Re: [PATCH -next] fork: annotate a data race in vm_area_dup()
From: Kirill A. Shutemov
Date: Tue Feb 18 2020 - 05:29:41 EST
On Mon, Feb 17, 2020 at 10:59:47PM -0500, Qian Cai wrote:
> > On Feb 17, 2020, at 5:31 PM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> > I'm confused. AFAICS both sides hold mmap_sem on write:
> > - vm_mmap_pgoff() takes mmap_sem for the write on the write side
> > - do_mprotect_pkey() takes mmap_sem for the write on the read side
> > What do I miss?
> Ah, good catch. I missed the locking for the read there. This is interesting because Marco
> did confirmed that the concurrency could happen,
> If that means KCSAN is not at fault, then I could think of two things,
> 1) someone downgrades the lock.
> I donât think that a case here. Only __do_munmap() will do that but I did not see how
> it will affect us here.
> 2) the reader and writer are two different processes.
> So, they held a different mmap_sem, but I canât see how could two processes shared
> the same vm_area_struct. Also, file->f_mapping->i_mmap was also stored in the
> writer, but I canât see how it was also loaded in the reader.
> Any ideas?
I think I've got this:
vm_area_dup() blindly copies all fields of orignal VMA to the new one.
This includes coping vm_area_struct::shared.rb which is normally protected
by i_mmap_lock. But this is fine because the read value will be
overwritten on the following __vma_link_file() under proper protectection.
So the fix is correct, but justificaiton is lacking.
Also, I would like to more fine-grained annotation: marking with
data_race() 200 bytes copy may hide other issues.
Kirill A. Shutemov