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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/24/26 16:48, 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);
>  

So instead of shared zeropages we'd now get zero-filled shmem pages.

The alternative would be to just convert it to a proper read-only COW mapping in
mmap code:
* Not clearing VM_MAYWRITE, but keeping VM_WRITE clear
* Clearing VMA_SHARED and VMA_MAYSHARE

Sure, someone could then mprotect(PROT_WRITE that thing) or
FOLL_FORCE|FOLL_WRITE to get anonymous memory. Just raising that as an alternative.

-- 
Cheers,

David