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SubjectRe: [PATCH 1/1] Fix permission checking by NFS client for open-create with mode 000
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Hi Frank

On Wed, Jul 9, 2014 at 5:54 PM, Frank S. Filz <ffilzlnx@mindspring.com> wrote:
> From: "Frank S. Filz" <ffilzlnx@mindspring.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@mindspring.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@primarydata.com


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