Re: [PATCH] ovl: fix mmap denywrite

[Christian König <christian.koenig@xxxxxxx>]

Am 22.06.21 um 14:30 schrieb Miklos Szeredi:
> Overlayfs did not honor positive i_writecount on realfile for VM_DENYWRITE
> mappings.  Similarly negative i_mmap_writable counts were ignored for
> VM_SHARED mappings.
>
> Fix by making vma_set_file() switch the temporary counts obtained and
> released by mmap_region().

Mhm, I don't fully understand the background but that looks like 
something specific to overlayfs to me.

So why are you changing the common helper?

Thanks,
Christian.

> Reported-by: Chengguang Xu <cgxu519@xxxxxxxxxxxx>
> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> ---
>   fs/overlayfs/file.c |    4 +++-
>   include/linux/mm.h  |    1 +
>   mm/mmap.c           |    2 +-
>   mm/util.c           |   38 +++++++++++++++++++++++++++++++++++++-
>   4 files changed, 42 insertions(+), 3 deletions(-)
>
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -430,7 +430,9 @@ 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_checkwrite(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
> @@ -2751,6 +2751,7 @@ static inline void vma_set_page_prot(str
>   #endif
>   
>   void vma_set_file(struct vm_area_struct *vma, struct file *file);
> +int vma_set_file_checkwrite(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
> @@ -1809,6 +1809,7 @@ 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;
>   
> @@ -1870,7 +1871,6 @@ 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 +314,48 @@ 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_checkwrite(struct vm_area_struct *vma, struct file *file)
>   {
> +	vm_flags_t vm_flags = vma->vm_flags;
> +	int err = 0;
> +
>   	/* Changing an anonymous vma with this is illegal */
>   	get_file(file);
> +
> +	/* Get temporary denial counts on replacement */
> +	if (vm_flags & VM_DENYWRITE) {
> +		err = deny_write_access(file);
> +		if (err)
> +			goto out_put;
> +	}
> +	if (vm_flags & VM_SHARED) {
> +		err = mapping_map_writable(file->f_mapping);
> +		if (err)
> +			goto out_allow;
> +	}
> +
>   	swap(vma->vm_file, file);
> +
> +	/* Undo temporary denial counts on replaced */
> +	if (vm_flags & VM_SHARED)
> +		mapping_unmap_writable(file->f_mapping);
> +out_allow:
> +	if (vm_flags & VM_DENYWRITE)
> +		allow_write_access(file);
> +out_put:
>   	fput(file);
> +	return err;
> +}
> +EXPORT_SYMBOL(vma_set_file_checkwrite);
> +
> +/*
> + * Change backing file, only valid to use during initial VMA setup.
> + */
> +void vma_set_file(struct vm_area_struct *vma, struct file *file)
> +{
> +	int err = vma_set_file_checkwrite(vma, file);
> +
> +	WARN_ON_ONCE(err);
>   }
>   EXPORT_SYMBOL(vma_set_file);
>