[PATCH] nfsd: use vfs setgid helper

[Christian Brauner <brauner@kernel.org>]

We've aligned setgid behavior over multiple kernel releases. The details
can be found in commit cf619f891971 ("Merge tag 'fs.ovl.setgid.v6.2' of
git://git.kernel.org/pub/scm/linux/kernel/git/vfs/idmapping") and
commit 426b4ca2d6a5 ("Merge tag 'fs.setgid.v6.0' of
git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux").
Consistent setgid stripping behavior is now encapsulated in the
setattr_should_drop_sgid() helper which is used by all filesystems that
strip setgid bits outside of vfs proper. Usually ATTR_KILL_SGID is
raised in e.g., chown_common() and is subject to the
setattr_should_drop_sgid() check to determine whether the setgid bit can
be retained. Since nfsd is raising ATTR_KILL_SGID unconditionally it
will cause notify_change() to strip it even if the caller had the
necessary privileges to retain it. Ensure that nfsd only raises
ATR_KILL_SGID if the caller lacks the necessary privileges to retain the
setgid bit.

Without this patch the setgid stripping tests in LTP will fail:

> As you can see, the problem is S_ISGID (0002000) was dropped on a
> non-group-executable file while chown was invoked by super-user, while

[...]

> fchown02.c:66: TFAIL: testfile2: wrong mode permissions 0100700, expected 0102700

[...]

> chown02.c:57: TFAIL: testfile2: wrong mode permissions 0100700, expected 0102700

With this patch all tests pass.

Reported-by: Sherry Yang <sherry.yang@oracle.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
ubuntu@imp1-vm:~/ltp-install$ sudo ./runltp -d /mnt -s chown02
INFO: ltp-pan reported all tests PASS
LTP Version: 20230127-112-gf41e8a2fa

ubuntu@imp1-vm:~/ltp-install$ sudo ./runltp -d /mnt -s fchown02
INFO: ltp-pan reported all tests PASS
LTP Version: 20230127-112-gf41e8a2fa

ubuntu@imp1-vm:~/src/git/xfstests$ sudo ./check -g perms
FSTYP         -- nfs
PLATFORM      -- Linux/x86_64 imp1-vm 6.3.0-nfs-setgid-3a3cfe624076 #20 SMP PREEMPT_DYNAMIC Tue May  2 12:35:51 UTC 2023
MKFS_OPTIONS  -- 127.0.0.1:/nfsscratch
MOUNT_OPTIONS -- -o vers=3,noacl 127.0.0.1:/nfsscratch /mnt/scratch
Passed all 41 tests
---
 fs/nfsd/vfs.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index bb9d47172162..c4ef24c5ffd0 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -388,7 +388,9 @@ nfsd_sanitize_attrs(struct inode *inode, struct iattr *iap)
 				iap->ia_mode &= ~S_ISGID;
 		} else {
 			/* set ATTR_KILL_* bits and let VFS handle it */
-			iap->ia_valid |= (ATTR_KILL_SUID | ATTR_KILL_SGID);
+			iap->ia_valid |= ATTR_KILL_SUID;
+			iap->ia_valid |=
+				setattr_should_drop_sgid(&nop_mnt_idmap, inode);
 		}
 	}
 }
-- 
2.34.1