NFSD: Map flex file layout IDs through the request's user namespace

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(-)

fs/nfsd/flexfilelayout.c +0 -0

index 9f532418cac8..c6e6b9106a83 100644
@@ -15,6 +15,7 @@

 #include "nfserr.h"
 #include "flexfilelayoutxdr.h"
+#include "auth.h"
 #include "pnfs.h"
 #include "vfs.h"

fs/nfsd/flexfilelayout.c +1 -1

@@ -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;

fs/nfsd/flexfilelayout.c +0 -0

@@ -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;

fs/nfsd/flexfilelayout.c +10 -7

-	/* 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)

fs/nfsd/flexfilelayoutxdr.c +0 -0

index 97d8a28dd3a0..c12bb7c371b0 100644
@@ -33,8 +33,8 @@ nfsd4_ff_encode_layoutget(struct xdr_stream *xdr,

  fh_len = 4 + xdr_align_size(fl->fh.size);

fs/nfsd/flexfilelayoutxdr.c +2 -2

-	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

fs/nfsd/flexfilelayoutxdr.h +3 -3

index 6d5a1066a903..3e1876d49db2 100644
@@ -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