bpf: implement dummy fops for bpf objects

syzkaller was able to trigger the following warning in
do_dentry_open():

  WARNING: CPU: 1 PID: 4508 at fs/open.c:778 do_dentry_open+0x4ad/0xe40 fs/open.c:778
  Kernel panic - not syncing: panic_on_warn set ...

  CPU: 1 PID: 4508 Comm: syz-executor867 Not tainted 4.17.0+ #90
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
  Call Trace:
  [...]
   vfs_open+0x139/0x230 fs/open.c:908
   do_last fs/namei.c:3370 [inline]
   path_openat+0x1717/0x4dc0 fs/namei.c:3511
   do_filp_open+0x249/0x350 fs/namei.c:3545
   do_sys_open+0x56f/0x740 fs/open.c:1101
   __do_sys_openat fs/open.c:1128 [inline]
   __se_sys_openat fs/open.c:1122 [inline]
   __x64_sys_openat+0x9d/0x100 fs/open.c:1122
   do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
   entry_SYSCALL_64_after_hwframe+0x49/0xbe

Problem was that prog and map inodes in bpf fs did not
implement a dummy file open operation that would return an
error. The patch in do_dentry_open() checks whether f_ops
are present and if not bails out with an error. While this
may be fine, we really shouldn't be throwing a warning
though. Thus follow the model similar to bad_file_ops and
reject the request unconditionally with -EIO.

Fixes: b2197755b2 ("bpf: add support for persistent maps/progs")
Reported-by: syzbot+2e7fcab0f56fdbb330b8@syzkaller.appspotmail.com
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/inode.c

@@ -295,6 +295,15 @@ static const struct file_operations bpffs_map_fops = {
 	.release	= bpffs_map_release,
 };
 
+static int bpffs_obj_open(struct inode *inode, struct file *file)
+{
+	return -EIO;
+}
+
+static const struct file_operations bpffs_obj_fops = {
+	.open		= bpffs_obj_open,
+};
+
 static int bpf_mkobj_ops(struct dentry *dentry, umode_t mode, void *raw,
 			 const struct inode_operations *iops,
 			 const struct file_operations *fops)
@@ -314,7 +323,8 @@ static int bpf_mkobj_ops(struct dentry *dentry, umode_t mode, void *raw,
 
 static int bpf_mkprog(struct dentry *dentry, umode_t mode, void *arg)
 {
-	return bpf_mkobj_ops(dentry, mode, arg, &bpf_prog_iops, NULL);
+	return bpf_mkobj_ops(dentry, mode, arg, &bpf_prog_iops,
+			     &bpffs_obj_fops);
 }
 
 static int bpf_mkmap(struct dentry *dentry, umode_t mode, void *arg)
@@ -322,7 +332,7 @@ static int bpf_mkmap(struct dentry *dentry, umode_t mode, void *arg)
 	struct bpf_map *map = arg;
 
 	return bpf_mkobj_ops(dentry, mode, arg, &bpf_map_iops,
-			     map->btf ? &bpffs_map_fops : NULL);
+			     map->btf ? &bpffs_map_fops : &bpffs_obj_fops);
 }
 
 static struct dentry *