fuse: introduce the notion of FUSE_HANDLE_KILLPRIV_V2

We already have FUSE_HANDLE_KILLPRIV flag that says that file server will
remove suid/sgid/caps on truncate/chown/write. But that's little different
from what Linux VFS implements.

To be consistent with Linux VFS behavior what we want is.

- caps are always cleared on chown/write/truncate
- suid is always cleared on chown, while for truncate/write it is cleared
  only if caller does not have CAP_FSETID.
- sgid is always cleared on chown, while for truncate/write it is cleared
  only if caller does not have CAP_FSETID as well as file has group execute
  permission.

As previous flag did not provide above semantics. Implement a V2 of the
protocol with above said constraints.

Server does not know if caller has CAP_FSETID or not. So for the case
of write()/truncate(), client will send information in special flag to
indicate whether to kill priviliges or not. These changes are in subsequent
patches.

FUSE_HANDLE_KILLPRIV_V2 relies on WRITE being sent to server to clear
suid/sgid/security.capability. But with ->writeback_cache, WRITES are
cached in guest. So it is not recommended to use FUSE_HANDLE_KILLPRIV_V2
and writeback_cache together. Though it probably might be good enough
for lot of use cases.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

2 files changed, 12 insertions, 1 deletions

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 8301c5056022..d414c787e362 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -636,6 +636,14 @@ struct fuse_conn {
 	unsigned int legacy_opts_show:1;
 
 	/*
+	 * fs kills suid/sgid/cap on write/chown/trunc. suid is killed on
+	 * write/trunc only if caller did not have CAP_FSETID.  sgid is killed
+	 * on write/truncate only if caller did not have CAP_FSETID as well as
+	 * file has group execute permission.
+	 */
+	unsigned handle_killpriv_v2:1;
+
+	/*
 	 * The following bitfields are only for optimization purposes
 	 * and hence races in setting them will not cause malfunction
 	 */
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index e7e9005b9b66..5a6102cd6473 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1038,6 +1038,8 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 			    !fuse_dax_check_alignment(fc, arg->map_alignment)) {
 				ok = false;
 			}
+			if (arg->flags & FUSE_HANDLE_KILLPRIV_V2)
+				fc->handle_killpriv_v2 = 1;
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -1080,7 +1082,8 @@ void fuse_send_init(struct fuse_mount *fm)
 		FUSE_WRITEBACK_CACHE | FUSE_NO_OPEN_SUPPORT |
 		FUSE_PARALLEL_DIROPS | FUSE_HANDLE_KILLPRIV | FUSE_POSIX_ACL |
 		FUSE_ABORT_ERROR | FUSE_MAX_PAGES | FUSE_CACHE_SYMLINKS |
-		FUSE_NO_OPENDIR_SUPPORT | FUSE_EXPLICIT_INVAL_DATA;
+		FUSE_NO_OPENDIR_SUPPORT | FUSE_EXPLICIT_INVAL_DATA |
+		FUSE_HANDLE_KILLPRIV_V2;
 #ifdef CONFIG_FUSE_DAX
 	if (fm->fc->dax)
 		ia->in.flags |= FUSE_MAP_ALIGNMENT;