---- 在 星期二, 2021-06-22 20:30:04 Miklos Szeredi <miklos@xxxxxxxxxx> 撰写 ----
> 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().
>
> 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;
I'm afraid that it may affect other overlayfs instances which share lower layers(no upper),
so could we just check those permissions for upper layer?
>
> 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;
I'm not sure the behavior of changing vma_file is always safe for vma merging case.
In vma merging case, before go to tag 'unmap_writable' the reference of vma->vm_file will be released by fput().
For overlayfs, it probably safe because overlayfs file will get another reference for lower/upper file.
Thanks,
Chengguang Xu
> 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);
>
>
>
>
