virtiofsd: passthrough_ll: create new files in caller's context

We need to create files in the caller's context. Otherwise after
creating a file, the caller might not be able to do file operations on
that file.

Changed effective uid/gid to caller's uid/gid, create file and then
switch back to uid/gid 0.

Use syscall(setresuid, ...) otherwise glibc does some magic to change EUID
in all threads, which is not what we want.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tools/virtiofsd/passthrough_ll.c

@@ -50,6 +50,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/file.h>
+#include <sys/syscall.h>
 #include <sys/xattr.h>
 #include <unistd.h>
 
@@ -83,6 +84,11 @@ struct lo_inode {
     uint64_t refcount; /* protected by lo->mutex */
 };
 
+struct lo_cred {
+    uid_t euid;
+    gid_t egid;
+};
+
 enum {
     CACHE_NEVER,
     CACHE_NORMAL,
@@ -383,6 +389,69 @@ static void lo_lookup(fuse_req_t req, fuse_ino_t parent, const char *name)
     }
 }
 
+/*
+ * On some archs, setres*id is limited to 2^16 but they
+ * provide setres*id32 variants that allow 2^32.
+ * Others just let setres*id do 2^32 anyway.
+ */
+#ifdef SYS_setresgid32
+#define OURSYS_setresgid SYS_setresgid32
+#else
+#define OURSYS_setresgid SYS_setresgid
+#endif
+
+#ifdef SYS_setresuid32
+#define OURSYS_setresuid SYS_setresuid32
+#else
+#define OURSYS_setresuid SYS_setresuid
+#endif
+
+/*
+ * Change to uid/gid of caller so that file is created with
+ * ownership of caller.
+ * TODO: What about selinux context?
+ */
+static int lo_change_cred(fuse_req_t req, struct lo_cred *old)
+{
+    int res;
+
+    old->euid = geteuid();
+    old->egid = getegid();
+
+    res = syscall(OURSYS_setresgid, -1, fuse_req_ctx(req)->gid, -1);
+    if (res == -1) {
+        return errno;
+    }
+
+    res = syscall(OURSYS_setresuid, -1, fuse_req_ctx(req)->uid, -1);
+    if (res == -1) {
+        int errno_save = errno;
+
+        syscall(OURSYS_setresgid, -1, old->egid, -1);
+        return errno_save;
+    }
+
+    return 0;
+}
+
+/* Regain Privileges */
+static void lo_restore_cred(struct lo_cred *old)
+{
+    int res;
+
+    res = syscall(OURSYS_setresuid, -1, old->euid, -1);
+    if (res == -1) {
+        fuse_log(FUSE_LOG_ERR, "seteuid(%u): %m\n", old->euid);
+        exit(1);
+    }
+
+    res = syscall(OURSYS_setresgid, -1, old->egid, -1);
+    if (res == -1) {
+        fuse_log(FUSE_LOG_ERR, "setegid(%u): %m\n", old->egid);
+        exit(1);
+    }
+}
+
 static void lo_mknod_symlink(fuse_req_t req, fuse_ino_t parent,
                              const char *name, mode_t mode, dev_t rdev,
                              const char *link)
@@ -391,12 +460,21 @@ static void lo_mknod_symlink(fuse_req_t req, fuse_ino_t parent,
     int saverr;
     struct lo_inode *dir = lo_inode(req, parent);
     struct fuse_entry_param e;
+    struct lo_cred old = {};
 
     saverr = ENOMEM;
 
+    saverr = lo_change_cred(req, &old);
+    if (saverr) {
+        goto out;
+    }
+
     res = mknod_wrapper(dir->fd, name, link, mode, rdev);
 
     saverr = errno;
+
+    lo_restore_cred(&old);
+
     if (res == -1) {
         goto out;
     }
@@ -794,26 +872,34 @@ static void lo_create(fuse_req_t req, fuse_ino_t parent, const char *name,
     struct lo_data *lo = lo_data(req);
     struct fuse_entry_param e;
     int err;
+    struct lo_cred old = {};
 
     if (lo_debug(req)) {
         fuse_log(FUSE_LOG_DEBUG, "lo_create(parent=%" PRIu64 ", name=%s)\n",
                  parent, name);
     }
 
-    fd = openat(lo_fd(req, parent), name, (fi->flags | O_CREAT) & ~O_NOFOLLOW,
-                mode);
-    if (fd == -1) {
-        return (void)fuse_reply_err(req, errno);
+    err = lo_change_cred(req, &old);
+    if (err) {
+        goto out;
     }
 
-    fi->fh = fd;
+    fd = openat(lo_fd(req, parent), name, (fi->flags | O_CREAT) & ~O_NOFOLLOW,
+                mode);
+    err = fd == -1 ? errno : 0;
+    lo_restore_cred(&old);
+
+    if (!err) {
+        fi->fh = fd;
+        err = lo_do_lookup(req, parent, name, &e);
+    }
     if (lo->cache == CACHE_NEVER) {
         fi->direct_io = 1;
     } else if (lo->cache == CACHE_ALWAYS) {
         fi->keep_cache = 1;
     }
 
-    err = lo_do_lookup(req, parent, name, &e);
+out:
     if (err) {
         fuse_reply_err(req, err);
     } else {