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 <[email protected]> --- 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 _______________________________________________ Virtio-fs mailing list [email protected] https://www.redhat.com/mailman/listinfo/virtio-fs
