[PATCH 01/25] virtiofsd: passthrough_ll: create new files in caller's context

[Dr. David Alan Gilbert (git)]

From: Vivek Goyal <address@hidden>

We need to create files in callers context. Otherwise afer creating file,
caller himself 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 backto 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 <address@hidden>
Signed-off-by: Miklos Szeredi <address@hidden>
---
 contrib/virtiofsd/passthrough_ll.c | 75 ++++++++++++++++++++++++++++--
 1 file changed, 71 insertions(+), 4 deletions(-)

diff --git a/contrib/virtiofsd/passthrough_ll.c 
b/contrib/virtiofsd/passthrough_ll.c
index 2a1a880224..2de35d4d3d 100644
--- a/contrib/virtiofsd/passthrough_ll.c
+++ b/contrib/virtiofsd/passthrough_ll.c
@@ -49,6 +49,7 @@
 #include <inttypes.h>
 #include <pthread.h>
 #include <sys/file.h>
+#include <sys/syscall.h>
 #include <sys/xattr.h>
 
 #include "passthrough_helpers.h"
@@ -78,6 +79,11 @@ struct lo_inode {
        uint64_t refcount; /* protected by lo->mutex */
 };
 
+struct lo_cred {
+       uid_t euid;
+       gid_t egid;
+};
+
 enum {
        CACHE_NEVER,
        CACHE_NORMAL,
@@ -376,6 +382,51 @@ static void lo_lookup(fuse_req_t req, fuse_ino_t parent, 
const char *name)
                fuse_reply_entry(req, &e);
 }
 
+/*
+ * 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(SYS_setresgid, -1,fuse_req_ctx(req)->gid, -1);
+       if (res == -1)
+               return errno;
+
+       res = syscall(SYS_setresuid, -1, fuse_req_ctx(req)->uid, -1);
+       if (res == -1) {
+               int errno_save = errno;
+
+               syscall(SYS_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(SYS_setresuid, -1, old->euid, -1);
+       if (res == -1) {
+               fuse_log(FUSE_LOG_ERR, "seteuid(%u): %m\n", old->euid);
+               exit(1);
+       }
+
+       res = syscall(SYS_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)
@@ -384,12 +435,20 @@ 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;
 
@@ -768,23 +827,31 @@ 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);
 
+       err = lo_change_cred(req, &old);
+       if (err)
+               goto out;
+
        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 = fd == -1 ? errno : 0;
+       lo_restore_cred(&old);
 
-       fi->fh = fd;
+       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
-- 
2.23.0