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Re: [PATCH v7 6/7] xfs: add capable check to free eofblocks ioctl

To: Dwight Engen <dwight.engen@xxxxxxxxxx>
Subject: Re: [PATCH v7 6/7] xfs: add capable check to free eofblocks ioctl
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Tue, 30 Jul 2013 08:33:39 -0400
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20130729230705.128e4509@xxxxxxxxxx>
References: <20130729230705.128e4509@xxxxxxxxxx>
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7
On 07/29/2013 11:07 PM, Dwight Engen wrote:
> Check for CAP_SYS_ADMIN since the caller can truncate preallocated
> blocks from files they do not own nor have write access to. A more
> fine grained access check was considered: require the caller to
> specify their own uid/gid and to use inode_permission to check for
> write, but this would not catch the case of an inode not reachable
> via path traversal from the callers mount namespace.
> Add check for read-only filesystem to free eofblocks ioctl.
> Signed-off-by: Dwight Engen <dwight.engen@xxxxxxxxxx>
> ---
>  fs/xfs/xfs_ioctl.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index 6e72eff..b1990ac 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -1613,6 +1613,12 @@ xfs_file_ioctl(
>               struct xfs_fs_eofblocks eofb;
>               struct xfs_eofblocks keofb;
> +             if (!capable(CAP_SYS_ADMIN))
> +                     return -EPERM;
> +

I see that we aren't using XFS_ERROR() on the EPERM returns in
xfs_file_ioctl(), but I see it used for other capability checks
elsewhere (i.e., down in xfs_growfs_data()). Perhaps somebody can chime
in as to the reasoning for that..? I guess it could be that we wouldn't
want to fire a BUG() at the interface point (ioctl()) on debug kernels
for every time a user attempts an operation they don't have the ability
to perform (e.g., we should notice on internal failures, not when
userspace sends us something wrong).

> +             if (IS_RDONLY(inode))
> +                     return -XFS_ERROR(EROFS);
> +

This should probably be consistent with the other read-only checks in
the ioctl code and check the xfs_mount structure:

        if (mp->m_flags & XFS_MOUNT_RDONLY)
                return -XFS_ERROR(EROFS);


>               if (copy_from_user(&eofb, arg, sizeof(eofb)))
>                       return -XFS_ERROR(EFAULT);

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