Re: mmap denywrite mess (Was: [GIT PULL] overlayfs fixes for 5.14-rc6)

[David Hildenbrand <david@redhat.com>]

On 11.08.21 16:11, Miklos Szeredi wrote:
> On Mon, Aug 09, 2021 at 02:25:17PM -0700, Linus Torvalds wrote:
> 
>> Ugh. Th edances with denywrite and mapping_unmap_writable are really
>> really annoying.
> 
> Attached version has error and success paths separated.  Was that your
> complaint?
> 
>> I get the feeling that the whole thing with deny_write_access and
>> mapping_map_writable could possibly be done after-the-fact somehow as
>> part of actually inserting the vma in the vma tree, rather than done
>> as the vma is prepared.
> 
> I don't know if that's doable or not.  The final denywrite count is obtained in
> __vma_link_file(), called after __vma_link().  The questions are:
> 
>   - does the order of those helper calls matter?
> 
>   - if it does, could the __vma_link() be safely undone after an unsuccessful
>     __vmal_link_file()?
> 
>> And most users of vma_set_file() probably really don't want that whole
>> thing at all (ie the DRM stuff that just switches out a local thing.
>> They also don't check for the new error cases you've added.
> 
> Christian König wants to follow up with those checks (which should be asserts,
> if the code wasn't buggy in the first place).
> 
>> So I really think this is quite questionable, and those cases should
>> probably have been done entirely inside ovlfs rather than polluting
>> the cases that don't care and don't check.
> 
> I don't get that.  mmap_region() currently drops the deny counts from the
> original file.  That doesn't work for overlayfs since it needs to take new temp
> counts on the override file.
> 
> So mmap_region() is changed to drop the counts on vma->vm_file, but then all
> callers of vma_set_file() will need to do that switch of temp counts, there's no
> way around that.
> 
> Thanks,
> Miklos
> 
> For reference, here's the previous discussion:
> 
> https://lore.kernel.org/linux-mm/YNHXzBgzRrZu1MrD@miu.piliscsaba.redhat.com/
> 
> ---
>   fs/overlayfs/file.c |    4 +++-
>   include/linux/mm.h  |    2 +-
>   mm/mmap.c           |    2 +-
>   mm/util.c           |   31 ++++++++++++++++++++++++++++++-
>   4 files changed, 35 insertions(+), 4 deletions(-)
> 
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -475,7 +475,9 @@ static int ovl_mmap(struct file *file, s
>   	if (WARN_ON(file != vma->vm_file))
>   		return -EIO;
>   
> -	vma_set_file(vma, realfile);
> +	ret = vma_set_file(vma, realfile);
> +	if (ret)
> +		return ret;
>   
>   	old_cred = ovl_override_creds(file_inode(file)->i_sb);
>   	ret = call_mmap(vma->vm_file, vma);
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2780,7 +2780,7 @@ static inline void vma_set_page_prot(str
>   }
>   #endif
>   
> -void vma_set_file(struct vm_area_struct *vma, struct file *file);
> +int /* __must_check */ vma_set_file(struct vm_area_struct *vma, struct file *file);
>   
>   #ifdef CONFIG_NUMA_BALANCING
>   unsigned long change_prot_numa(struct vm_area_struct *vma,
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1806,6 +1806,7 @@ unsigned long mmap_region(struct file *f
>   		 */
>   		vma->vm_file = get_file(file);
>   		error = call_mmap(file, vma);
> +		file = vma->vm_file;
>   		if (error)
>   			goto unmap_and_free_vma;
>   
> @@ -1867,7 +1868,6 @@ unsigned long mmap_region(struct file *f
>   		if (vm_flags & VM_DENYWRITE)
>   			allow_write_access(file);
>   	}
> -	file = vma->vm_file;
>   out:
>   	perf_event_mmap(vma);
>   
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -314,12 +314,41 @@ int vma_is_stack_for_current(struct vm_a
>   /*
>    * Change backing file, only valid to use during initial VMA setup.
>    */
> -void vma_set_file(struct vm_area_struct *vma, struct file *file)
> +int vma_set_file(struct vm_area_struct *vma, struct file *file)
>   {
> +	vm_flags_t vm_flags = vma->vm_flags;
> +	int err;
> +
> +	/* Get temporary denial counts on replacement */
> +	if (vm_flags & VM_DENYWRITE) {
> +		err = deny_write_access(file);
> +		if (err)
> +			return err;
> +	}
> +	if (vm_flags & VM_SHARED) {
> +		err = mapping_map_writable(file->f_mapping);
> +		if (err)
> +			goto undo_denywrite;
> +	}
> +
>   	/* Changing an anonymous vma with this is illegal */
>   	get_file(file);
>   	swap(vma->vm_file, file);
> +
> +	/* Undo temporary denial counts on replaced */
> +	if (vm_flags & VM_SHARED)
> +		mapping_unmap_writable(file->f_mapping);
> +
> +	if (vm_flags & VM_DENYWRITE)
> +		allow_write_access(file);
> +
>   	fput(file);
> +	return 0;
> +
> +undo_denywrite:
> +	if (vm_flags & VM_DENYWRITE)
> +		allow_write_access(file);
> +	return err;
>   }
>   EXPORT_SYMBOL(vma_set_file);
>   
> 

I proposed a while ago to get rid of VM_DENYWRITE completely:

https://lkml.kernel.org/r/20210423131640.20080-1-david@redhat.com

I haven't looked how much it still applies to current upstream, but 
maybe that might help cleaning up that code.

-- 
Thanks,

David / dhildenb