nfsd: simplify fh_verify access checks

All nfsd security depends on the security checks in fh_verify, and
especially on nfsd_setuser().

It therefore bothers me that the nfsd_setuser call may be made from
three different places, depending on whether the filehandle has already
been mapped to a dentry, and on whether subtreechecking is in force.

Instead, make an unconditional call in fh_verify(), so it's trivial to
verify that the call always occurs.

That leaves us with a redundant nfsd_setuser() call in the subtreecheck
case--it needs the correct user set earlier in order to check execute
permissions on the path to this filehandle--but I'm willing to accept
that minor inefficiency in the subtreecheck case in return for more
straightforward permission checking.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

1 files changed, 21 insertions, 29 deletions

diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
index d0d8a217a3e..a77efb8c224 100644
--- a/fs/nfsd/nfsfh.c
+++ b/fs/nfsd/nfsfh.c
@@ -233,14 +233,6 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst *rqstp, struct svc_fh *fhp)
                 goto out;
         }
 
-        if (exp->ex_flags & NFSEXP_NOSUBTREECHECK) {
-                error = nfsd_setuser_and_check_port(rqstp, exp);
-                if (error) {
-                        dput(dentry);
-                        goto out;
-                }
-        }
-
         if (S_ISDIR(dentry->d_inode->i_mode) &&
                         (dentry->d_flags & DCACHE_DISCONNECTED)) {
                 printk("nfsd: find_fh_dentry returned a DISCONNECTED directory: %s/%s\n",
@@ -295,28 +287,28 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, int type, int access)
                 error = nfsd_set_fh_dentry(rqstp, fhp);
                 if (error)
                         goto out;
-                dentry = fhp->fh_dentry;
-                exp = fhp->fh_export;
-        } else {
-                /*
-                 * just rechecking permissions
-                 * (e.g. nfsproc_create calls fh_verify, then nfsd_create
-                 * does as well)
-                 */
-                dprintk("nfsd: fh_verify - just checking\n");
-                dentry = fhp->fh_dentry;
-                exp = fhp->fh_export;
-                /*
-                 * Set user creds for this exportpoint; necessary even
-                 * in the "just checking" case because this may be a
-                 * filehandle that was created by fh_compose, and that
-                 * is about to be used in another nfsv4 compound
-                 * operation.
-                 */
-                error = nfsd_setuser_and_check_port(rqstp, exp);
-                if (error)
-                        goto out;
         }
+        dentry = fhp->fh_dentry;
+        exp = fhp->fh_export;
+        /*
+         * We still have to do all these permission checks, even when
+         * fh_dentry is already set:
+         *      - fh_verify may be called multiple times with different
+         *        "access" arguments (e.g. nfsd_proc_create calls
+         *        fh_verify(...,NFSD_MAY_EXEC) first, then later (in
+         *        nfsd_create) calls fh_verify(...,NFSD_MAY_CREATE).
+         *      - in the NFSv4 case, the filehandle may have been filled
+         *        in by fh_compose, and given a dentry, but further
+         *        compound operations performed with that filehandle
+         *        still need permissions checks.  In the worst case, a
+         *        mountpoint crossing may have changed the export
+         *        options, and we may now need to use a different uid
+         *        (for example, if different id-squashing options are in
+         *        effect on the new filesystem).
+         */
+        error = nfsd_setuser_and_check_port(rqstp, exp);
+        if (error)
+                goto out;
 
         error = nfsd_mode_check(rqstp, dentry->d_inode->i_mode, type);
         if (error)