filesystem_limit/snapshot_limit is incorrectly enforced against root

The filesystem_limit and snapshot_limit properties limit the number of
filesystems or snapshots that can be created below this dataset.
According to the manpage, "The limit is not enforced if the user is
allowed to change the limit."  Two types of users are allowed to change
the limit:

1. Those that have been delegated the `filesystem_limit` or
`snapshot_limit` permission, e.g. with
`zfs allow USER filesystem_limit DATASET`.  This works properly.

2. A user with elevated system privileges (e.g. root).  This does not
work - the root user will incorrectly get an error when trying to create
a snapshot/filesystem, if it exceeds the `_limit` property.

The problem is that `priv_policy_ns()` does not work if the `cred_t` is
not that of the current process.  This happens when
`dsl_enforce_ds_ss_limits()` is called in syncing context (as part of a
sync task's check func) to determine the permissions of the
corresponding user process.

This commit fixes the issue by passing the `task_struct` (typedef'ed as
a `proc_t`) to syncing context, and then using `has_capability()` to
determine if that process is privileged.  Note that we still need to
pass the `cred_t` to syncing context so that we can check if the user
was delegated this permission with `zfs allow`.

This problem only impacts Linux.  Wrappers are added to FreeBSD but it
continues to use `priv_check_cred()`, which works on arbitrary `cred_t`.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
Closes #8226
Closes #10545 

1 files changed, 8 insertions, 6 deletions

diff --git a/module/zfs/dsl_dataset.c b/module/zfs/dsl_dataset.c
index 4986bc449..c04f47d36 100644
--- a/module/zfs/dsl_dataset.c
+++ b/module/zfs/dsl_dataset.c
@@ -1465,7 +1465,7 @@ dsl_dataset_snapshot_reserve_space(dsl_dataset_t *ds, dmu_tx_t *tx)
 
 int
 dsl_dataset_snapshot_check_impl(dsl_dataset_t *ds, const char *snapname,
-    dmu_tx_t *tx, boolean_t recv, uint64_t cnt, cred_t *cr)
+    dmu_tx_t *tx, boolean_t recv, uint64_t cnt, cred_t *cr, proc_t *proc)
 {
 	int error;
 	uint64_t value;
@@ -1510,7 +1510,7 @@ dsl_dataset_snapshot_check_impl(dsl_dataset_t *ds, const char *snapname,
 	 */
 	if (cnt != 0 && cr != NULL) {
 		error = dsl_fs_ss_limit_check(ds->ds_dir, cnt,
-		    ZFS_PROP_SNAPSHOT_LIMIT, NULL, cr);
+		    ZFS_PROP_SNAPSHOT_LIMIT, NULL, cr, proc);
 		if (error != 0)
 			return (error);
 	}
@@ -1611,7 +1611,7 @@ dsl_dataset_snapshot_check(void *arg, dmu_tx_t *tx)
 			if (error == 0) {
 				error = dsl_fs_ss_limit_check(ds->ds_dir, cnt,
 				    ZFS_PROP_SNAPSHOT_LIMIT, NULL,
-				    ddsa->ddsa_cr);
+				    ddsa->ddsa_cr, ddsa->ddsa_proc);
 				dsl_dataset_rele(ds, FTAG);
 			}
 
@@ -1649,7 +1649,7 @@ dsl_dataset_snapshot_check(void *arg, dmu_tx_t *tx)
 		if (error == 0) {
 			/* passing 0/NULL skips dsl_fs_ss_limit_check */
 			error = dsl_dataset_snapshot_check_impl(ds,
-			    atp + 1, tx, B_FALSE, 0, NULL);
+			    atp + 1, tx, B_FALSE, 0, NULL, NULL);
 			dsl_dataset_rele(ds, FTAG);
 		}
 
@@ -1927,6 +1927,7 @@ dsl_dataset_snapshot(nvlist_t *snaps, nvlist_t *props, nvlist_t *errors)
 	ddsa.ddsa_props = props;
 	ddsa.ddsa_errors = errors;
 	ddsa.ddsa_cr = CRED();
+	ddsa.ddsa_proc = curproc;
 
 	if (error == 0) {
 		error = dsl_sync_task(firstname, dsl_dataset_snapshot_check,
@@ -1974,7 +1975,7 @@ dsl_dataset_snapshot_tmp_check(void *arg, dmu_tx_t *tx)
 
 	/* NULL cred means no limit check for tmp snapshot */
 	error = dsl_dataset_snapshot_check_impl(ds, ddsta->ddsta_snapname,
-	    tx, B_FALSE, 0, NULL);
+	    tx, B_FALSE, 0, NULL, NULL);
 	if (error != 0) {
 		dsl_dataset_rele(ds, FTAG);
 		return (error);
@@ -3477,7 +3478,7 @@ dsl_dataset_promote_check(void *arg, dmu_tx_t *tx)
 
 	/* Check that there is enough space and limit headroom here */
 	err = dsl_dir_transfer_possible(origin_ds->ds_dir, hds->ds_dir,
-	    0, ss_mv_cnt, ddpa->used, ddpa->cr);
+	    0, ss_mv_cnt, ddpa->used, ddpa->cr, ddpa->proc);
 	if (err != 0)
 		goto out;
 
@@ -3904,6 +3905,7 @@ dsl_dataset_promote(const char *name, char *conflsnap)
 	ddpa.ddpa_clonename = name;
 	ddpa.err_ds = fnvlist_alloc();
 	ddpa.cr = CRED();
+	ddpa.proc = curproc;
 
 	error = dsl_sync_task(name, dsl_dataset_promote_check,
 	    dsl_dataset_promote_sync, &ddpa,