Re: [PATCH 1/1] selinux,smack: don't bypass permissions check in inode_setsecctx hook

[Chuck Lever <chuck.lever@oracle.com>]

On Wed, Aug 28, 2024 at 03:51:29PM -0400, Scott Mayhew wrote:
> Marek Gresko reports that the root user on an NFS client is able to
> change the security labels on files on an NFS filesystem that is
> exported with root squashing enabled.
> 
> The end of the kerneldoc comment for __vfs_setxattr_noperm() states:
> 
>  *  This function requires the caller to lock the inode's i_mutex before it
>  *  is executed. It also assumes that the caller will make the appropriate
>  *  permission checks.
> 
> nfsd_setattr() does do permissions checking via fh_verify() and
> nfsd_permission(), but those don't do all the same permissions checks
> that are done by security_inode_setxattr() and its related LSM hooks do.
> 
> Since nfsd_setattr() is the only consumer of security_inode_setsecctx(),
> simplest solution appears to be to replace the call to
> __vfs_setxattr_noperm() with a call to __vfs_setxattr_locked().  This
> fixes the above issue and has the added benefit of causing nfsd to
> recall conflicting delegations on a file when a client tries to change
> its security label.
> 
> Reported-by: Marek Gresko <marek.gresko@protonmail.com>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=218809
> Signed-off-by: Scott Mayhew <smayhew@redhat.com>
> ---
>  security/selinux/hooks.c   | 4 ++--
>  security/smack/smack_lsm.c | 4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index bfa61e005aac..400eca4ad0fb 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -6660,8 +6660,8 @@ static int selinux_inode_notifysecctx(struct inode *inode, void *ctx, u32 ctxlen
>   */
>  static int selinux_inode_setsecctx(struct dentry *dentry, void *ctx, u32 ctxlen)
>  {
> -	return __vfs_setxattr_noperm(&nop_mnt_idmap, dentry, XATTR_NAME_SELINUX,
> -				     ctx, ctxlen, 0);
> +	return __vfs_setxattr_locked(&nop_mnt_idmap, dentry, XATTR_NAME_SELINUX,
> +				     ctx, ctxlen, 0, NULL);
>  }
>  
>  static int selinux_inode_getsecctx(struct inode *inode, void **ctx, u32 *ctxlen)
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 4164699cd4f6..002a1b9ed83a 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -4880,8 +4880,8 @@ static int smack_inode_notifysecctx(struct inode *inode, void *ctx, u32 ctxlen)
>  
>  static int smack_inode_setsecctx(struct dentry *dentry, void *ctx, u32 ctxlen)
>  {
> -	return __vfs_setxattr_noperm(&nop_mnt_idmap, dentry, XATTR_NAME_SMACK,
> -				     ctx, ctxlen, 0);
> +	return __vfs_setxattr_locked(&nop_mnt_idmap, dentry, XATTR_NAME_SMACK,
> +				     ctx, ctxlen, 0, NULL);
>  }
>  
>  static int smack_inode_getsecctx(struct inode *inode, void **ctx, u32 *ctxlen)
> -- 
> 2.46.0
> 

Nice, thorough work, Scott.

Reviewed-by: Chuck Lever <chuck.lever@oracle.com>

-- 
Chuck Lever