Re: [PATCH v2 4/4] vfs: don't allow most setxattr to immutable files

[Amir Goldstein <amir73il@xxxxxxxxx>]

On Mon, Jul 1, 2019 at 7:31 PM Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote:
>
> From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
>
> The chattr manpage has this to say about immutable files:
>
> "A file with the 'i' attribute cannot be modified: it cannot be deleted
> or renamed, no link can be created to this file, most of the file's
> metadata can not be modified, and the file can not be opened in write
> mode."
>
> However, we don't actually check the immutable flag in the setattr code,
> which means that we can update inode flags and project ids and extent
> size hints on supposedly immutable files.  Therefore, reject setflags
> and fssetxattr calls on an immutable file if the file is immutable and
> will remain that way.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> ---
> v2: use memcmp instead of open coding a bunch of checks


Thanks,

Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>


> ---
>  fs/inode.c |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/fs/inode.c b/fs/inode.c
> index cf07378e5731..31f694e405fe 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2214,6 +2214,14 @@ int vfs_ioc_setflags_prepare(struct inode *inode, unsigned int oldflags,
>             !capable(CAP_LINUX_IMMUTABLE))
>                 return -EPERM;
>
> +       /*
> +        * We aren't allowed to change any other flags if the immutable flag is
> +        * already set and is not being unset.
> +        */
> +       if ((oldflags & FS_IMMUTABLE_FL) && (flags & FS_IMMUTABLE_FL) &&
> +           oldflags != flags)
> +               return -EPERM;
> +
>         /*
>          * Now that we're done checking the new flags, flush all pending IO and
>          * dirty mappings before setting S_IMMUTABLE on an inode via
> @@ -2284,6 +2292,15 @@ int vfs_ioc_fssetxattr_check(struct inode *inode, const struct fsxattr *old_fa,
>             !(S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode)))
>                 return -EINVAL;
>
> +       /*
> +        * We aren't allowed to change any fields if the immutable flag is
> +        * already set and is not being unset.
> +        */
> +       if ((old_fa->fsx_xflags & FS_XFLAG_IMMUTABLE) &&
> +           (fa->fsx_xflags & FS_XFLAG_IMMUTABLE) &&
> +           memcmp(fa, old_fa, offsetof(struct fsxattr, fsx_pad)))
> +               return -EPERM;
> +
>         /* Extent size hints of zero turn off the flags. */
>         if (fa->fsx_extsize == 0)
>                 fa->fsx_xflags &= ~(FS_XFLAG_EXTSIZE | FS_XFLAG_EXTSZINHERIT);