Re: [PATCH] drivers/char/mem: mmap readonly MAP_SHARED-/dev/zero correctly

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Sep 24, 2026 at 04:29:23PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Thu, Sep 24, 2026 at 05:08:49PM +0200, Greg Kroah-Hartman wrote:
> > On Thu, Sep 24, 2026 at 03:48:24PM +0100, Lorenzo Stoakes (ARM) wrote:
> > > Rather surprisingly, opening /dev/zero read-only then mmap()'ing it
> > > MAP_SHARED gets you true anonymous memory (albeit in a VMA with
> > > non-NULL vma->vm_file).
> > >
> > > This is a by-product of MAP_PRIVATE-/dev/zero being how anonymous memory
> > > was mapped in Linux's distant past.
> > >
> > > It happens because mmap_zero_prepare() gates on VMA_SHARED_BIT and when
> > > mapping a read-only file MAP_SHARED, do_mmap() clears VMA_SHARED_BIT and
> > > VMA_MAYWRITE_BIT.
> > >
> > > The gating is incorrect - the (poorly named) VMA_MAYSHARE_BIT flag exists
> > > explicitly to tell you if something was originally mapped MAP_SHARED.
> > >
> > > So the fix is simple - gate on this instead.
> > >
> > > This isn't exactly a common use case, but it's unexpected behaviour which
> > > now causes an assert if CONFIG_DEBUG_VM is set.
> > >
> > > While this bug has existed since the dawn of time for linux (or at least
> > > since 2.6.12), it hasn't caused issues in the past, so while it's incorrect
> > > behaviour, it doesn't seem necessary to backport that far.
> > >
> > > The mapping is now accounted at mmap time and can fail with -ENOMEM under
> > > strict overcommit, and read faults allocate folios. However this is normal
> > > behaviour for a read-only shmem mapping.
> > >
> > > Commit 93c0c8dc87f6 ("mm/rmap: use anon pgoff to track MAP_PRIVATE
> > > file-backed anon folios") is the first patch at which the debug assert
> > > fires, so target that instead.
> > >
> > > Fixes: 93c0c8dc87f6 ("mm/rmap: use anon pgoff to track MAP_PRIVATE file-backed anon folios")
> > > Reported-by: syzbot+c181d3198e98f8aef8b9@syzkaller.appspotmail.com
> > > Closes: https://lore.kernel.org/linux-mm/6ab4ae75.80e1c6cc.1e8e5f.000d.GAE@google.com/
> > > Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> > > ---
> > >  drivers/char/mem.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> > > index 63253d1de5d7..5b93c92c2cf1 100644
> > > --- a/drivers/char/mem.c
> > > +++ b/drivers/char/mem.c
> > > @@ -503,7 +503,7 @@ static int mmap_zero_prepare(struct vm_area_desc *desc)
> > >  #ifndef CONFIG_MMU
> > >  	return -ENOSYS;
> > >  #endif
> > > -	if (vma_desc_test(desc, VMA_SHARED_BIT))
> > > +	if (vma_desc_test(desc, VMA_MAYSHARE_BIT))
> > >  		return shmem_zero_setup_desc(desc);
> >
> > What's the odds that something "big" actually relies on this?
> 
> I think it'd be fairly crazy to, unless people really care about read-faulting a
> lot of zero pages which they want to somehow not account properly.
> 
> Anything's possible but I think on this, right to take a (v. small) risk!
> 
> >
> > I'm all for the change, but be prepared in about 5 years for people to
> > start complaining :)
> >
> > Want me to take it through my tree?
> 
> Sure, thanks, though there will be a conflict against a patch in mm.
> 
> Though it's really that the file is moved (to mm/), so actually should be an
> easy resolution?

No problem, you can take it if that makes it easier:

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>