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Re: [PATCH] nfsd: revoking of suid/sgid bits after chown() in a consiste

To: Stanislav Kholmanskikh <stanislav.kholmanskikh@xxxxxxxxxx>
Subject: Re: [PATCH] nfsd: revoking of suid/sgid bits after chown() in a consistent way
From: "J. Bruce Fields" <bfields@xxxxxxxxxxxx>
Date: Thu, 12 Dec 2013 11:01:28 -0500
Cc: linux-nfs@xxxxxxxxxxxxxxx, vasily.isaenko@xxxxxxxxxx, hch@xxxxxxxxxxxxx, xfs@xxxxxxxxxxx, sprabhu@xxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1386756996-28083-1-git-send-email-stanislav.kholmanskikh@xxxxxxxxxx>
References: <20131206204747.GB12613@xxxxxxxxxxxx> <1386756996-28083-1-git-send-email-stanislav.kholmanskikh@xxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
Thanks, applying for 3.14.--b.

On Wed, Dec 11, 2013 at 02:16:36PM +0400, Stanislav Kholmanskikh wrote:
> There is an inconsistency in the handling of SUID/SGID file
> bits after chown() between NFS and other local file systems.
> 
> Local file systems (for example, ext3, ext4, xfs, btrfs) revoke
> SUID/SGID bits after chown() on a regular file even if
> the owner/group of the file has not been changed:
> 
> ~# touch file; chmod ug+s file; chmod u+x file
> ~# ls -l file
> -rwsr-Sr-- 1 root root 0 Dec  6 04:49 file
> ~# chown root file; ls -l file
> -rwxr-Sr-- 1 root root 0 Dec  6 04:49 file
> 
> but NFS doesn't do that:
> 
> ~# touch file; chmod ug+s file; chmod u+x file
> ~# ls -l file
> -rwsr-Sr-- 1 root root 0 Dec  6 04:49 file
> ~# chown root file; ls -l file
> -rwsr-Sr-- 1 root root 0 Dec  6 04:49 file
> 
> NFS does that only if the owner/group has been changed:
> 
> ~# touch file; chmod ug+s file; chmod u+x file
> ~# ls -l file
> -rwsr-Sr-- 1 root root 0 Dec  6 05:02 file
> ~# chown bin file; ls -l file
> -rwxr-Sr-- 1 bin root 0 Dec  6 05:02 file
> 
> See: http://pubs.opengroup.org/onlinepubs/9699919799/functions/chown.html
> 
>  "If the specified file is a regular file, one or more of
>  the S_IXUSR, S_IXGRP, or S_IXOTH bits of the file mode are set,
>  and the process has appropriate privileges, it is
>  implementation-defined whether the set-user-ID and set-group-ID
>  bits are altered."
> 
> So both variants are acceptable by POSIX.
> 
> This patch makes NFS to behave like local file systems.
> 
> Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@xxxxxxxxxx>
> ---
>  fs/nfsd/vfs.c |    3 +--
>  1 files changed, 1 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 72cb28e..8226991 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -348,8 +348,7 @@ nfsd_sanitize_attrs(struct inode *inode, struct iattr 
> *iap)
>  
>       /* Revoke setuid/setgid on chown */
>       if (!S_ISDIR(inode->i_mode) &&
> -         (((iap->ia_valid & ATTR_UID) && !uid_eq(iap->ia_uid, inode->i_uid)) 
> ||
> -          ((iap->ia_valid & ATTR_GID) && !gid_eq(iap->ia_gid, 
> inode->i_gid)))) {
> +         ((iap->ia_valid & ATTR_UID) || (iap->ia_valid & ATTR_GID))) {
>               iap->ia_valid |= ATTR_KILL_PRIV;
>               if (iap->ia_valid & ATTR_MODE) {
>                       /* we're setting mode too, just clear the s*id bits */
> -- 
> 1.7.1
> 

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