Re: [PATCH v2 02/30] fuse: Clear setuid bit even in cache=never path

Miklos Szeredi
2019-05-20T14:41:48Z

On Wed, May 15, 2019 at 03:26:47PM -0400, Vivek Goyal wrote:
> If fuse daemon is started with cache=never, fuse falls back to direct IO.
> In that write path we don't call file_remove_privs() and that means setuid
> bit is not cleared if unpriviliged user writes to a file with setuid bit set.
>
> pjdfstest chmod test 12.t tests this and fails.

I think better sulution is to tell the server if the suid bit needs to be
removed, so it can do so in a race free way.

Here's the kernel patch, and I'll reply with the libfuse patch.

---
fs/fuse2/file.c | 2 ++
include/uapi/linux/fuse.h | 3 +++
2 files changed, 5 insertions(+)

--- a/fs/fuse2/file.c
+++ b/fs/fuse2/file.c
@@ -363,6 +363,8 @@ static ssize_t fuse_send_write(struct fu
inarg->flags |= O_DSYNC;
if (iocb->ki_flags & IOCB_SYNC)
inarg->flags |= O_SYNC;
+ if (!capable(CAP_FSETID))
+ inarg->write_flags |= FUSE_WRITE_KILL_PRIV;
req->inh.opcode = FUSE_WRITE;
req->inh.nodeid = ff->nodeid;
req->inh.len = req->inline_inlen + count;
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -125,6 +125,7 @@
*
* 7.29
* - add FUSE_NO_OPENDIR_SUPPORT flag
+ * - add FUSE_WRITE_KILL_PRIV flag
*/

#ifndef _LINUX_FUSE_H
@@ -318,9 +319,11 @@ struct fuse_file_lock {
*
* FUSE_WRITE_CACHE: delayed write from page cache, file handle is guessed
* FUSE_WRITE_LOCKOWNER: lock_owner field is valid
+ * FUSE_WRITE_KILL_PRIV: kill suid and sgid bits
*/
#define FUSE_WRITE_CACHE (1 << 0)
#define FUSE_WRITE_LOCKOWNER (1 << 1)
+#define FUSE_WRITE_KILL_PRIV (1 << 2)

/**
* Read flags