Re: [PATCH] NFSD: Map flex file layout IDs through the request's user namespace

On Mon, 2026-07-20 at 12:37 -0400, Chuck Lever wrote:
> nfsd4_ff_proc_layoutget() and nfsd4_ff_encode_layoutget() translate
> the file's owner and group with init_user_ns, but every other identity
> nfsd places on the wire goes through nfsd_user_namespace(). When the
> transport carries a credential from a non-initial user namespace, the
> flex file layout reports host-global IDs. The client copies those IDs
> into the AUTH_SYS credential it presents to the data server, and
> svcauth_unix_accept() resolves that credential in the transport's
> namespace, so data server I/O runs under an identity unrelated to the
> file's owner.
> 
> Switching to the request's namespace introduces a second hazard.
> from_kuid() returns (uid_t)-1 when the target namespace has no
> mapping for the owner, and the IOMODE_READ arm adds one to that
> result to derive an identity for which the data server denies
> writes. The addition would wrap to zero, handing the client uid 0
> instead of an identity distinct from the owner.
> 
> Translate both IDs in nfsd4_ff_proc_layoutget(), which has the
> svc_rqst, and carry the wire values in struct pnfs_ff_layout.
> from_kuid_munged() substitutes overflowuid for an unmapped owner and
> thus never returns (uid_t)-1, so the increment cannot wrap to zero.
> 
> Fixes: 9b9960a0ca47 ("nfsd: Add a super simple flex file server")
> Reported-by: sashiko-bot <[email protected]>
> Closes: https://sashiko.dev/#/patchset/[email protected]?part=3
> Signed-off-by: Chuck Lever <[email protected]>
> ---
>  fs/nfsd/flexfilelayout.c    | 20 ++++++++++++--------
>  fs/nfsd/flexfilelayoutxdr.c |  4 ++--
>  fs/nfsd/flexfilelayoutxdr.h |  5 +++--
>  3 files changed, 17 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/nfsd/flexfilelayout.c b/fs/nfsd/flexfilelayout.c
> index 9f532418cac8..c6e6b9106a83 100644
> --- a/fs/nfsd/flexfilelayout.c
> +++ b/fs/nfsd/flexfilelayout.c
> @@ -15,6 +15,7 @@
>  
>  #include "nfserr.h"
>  #include "flexfilelayoutxdr.h"
> +#include "auth.h"
>  #include "pnfs.h"
>  #include "vfs.h"
>  
> @@ -24,10 +25,10 @@ static __be32
>  nfsd4_ff_proc_layoutget(struct svc_rqst *rqstp, struct inode *inode,
>  		const struct svc_fh *fhp, struct nfsd4_layoutget *args)
>  {
> +	struct user_namespace *userns = nfsd_user_namespace(rqstp);
>  	struct nfsd4_layout_seg *seg = &args->lg_seg;
>  	u32 device_generation = 0;
>  	int error;
> -	uid_t u;
>  
>  	struct pnfs_ff_layout *fl;
>  
> @@ -50,13 +51,16 @@ nfsd4_ff_proc_layoutget(struct svc_rqst *rqstp, struct inode *inode,
>  	fl->flags = FF_FLAGS_NO_LAYOUTCOMMIT | FF_FLAGS_NO_IO_THRU_MDS |
>  		    FF_FLAGS_NO_READ_IO;
>  
> -	/* Do not allow a IOMODE_READ segment to have write pemissions */
> -	if (seg->iomode == IOMODE_READ) {
> -		u = from_kuid(&init_user_ns, inode->i_uid) + 1;
> -		fl->uid = make_kuid(&init_user_ns, u);
> -	} else
> -		fl->uid = inode->i_uid;
> -	fl->gid = inode->i_gid;
> +	fl->uid = from_kuid_munged(userns, inode->i_uid);
> +	fl->gid = from_kgid_munged(userns, inode->i_gid);
> +
> +	/*
> +	 * Do not allow an IOMODE_READ segment to have write permissions.
> +	 * The group is left intact so group-readable files stay readable;
> +	 * nfsd_setuser() squashes an unmapped uid to the export's anon ID.
> +	 */
> +	if (seg->iomode == IOMODE_READ)
> +		fl->uid++;
>  
>  	error = nfsd4_set_deviceid(&fl->deviceid, fhp, device_generation);
>  	if (error)
> diff --git a/fs/nfsd/flexfilelayoutxdr.c b/fs/nfsd/flexfilelayoutxdr.c
> index 97d8a28dd3a0..c12bb7c371b0 100644
> --- a/fs/nfsd/flexfilelayoutxdr.c
> +++ b/fs/nfsd/flexfilelayoutxdr.c
> @@ -33,8 +33,8 @@ nfsd4_ff_encode_layoutget(struct xdr_stream *xdr,
>  
>  	fh_len = 4 + xdr_align_size(fl->fh.size);
>  
> -	uid.len = sprintf(uid.buf, "%u", from_kuid(&init_user_ns, fl->uid));
> -	gid.len = sprintf(gid.buf, "%u", from_kgid(&init_user_ns, fl->gid));
> +	uid.len = sprintf(uid.buf, "%u", fl->uid);
> +	gid.len = sprintf(gid.buf, "%u", fl->gid);
>  
>  	/* data server entry: deviceid + efficiency + stateid + fh list +
>  	 * user + group + flags + stats_collect_hint
> diff --git a/fs/nfsd/flexfilelayoutxdr.h b/fs/nfsd/flexfilelayoutxdr.h
> index 6d5a1066a903..3e1876d49db2 100644
> --- a/fs/nfsd/flexfilelayoutxdr.h
> +++ b/fs/nfsd/flexfilelayoutxdr.h
> @@ -35,8 +35,9 @@ struct pnfs_ff_device_addr {
>  struct pnfs_ff_layout {
>  	u32				flags;
>  	u32				stats_collect_hint;
> -	kuid_t				uid;
> -	kgid_t				gid;
> +	/* Values to encode; nfsd4_ff_proc_layoutget() has mapped these */
> +	u32				uid;
> +	u32				gid;
>  	struct nfsd4_deviceid		deviceid;
>  	stateid_t			stateid;
>  	struct nfs_fh			fh;
> --
> 2.54.0

Reviewed-by: Jeff Layton <[email protected]>