virtiofsd: Fix xattr operations

Current virtiofsd has problems about xattr operations and
they does not work properly for directory/symlink/special file.

The fundamental cause is that virtiofsd uses openat() + f...xattr()
systemcalls for xattr operation but we should not open symlink/special
file in the daemon. Therefore the function is restricted.

Fix this problem by:
 1. during setup of each thread, call unshare(CLONE_FS)
 2. in xattr operations (i.e. lo_getxattr), if inode is not a regular
    file or directory, use fchdir(proc_loot_fd) + ...xattr() +
    fchdir(root.fd) instead of openat() + f...xattr()

    (Note: for a regular file/directory openat() + f...xattr()
     is still used for performance reason)

With this patch, xfstests generic/062 passes on virtiofs.

This fix is suggested by Miklos Szeredi and Stefan Hajnoczi.
The original discussion can be found here:
  https://www.redhat.com/archives/virtio-fs/2019-October/msg00046.html

Signed-off-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
Message-Id: <20200227055927.24566-3-misono.tomohiro@jp.fujitsu.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tools/virtiofsd/fuse_virtio.c

@@ -426,6 +426,8 @@ err:
     return ret;
 }
 
+static __thread bool clone_fs_called;
+
 /* Process one FVRequest in a thread pool */
 static void fv_queue_worker(gpointer data, gpointer user_data)
 {
@@ -441,6 +443,17 @@ static void fv_queue_worker(gpointer data, gpointer user_data)
 
     assert(se->bufsize > sizeof(struct fuse_in_header));
 
+    if (!clone_fs_called) {
+        int ret;
+
+        /* unshare FS for xattr operation */
+        ret = unshare(CLONE_FS);
+        /* should not fail */
+        assert(ret == 0);
+
+        clone_fs_called = true;
+    }
+
     /*
      * An element contains one request and the space to send our response
      * They're spread over multiple descriptors in a scatter/gather set

tools/virtiofsd/passthrough_ll.c

@@ -123,7 +123,7 @@ struct lo_inode {
     pthread_mutex_t plock_mutex;
     GHashTable *posix_locks; /* protected by lo_inode->plock_mutex */
 
-    bool is_symlink;
+    mode_t filetype;
 };
 
 struct lo_cred {
@@ -695,7 +695,7 @@ static int utimensat_empty(struct lo_data *lo, struct lo_inode *inode,
     struct lo_inode *parent;
     char path[PATH_MAX];
 
-    if (inode->is_symlink) {
+    if (S_ISLNK(inode->filetype)) {
         res = utimensat(inode->fd, "", tv, AT_EMPTY_PATH);
         if (res == -1 && errno == EINVAL) {
             /* Sorry, no race free way to set times on symlink. */
@@ -928,7 +928,8 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
             goto out_err;
         }
 
-        inode->is_symlink = S_ISLNK(e->attr.st_mode);
+        /* cache only filetype */
+        inode->filetype = (e->attr.st_mode & S_IFMT);
 
         /*
          * One for the caller and one for nlookup (released in
@@ -1135,7 +1136,7 @@ static int linkat_empty_nofollow(struct lo_data *lo, struct lo_inode *inode,
     struct lo_inode *parent;
     char path[PATH_MAX];
 
-    if (inode->is_symlink) {
+    if (S_ISLNK(inode->filetype)) {
         res = linkat(inode->fd, "", dfd, name, AT_EMPTY_PATH);
         if (res == -1 && (errno == ENOENT || errno == EINVAL)) {
             /* Sorry, no race free way to hard-link a symlink. */
@@ -2189,12 +2190,6 @@ static void lo_getxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
     fuse_log(FUSE_LOG_DEBUG, "lo_getxattr(ino=%" PRIu64 ", name=%s size=%zd)\n",
              ino, name, size);
 
-    if (inode->is_symlink) {
-        /* Sorry, no race free way to getxattr on symlink. */
-        saverr = EPERM;
-        goto out;
-    }
-
     if (size) {
         value = malloc(size);
         if (!value) {
@@ -2203,12 +2198,25 @@ static void lo_getxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
     }
 
     sprintf(procname, "%i", inode->fd);
+    /*
+     * It is not safe to open() non-regular/non-dir files in file server
+     * unless O_PATH is used, so use that method for regular files/dir
+     * only (as it seems giving less performance overhead).
+     * Otherwise, call fchdir() to avoid open().
+     */
+    if (S_ISREG(inode->filetype) || S_ISDIR(inode->filetype)) {
         fd = openat(lo->proc_self_fd, procname, O_RDONLY);
         if (fd < 0) {
             goto out_err;
         }
-
         ret = fgetxattr(fd, name, value, size);
+    } else {
+        /* fchdir should not fail here */
+        assert(fchdir(lo->proc_self_fd) == 0);
+        ret = getxattr(procname, name, value, size);
+        assert(fchdir(lo->root.fd) == 0);
+    }
+
     if (ret == -1) {
         goto out_err;
     }
@@ -2262,12 +2270,6 @@ static void lo_listxattr(fuse_req_t req, fuse_ino_t ino, size_t size)
     fuse_log(FUSE_LOG_DEBUG, "lo_listxattr(ino=%" PRIu64 ", size=%zd)\n", ino,
              size);
 
-    if (inode->is_symlink) {
-        /* Sorry, no race free way to listxattr on symlink. */
-        saverr = EPERM;
-        goto out;
-    }
-
     if (size) {
         value = malloc(size);
         if (!value) {
@@ -2276,12 +2278,19 @@ static void lo_listxattr(fuse_req_t req, fuse_ino_t ino, size_t size)
     }
 
     sprintf(procname, "%i", inode->fd);
+    if (S_ISREG(inode->filetype) || S_ISDIR(inode->filetype)) {
         fd = openat(lo->proc_self_fd, procname, O_RDONLY);
         if (fd < 0) {
             goto out_err;
         }
-
         ret = flistxattr(fd, value, size);
+    } else {
+        /* fchdir should not fail here */
+        assert(fchdir(lo->proc_self_fd) == 0);
+        ret = listxattr(procname, value, size);
+        assert(fchdir(lo->root.fd) == 0);
+    }
+
     if (ret == -1) {
         goto out_err;
     }
@@ -2335,20 +2344,21 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
     fuse_log(FUSE_LOG_DEBUG, "lo_setxattr(ino=%" PRIu64
              ", name=%s value=%s size=%zd)\n", ino, name, value, size);
 
-    if (inode->is_symlink) {
-        /* Sorry, no race free way to setxattr on symlink. */
-        saverr = EPERM;
-        goto out;
-    }
-
     sprintf(procname, "%i", inode->fd);
-    fd = openat(lo->proc_self_fd, procname, O_RDWR);
+    if (S_ISREG(inode->filetype) || S_ISDIR(inode->filetype)) {
+        fd = openat(lo->proc_self_fd, procname, O_RDONLY);
         if (fd < 0) {
             saverr = errno;
             goto out;
         }
-
         ret = fsetxattr(fd, name, value, size, flags);
+    } else {
+        /* fchdir should not fail here */
+        assert(fchdir(lo->proc_self_fd) == 0);
+        ret = setxattr(procname, name, value, size, flags);
+        assert(fchdir(lo->root.fd) == 0);
+    }
+
     saverr = ret == -1 ? errno : 0;
 
 out:
@@ -2383,20 +2393,21 @@ static void lo_removexattr(fuse_req_t req, fuse_ino_t ino, const char *name)
     fuse_log(FUSE_LOG_DEBUG, "lo_removexattr(ino=%" PRIu64 ", name=%s)\n", ino,
              name);
 
-    if (inode->is_symlink) {
-        /* Sorry, no race free way to setxattr on symlink. */
-        saverr = EPERM;
-        goto out;
-    }
-
     sprintf(procname, "%i", inode->fd);
-    fd = openat(lo->proc_self_fd, procname, O_RDWR);
+    if (S_ISREG(inode->filetype) || S_ISDIR(inode->filetype)) {
+        fd = openat(lo->proc_self_fd, procname, O_RDONLY);
         if (fd < 0) {
             saverr = errno;
             goto out;
         }
-
         ret = fremovexattr(fd, name);
+    } else {
+        /* fchdir should not fail here */
+        assert(fchdir(lo->proc_self_fd) == 0);
+        ret = removexattr(procname, name);
+        assert(fchdir(lo->root.fd) == 0);
+    }
+
     saverr = ret == -1 ? errno : 0;
 
 out:
@@ -2796,7 +2807,7 @@ static void setup_root(struct lo_data *lo, struct lo_inode *root)
         exit(1);
     }
 
-    root->is_symlink = false;
+    root->filetype = S_IFDIR;
     root->fd = fd;
     root->key.ino = stat.st_ino;
     root->key.dev = stat.st_dev;

tools/virtiofsd/seccomp.c

@@ -41,6 +41,7 @@ static const int syscall_whitelist[] = {
     SCMP_SYS(exit),
     SCMP_SYS(exit_group),
     SCMP_SYS(fallocate),
+    SCMP_SYS(fchdir),
     SCMP_SYS(fchmodat),
     SCMP_SYS(fchownat),
     SCMP_SYS(fcntl),
@@ -62,7 +63,9 @@ static const int syscall_whitelist[] = {
     SCMP_SYS(getpid),
     SCMP_SYS(gettid),
     SCMP_SYS(gettimeofday),
+    SCMP_SYS(getxattr),
     SCMP_SYS(linkat),
+    SCMP_SYS(listxattr),
     SCMP_SYS(lseek),
     SCMP_SYS(madvise),
     SCMP_SYS(mkdirat),
@@ -85,6 +88,7 @@ static const int syscall_whitelist[] = {
     SCMP_SYS(recvmsg),
     SCMP_SYS(renameat),
     SCMP_SYS(renameat2),
+    SCMP_SYS(removexattr),
     SCMP_SYS(rt_sigaction),
     SCMP_SYS(rt_sigprocmask),
     SCMP_SYS(rt_sigreturn),
@@ -98,10 +102,12 @@ static const int syscall_whitelist[] = {
     SCMP_SYS(setresuid32),
 #endif
     SCMP_SYS(set_robust_list),
+    SCMP_SYS(setxattr),
     SCMP_SYS(symlinkat),
     SCMP_SYS(time), /* Rarely needed, except on static builds */
     SCMP_SYS(tgkill),
     SCMP_SYS(unlinkat),
+    SCMP_SYS(unshare),
     SCMP_SYS(utimensat),
     SCMP_SYS(write),
     SCMP_SYS(writev),