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@xxxxxxxxxx>
---
fs/fuse/fuse_i.h | 6 ++++++
fs/fuse/inode.c | 5 ++++-
include/uapi/linux/fuse.h | 7 +++++++
3 files changed, 17 insertions(+), 1 deletion(-)
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index dbaae2f6c73e..3dd1578be405 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -631,6 +631,12 @@ struct fuse_conn {
/* show legacy mount options */
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 d252237219bf..20740b61f12b 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -993,6 +993,8 @@ static void process_init_reply(struct fuse_conn *fc, 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;
@@ -1035,7 +1037,8 @@ void fuse_send_init(struct fuse_conn *fc)
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 (fc->dax)
ia->in.flags |= FUSE_MAP_ALIGNMENT;
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index 8899e4862309..3ae3f222a0ed 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -172,6 +172,7 @@
* - add FUSE_WRITE_KILL_PRIV flag
* - add FUSE_SETUPMAPPING and FUSE_REMOVEMAPPING
* - add map_alignment to fuse_init_out, add FUSE_MAP_ALIGNMENT flag
+ * - add FUSE_HANDLE_KILLPRIV_V2
*/
#ifndef _LINUX_FUSE_H
@@ -316,6 +317,11 @@ struct fuse_file_lock {
* FUSE_MAP_ALIGNMENT: init_out.map_alignment contains log2(byte alignment) for
* foffset and moffset fields in struct
* fuse_setupmapping_out and fuse_removemapping_one.
+ * FUSE_HANDLE_KILLPRIV_V2: fs kills suid/sgid/cap on write/chown/trunc.
+ * Upon write/truncate suid/sgid is only killed if caller
+ * does not have CAP_FSETID. Additionally upon
+ * write/truncate sgid is killed only if file has group
+ * execute permission. (Same as Linux VFS behavior).
*/
#define FUSE_ASYNC_READ (1 << 0)
#define FUSE_POSIX_LOCKS (1 << 1)
@@ -344,6 +350,7 @@ struct fuse_file_lock {
#define FUSE_NO_OPENDIR_SUPPORT (1 << 24)
#define FUSE_EXPLICIT_INVAL_DATA (1 << 25)
#define FUSE_MAP_ALIGNMENT (1 << 26)
+#define FUSE_HANDLE_KILLPRIV_V2 (1 << 27)
/**
* CUSE INIT request/reply flags
--
2.25.4
