linux-kernel - Re: [PATCH 1/1] Fix permission checking by NFS client for open-create with mode 000

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Date:	Wed, 9 Jul 2014 18:17:44 -0400
From:	Trond Myklebust <trond.myklebust@...marydata.com>
To:	"Frank S. Filz" <ffilzlnx@...dspring.com>
Cc:	Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
	Linux Kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] Fix permission checking by NFS client for open-create
 with mode 000

Hi Frank

On Wed, Jul 9, 2014 at 5:54 PM, Frank S. Filz <ffilzlnx@...dspring.com> wrote:
> From: "Frank S. Filz" <ffilzlnx@...dspring.com>
>
> The NFS v4 client sends a COMPOUND with an OPEN and an ACCESS.
>
> The ACCESS is required to verify an open for read is actually
> allowed because RFC 3530 indicates OPEN for read only must succeed
> for an execute only file.
>
> The old code expected to have read access if the requested access
> was O_RDWR.
>
> We can expect the OPEN to properly permission check as long as
> the open is O_WRONLY or O_RDWR.
>
> Signed-off-by: Frank S. Filz <ffilzlnx@...dspring.com>
> ---
>  fs/nfs/nfs4proc.c | 25 ++++++++++++++++++++-----
>  1 file changed, 20 insertions(+), 5 deletions(-)
>
> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
> index 4bf3d97..9742054 100644
> --- a/fs/nfs/nfs4proc.c
> +++ b/fs/nfs/nfs4proc.c
> @@ -1966,15 +1966,30 @@ static int nfs4_opendata_access(struct rpc_cred *cred,
>                 return 0;
>
>         mask = 0;
> -       /* don't check MAY_WRITE - a newly created file may not have
> -        * write mode bits, but POSIX allows the creating process to write.
> -        * use openflags to check for exec, because fmode won't
> -        * always have FMODE_EXEC set when file open for exec. */
> +       /* Don't trust the permission check on OPEN if open for exec or for
> +        * read only. Since FMODE_EXEC doesn't go across the wire, the server
> +        * has no way to distinguish between an open to read an executable file
> +        * and an open to read a readable file. Write access is properly checked
> +        * and permission SHOULD always be granted if the file was created as a
> +        * result of this OPEN, no matter what mode the file was created with.
> +        *
> +        * NOTE: If the case of a OPEN CREATE READ-ONLY with a mode that does
> +        *       not allow read access, this test will produce an incorrect
> +        *       EACCES error.
> +        */
>         if (openflags & __FMODE_EXEC) {
>                 /* ONLY check for exec rights */
>                 mask = MAY_EXEC;
> -       } else if (fmode & FMODE_READ)
> +       } else if (!(fmode & FMODE_WRITE)) {
> +               /* In case the file was execute only, check for read permission
> +                * ONLY if write access was not requested. It is expected that
> +                * an OPEN for write will fail if the file is execute only.
> +                * Note that if the file was newly created, the fmode SHOULD
> +                * include FMODE_WRITE, especially if the file will be created
> +                * with a restrictive mode.
> +                */
>                 mask = MAY_READ;
> +       }

This looks wrong. AFAICS it will allow you to open an existing file
which has -wx permissions (i.e. no read permissions) for O_RDWR. That
should not be permitted under POSIX rules.

>
>         cache.cred = cred;
>         cache.jiffies = jiffies;
> --
> 1.8.3.1
>



-- 
Trond Myklebust

Linux NFS client maintainer, PrimaryData

trond.myklebust@...marydata.com
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