reg-files: Do not try to linkat with wrong user

We link files to each other at restore time to restore
unlinked paths. Kernel has strange secutiry restrictions
about linkat we use. If the fsuid of the caller doesn't
equals the uid of the file and the file is not "safe"
one, then only global CAP_CHOWN will be allowed to link().

This brings problems in user namespaces -- uns root is
not allowed to linkat any file, unlike global root.

Fortunately, we can change the fsuid temporarily and
still linkat the file we want. Hopefully this hack will
go away some day soon, when the kernel will have saner
checks for linkat capabilities.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>

files-reg.c

 #include <stdlib.h>
 #include <unistd.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <string.h>
 #include <sys/mman.h>
 #include <sys/types.h>
@@ -33,6 +34,8 @@
 #include "files-reg.h"
 #include "plugin.h"
 
+int setfsuid(uid_t fsuid);
+
 /*
  * Ghost files are those not visible from the FS. Dumping them is
  * nasty and the only way we have -- just carry its contents with
@@ -187,6 +190,7 @@ static int open_remap_ghost(struct reg_file_info *rfi,
 	gf->id = rfe->remap_id;
 	gf->remap.users = 0;
 	gf->remap.is_dir = S_ISDIR(gfe->mode);
+	gf->remap.owner = gfe->uid;
 	list_add_tail(&gf->list, &ghost_files);
 gf_found:
 	rfi->remap = &gf->remap;
@@ -208,6 +212,7 @@ static int open_remap_linked(struct reg_file_info *rfi,
 	struct file_remap *rm;
 	struct file_desc *rdesc;
 	struct reg_file_info *rrfi;
+	uid_t owner = -1;
 
 	rdesc = find_file_desc_raw(FD_TYPES__REG, rfe->remap_id);
 	if (!rdesc) {
@@ -222,9 +227,23 @@ static int open_remap_linked(struct reg_file_info *rfi,
 	rrfi = container_of(rdesc, struct reg_file_info, d);
 	pr_info("Remapped %s -> %s\n", rfi->path, rrfi->path);
 
+	if (root_ns_mask & CLONE_NEWUSER) {
+		int rfd;
+		struct stat st;
+
+		rfd = mntns_get_root_by_mnt_id(rfi->rfe->mnt_id);
+		if (fstatat(rfd, rrfi->path, &st, AT_SYMLINK_NOFOLLOW)) {
+			pr_perror("Can't get owner of link remap %s", rrfi->path);
+			return -1;
+		}
+
+		owner = st.st_uid;
+	}
+
 	rm->path = rrfi->path;
 	rm->users = 0;
 	rm->is_dir = false;
+	rm->owner = owner;
 	rm->mnt_id = rfi->rfe->mnt_id;
 	rfi->remap = rm;
 	return 0;
@@ -857,6 +876,47 @@ static void convert_path_from_another_mp(char *src, char *dst, int dlen,
 				src + off);
 }
 
+static int linkat_hard(int odir, char *opath, int ndir, char *npath, uid_t owner)
+{
+	int ret, old_fsuid = -1;
+
+	if (root_ns_mask & CLONE_NEWUSER)
+		/*
+		 * Kernel has strange secutiry restrictions about
+		 * linkat. If the fsuid of the caller doesn't equals
+		 * the uid of the file and the file is not "safe"
+		 * one, then only global CAP_CHOWN will be allowed
+		 * to link().
+		 *
+		 * Next, when we're in user namespace we're ns root,
+		 * but not global CAP_CHOWN. Thus, even though we
+		 * ARE ns root, we will not be allowed to link() at
+		 * files that belong to regular users %)
+		 *
+		 * Fortunately, the setfsuid() requires ns-level
+		 * CAP_SETUID which we have.
+		 */
+
+		old_fsuid = setfsuid(owner);
+
+	ret = linkat(odir, opath, ndir, npath, 0);
+	if (ret < 0)
+		pr_perror("Can't link %s -> %s", opath, npath);
+
+	if (root_ns_mask & CLONE_NEWUSER) {
+		setfsuid(old_fsuid);
+		if (setfsuid(-1) != old_fsuid)
+			pr_warn("Failed to restore old fsuid!\n");
+			/*
+			 * Don't fail here. We still have chances to run till
+			 * the pie/restorer, and if _this_ guy fails to set
+			 * the proper fsuid, then we'll abort the restore.
+			 */
+	}
+
+	return ret;
+}
+
 /*
  * This routine properly resolves d's path handling ghost/link-remaps.
  * The open_cb is a routine that does actual open, it differs for
@@ -909,7 +969,7 @@ out:
 	mntns_root = mntns_get_root_fd(tmi->nsid);
 
 out_root:
-	return linkat(mntns_root, rpath, mntns_root, path, 0);
+	return linkat_hard(mntns_root, rpath, mntns_root, path, rfi->remap->owner);
 }
 
 int open_path(struct file_desc *d,
@@ -936,7 +996,7 @@ int open_path(struct file_desc *d,
 			static char tmp_path[PATH_MAX];
 
 			if (errno != EEXIST) {
-				pr_perror("Can't link %s -> %s", rfi->path,
+				pr_err("Can't link %s -> %s", rfi->path,
 						rfi->remap->path);
 				return -1;
 			}

include/files-reg.h

@@ -16,6 +16,7 @@ struct file_remap {
 	bool is_dir;
 	int  mnt_id;
 	unsigned int users;
+	uid_t owner;
 };
 
 struct reg_file_info {

test/zdtm.sh

@@ -252,9 +252,6 @@ generate_test_list()
 		ns/static/sched_prio00
 		ns/static/sched_policy00
 		ns/static/fanotify00
-		ns/static/fifo-ghost
-		ns/static/unlink_fifo
-		ns/static/unlink_fifo_wronly
 		ns/static/dumpable02
 		ns/static/deleted_dev
 		ns/static/tempfs