin <[email protected]> wrote: > >> The syzbot reported the below general protection fault: >> >> general protection fault, probably for non-canonical address >> 0xe00eeaee0000003b: 0000 [#1] PREEMPT SMP KASAN >> KASAN: maybe wild-memory-access in range >> [0x00777770000001d8-0x00777770000001df] >> CPU: 1 PID: 10488 Comm: syz-executor721 Not tainted >> 5.9.0-rc3-syzkaller #0 >> Trace: >> free_pgtables+0x1b3/0x2f0 mm/memory.c:415 >> exit_mmap+0x2c0/0x530 mm/mmap.c:3184 >> __mmput+0x122/0x470 kernel/fork.c:1076 >> >> It's because the ->mmap() callback can change vma->vm_file and fput >> the original file. But the commit d70cec898324 ("mm: mmap: merge vma >> after >> call_mmap() if possible") failed to catch this case and always fput() >> the original file, hence add an extra fput(). >> >> ... >> >> --- a/mm/mmap.c >> +++ b/mm/mmap.c >> @@ -1815,7 +1815,11 @@ unsigned long mmap_region(struct file *file, unsigned >> long addr, >> merge = vma_merge(mm, prev, vma->vm_start, vma->vm_end, >> vma->vm_flags, >> NULL, vma->vm_file, vma->vm_pgoff, NULL, >> NULL_VM_UFFD_CTX); >> if (merge) { >> - fput(file); >> + /* ->mmap() can change vma->vm_file and fput >> the original file. So >> + * fput the vma->vm_file here or we would add >> an extra fput for file >> + * and cause general protection fault >> ultimately. >> + */ >> + fput(vma->vm_file); >> vm_area_free(vma); >> vma = merge; >> /* Update vm_flags and possible addr to pick up >> the change. We >> don't > >What about the case where this code block does its `goto unmap_writable'? > > /* Once vma denies write, undo our temporary denial count */ > if (file) { >unmap_writable: > if (vm_flags & VM_SHARED) > mapping_unmap_writable(file->f_mapping); > if (vm_flags & VM_DENYWRITE) > allow_write_access(file); > } > file = vma->vm_file; > >is this using the correct file? I think it is, but please do check. >
Many thanks for your reply. Yes, I think so too. We do deny_write_access and mapping_map_writable on @file, so we should undo all of this on @file. Since @file is unchanged over the second vma_merge() time, we'are using the correct @file to undo our temporary denial count. But how should I check this explicitly ? I can't find out a way to do this. Could you please figure it out for me? Thanks again.
