[PATCH 5.15 00/11] sgid fixes for 5.15.y

[PATCH 5.15 00/11] sgid fixes for 5.15.y

[Leah Rumancik]

Hello,

I finished testing the sgid fixes which Amir graciously backported to
5.15. This series fixes the previously failing generic/673 and
generic/68[3-7]. No regressions were seen in the 25 runs of the auto
group x 8 configs. I also did some extra runs on the perms group and
no regressions there either. The corresponding fixes are already in
6.1.y.

- Leah

Christian Brauner (5):
  attr: add in_group_or_capable()
  fs: move should_remove_suid()
  attr: add setattr_should_drop_sgid()
  attr: use consistent sgid stripping checks
  fs: use consistent setgid checks in is_sxid()

Darrick J. Wong (1):
  xfs: use setattr_copy to set vfs inode attributes

Dave Chinner (3):
  xfs: remove XFS_PREALLOC_SYNC
  xfs: fallocate() should call file_modified()
  xfs: set prealloc flag in xfs_alloc_file_space()

Yang Xu (2):
  fs: add mode_strip_sgid() helper
  fs: move S_ISGID stripping into the vfs_*() helpers

 Documentation/trace/ftrace.rst |  2 +-
 fs/attr.c                      | 72 +++++++++++++++++++++++++--
 fs/fuse/file.c                 |  2 +-
 fs/inode.c                     | 90 ++++++++++++++++++++--------------
 fs/internal.h                  | 10 +++-
 fs/namei.c                     | 82 ++++++++++++++++++++++++++-----
 fs/ocfs2/file.c                |  4 +-
 fs/ocfs2/namei.c               |  1 +
 fs/open.c                      |  8 +--
 fs/xfs/xfs_bmap_util.c         |  9 ++--
 fs/xfs/xfs_file.c              | 24 +++++----
 fs/xfs/xfs_iops.c              | 56 ++-------------------
 fs/xfs/xfs_pnfs.c              |  9 ++--
 include/linux/fs.h             |  6 ++-
 14 files changed, 235 insertions(+), 140 deletions(-)

-- 
2.40.0.rc0.216.gc4246ad0f0-goog

[PATCH 5.15 01/11] xfs: use setattr_copy to set vfs inode attributes

[Leah Rumancik]

From: "Darrick J. Wong" <djwong@kernel.org>

commit e014f37db1a2d109afa750042ac4d69cf3e3d88e upsream.

Filipe Manana pointed out that XFS' behavior w.r.t. setuid/setgid
revocation isn't consistent with btrfs[1] or ext4.  Those two
filesystems use the VFS function setattr_copy to convey certain
attributes from struct iattr into the VFS inode structure.

Andrey Zhadchenko reported[2] that XFS uses the wrong user namespace to
decide if it should clear setgid and setuid on a file attribute update.
This is a second symptom of the problem that Filipe noticed.

XFS, on the other hand, open-codes setattr_copy in xfs_setattr_mode,
xfs_setattr_nonsize, and xfs_setattr_time.  Regrettably, setattr_copy is
/not/ a simple copy function; it contains additional logic to clear the
setgid bit when setting the mode, and XFS' version no longer matches.

The VFS implements its own setuid/setgid stripping logic, which
establishes consistent behavior.  It's a tad unfortunate that it's
scattered across notify_change, should_remove_suid, and setattr_copy but
XFS should really follow the Linux VFS.  Adapt XFS to use the VFS
functions and get rid of the old functions.

[1] https://lore.kernel.org/fstests/CAL3q7H47iNQ=Wmk83WcGB-KBJVOEtR9+qGczzCeXJ9Y2KCV25Q@mail.gmail.com/
[2] https://lore.kernel.org/linux-xfs/20220221182218.748084-1-andrey.zhadchenko@virtuozzo.com/

Fixes: 7fa294c8991c ("userns: Allow chown and setgid preservation")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Tested-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/xfs/xfs_iops.c | 56 +++--------------------------------------------
 fs/xfs/xfs_pnfs.c |  3 ++-
 2 files changed, 5 insertions(+), 54 deletions(-)

diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index a607d6aca5c4..1eb71275e5b0 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -634,37 +634,6 @@ xfs_vn_getattr(
 	return 0;
 }
 
-static void
-xfs_setattr_mode(
-	struct xfs_inode	*ip,
-	struct iattr		*iattr)
-{
-	struct inode		*inode = VFS_I(ip);
-	umode_t			mode = iattr->ia_mode;
-
-	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
-
-	inode->i_mode &= S_IFMT;
-	inode->i_mode |= mode & ~S_IFMT;
-}
-
-void
-xfs_setattr_time(
-	struct xfs_inode	*ip,
-	struct iattr		*iattr)
-{
-	struct inode		*inode = VFS_I(ip);
-
-	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
-
-	if (iattr->ia_valid & ATTR_ATIME)
-		inode->i_atime = iattr->ia_atime;
-	if (iattr->ia_valid & ATTR_CTIME)
-		inode->i_ctime = iattr->ia_ctime;
-	if (iattr->ia_valid & ATTR_MTIME)
-		inode->i_mtime = iattr->ia_mtime;
-}
-
 static int
 xfs_vn_change_ok(
 	struct user_namespace	*mnt_userns,
@@ -763,16 +732,6 @@ xfs_setattr_nonsize(
 		gid = (mask & ATTR_GID) ? iattr->ia_gid : igid;
 		uid = (mask & ATTR_UID) ? iattr->ia_uid : iuid;
 
-		/*
-		 * CAP_FSETID overrides the following restrictions:
-		 *
-		 * The set-user-ID and set-group-ID bits of a file will be
-		 * cleared upon successful return from chown()
-		 */
-		if ((inode->i_mode & (S_ISUID|S_ISGID)) &&
-		    !capable(CAP_FSETID))
-			inode->i_mode &= ~(S_ISUID|S_ISGID);
-
 		/*
 		 * Change the ownerships and register quota modifications
 		 * in the transaction.
@@ -784,7 +743,6 @@ xfs_setattr_nonsize(
 				olddquot1 = xfs_qm_vop_chown(tp, ip,
 							&ip->i_udquot, udqp);
 			}
-			inode->i_uid = uid;
 		}
 		if (!gid_eq(igid, gid)) {
 			if (XFS_IS_GQUOTA_ON(mp)) {
@@ -795,15 +753,10 @@ xfs_setattr_nonsize(
 				olddquot2 = xfs_qm_vop_chown(tp, ip,
 							&ip->i_gdquot, gdqp);
 			}
-			inode->i_gid = gid;
 		}
 	}
 
-	if (mask & ATTR_MODE)
-		xfs_setattr_mode(ip, iattr);
-	if (mask & (ATTR_ATIME|ATTR_CTIME|ATTR_MTIME))
-		xfs_setattr_time(ip, iattr);
-
+	setattr_copy(mnt_userns, inode, iattr);
 	xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
 
 	XFS_STATS_INC(mp, xs_ig_attrchg);
@@ -1028,11 +981,8 @@ xfs_setattr_size(
 		xfs_inode_clear_eofblocks_tag(ip);
 	}
 
-	if (iattr->ia_valid & ATTR_MODE)
-		xfs_setattr_mode(ip, iattr);
-	if (iattr->ia_valid & (ATTR_ATIME|ATTR_CTIME|ATTR_MTIME))
-		xfs_setattr_time(ip, iattr);
-
+	ASSERT(!(iattr->ia_valid & (ATTR_UID | ATTR_GID)));
+	setattr_copy(mnt_userns, inode, iattr);
 	xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
 
 	XFS_STATS_INC(mp, xs_ig_attrchg);
diff --git a/fs/xfs/xfs_pnfs.c b/fs/xfs/xfs_pnfs.c
index 5e1d29d8b2e7..8865f7d4404a 100644
--- a/fs/xfs/xfs_pnfs.c
+++ b/fs/xfs/xfs_pnfs.c
@@ -283,7 +283,8 @@ xfs_fs_commit_blocks(
 	xfs_trans_ijoin(tp, ip, XFS_ILOCK_EXCL);
 	xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
 
-	xfs_setattr_time(ip, iattr);
+	ASSERT(!(iattr->ia_valid & (ATTR_UID | ATTR_GID)));
+	setattr_copy(&init_user_ns, inode, iattr);
 	if (update_isize) {
 		i_size_write(inode, iattr->ia_size);
 		ip->i_disk_size = iattr->ia_size;
-- 
2.40.0.rc0.216.gc4246ad0f0-goog

[PATCH 5.15 02/11] xfs: remove XFS_PREALLOC_SYNC

[Leah Rumancik]

From: Dave Chinner <dchinner@redhat.com>

commit 472c6e46f589c26057596dcba160712a5b3e02c5 upstream.

[partial backport for dependency -
 xfs_ioc_space() still uses XFS_PREALLOC_SYNC]

Callers can acheive the same thing by calling xfs_log_force_inode()
after making their modifications. There is no need for
xfs_update_prealloc_flags() to do this.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Tested-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/xfs/xfs_file.c | 13 +++++++------
 fs/xfs/xfs_pnfs.c |  6 ++++--
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 240eb932c014..752b676c92e3 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -95,8 +95,6 @@ xfs_update_prealloc_flags(
 		ip->i_diflags &= ~XFS_DIFLAG_PREALLOC;
 
 	xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
-	if (flags & XFS_PREALLOC_SYNC)
-		xfs_trans_set_sync(tp);
 	return xfs_trans_commit(tp);
 }
 
@@ -1059,9 +1057,6 @@ xfs_file_fallocate(
 		}
 	}
 
-	if (file->f_flags & O_DSYNC)
-		flags |= XFS_PREALLOC_SYNC;
-
 	error = xfs_update_prealloc_flags(ip, flags);
 	if (error)
 		goto out_unlock;
@@ -1084,8 +1079,14 @@ xfs_file_fallocate(
 	 * leave shifted extents past EOF and hence losing access to
 	 * the data that is contained within them.
 	 */
-	if (do_file_insert)
+	if (do_file_insert) {
 		error = xfs_insert_file_space(ip, offset, len);
+		if (error)
+			goto out_unlock;
+	}
+
+	if (file->f_flags & O_DSYNC)
+		error = xfs_log_force_inode(ip);
 
 out_unlock:
 	xfs_iunlock(ip, iolock);
diff --git a/fs/xfs/xfs_pnfs.c b/fs/xfs/xfs_pnfs.c
index 8865f7d4404a..3a82a13d880c 100644
--- a/fs/xfs/xfs_pnfs.c
+++ b/fs/xfs/xfs_pnfs.c
@@ -164,10 +164,12 @@ xfs_fs_map_blocks(
 		 * that the blocks allocated and handed out to the client are
 		 * guaranteed to be present even after a server crash.
 		 */
-		error = xfs_update_prealloc_flags(ip,
-				XFS_PREALLOC_SET | XFS_PREALLOC_SYNC);
+		error = xfs_update_prealloc_flags(ip, XFS_PREALLOC_SET);
+		if (!error)
+			error = xfs_log_force_inode(ip);
 		if (error)
 			goto out_unlock;
+
 	} else {
 		xfs_iunlock(ip, lock_flags);
 	}
-- 
2.40.0.rc0.216.gc4246ad0f0-goog

[PATCH 5.15 03/11] xfs: fallocate() should call file_modified()

[Leah Rumancik]

From: Dave Chinner <dchinner@redhat.com>

commit fbe7e520036583a783b13ff9744e35c2a329d9a4 upsream.

In XFS, we always update the inode change and modification time when
any fallocate() operation succeeds.  Furthermore, as various
fallocate modes can change the file contents (extending EOF,
punching holes, zeroing things, shifting extents), we should drop
file privileges like suid just like we do for a regular write().
There's already a VFS helper that figures all this out for us, so
use that.

The net effect of this is that we no longer drop suid/sgid if the
caller is root, but we also now drop file capabilities.

We also move the xfs_update_prealloc_flags() function so that it now
is only called by the scope that needs to set the the prealloc flag.

Based on a patch from Darrick Wong.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Tested-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/xfs/xfs_file.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 752b676c92e3..020e0a412287 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -954,6 +954,10 @@ xfs_file_fallocate(
 			goto out_unlock;
 	}
 
+	error = file_modified(file);
+	if (error)
+		goto out_unlock;
+
 	if (mode & FALLOC_FL_PUNCH_HOLE) {
 		error = xfs_free_file_space(ip, offset, len);
 		if (error)
@@ -1055,11 +1059,12 @@ xfs_file_fallocate(
 			if (error)
 				goto out_unlock;
 		}
-	}
 
-	error = xfs_update_prealloc_flags(ip, flags);
-	if (error)
-		goto out_unlock;
+		error = xfs_update_prealloc_flags(ip, XFS_PREALLOC_SET);
+		if (error)
+			goto out_unlock;
+
+	}
 
 	/* Change file size if needed */
 	if (new_size) {
-- 
2.40.0.rc0.216.gc4246ad0f0-goog

[PATCH 5.15 04/11] xfs: set prealloc flag in xfs_alloc_file_space()

[Leah Rumancik]

From: Dave Chinner <dchinner@redhat.com>

commit 0b02c8c0d75a738c98c35f02efb36217c170d78c upsream.

Now that we only call xfs_update_prealloc_flags() from
xfs_file_fallocate() in the case where we need to set the
preallocation flag, do this in xfs_alloc_file_space() where we
already have the inode joined into a transaction and get
rid of the call to xfs_update_prealloc_flags() from the fallocate
code.

This also means that we now correctly avoid setting the
XFS_DIFLAG_PREALLOC flag when xfs_is_always_cow_inode() is true, as
these inodes will never have preallocated extents.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Tested-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/xfs/xfs_bmap_util.c | 9 +++------
 fs/xfs/xfs_file.c      | 8 --------
 2 files changed, 3 insertions(+), 14 deletions(-)

diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
index 73a36b7be3bd..fd2ad6a3019c 100644
--- a/fs/xfs/xfs_bmap_util.c
+++ b/fs/xfs/xfs_bmap_util.c
@@ -851,9 +851,6 @@ xfs_alloc_file_space(
 			rblocks = 0;
 		}
 
-		/*
-		 * Allocate and setup the transaction.
-		 */
 		error = xfs_trans_alloc_inode(ip, &M_RES(mp)->tr_write,
 				dblocks, rblocks, false, &tp);
 		if (error)
@@ -870,9 +867,9 @@ xfs_alloc_file_space(
 		if (error)
 			goto error;
 
-		/*
-		 * Complete the transaction
-		 */
+		ip->i_diflags |= XFS_DIFLAG_PREALLOC;
+		xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
+
 		error = xfs_trans_commit(tp);
 		xfs_iunlock(ip, XFS_ILOCK_EXCL);
 		if (error)
diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 020e0a412287..8cd0c3df253f 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -909,7 +909,6 @@ xfs_file_fallocate(
 	struct inode		*inode = file_inode(file);
 	struct xfs_inode	*ip = XFS_I(inode);
 	long			error;
-	enum xfs_prealloc_flags	flags = 0;
 	uint			iolock = XFS_IOLOCK_EXCL | XFS_MMAPLOCK_EXCL;
 	loff_t			new_size = 0;
 	bool			do_file_insert = false;
@@ -1007,8 +1006,6 @@ xfs_file_fallocate(
 		}
 		do_file_insert = true;
 	} else {
-		flags |= XFS_PREALLOC_SET;
-
 		if (!(mode & FALLOC_FL_KEEP_SIZE) &&
 		    offset + len > i_size_read(inode)) {
 			new_size = offset + len;
@@ -1059,11 +1056,6 @@ xfs_file_fallocate(
 			if (error)
 				goto out_unlock;
 		}
-
-		error = xfs_update_prealloc_flags(ip, XFS_PREALLOC_SET);
-		if (error)
-			goto out_unlock;
-
 	}
 
 	/* Change file size if needed */
-- 
2.40.0.rc0.216.gc4246ad0f0-goog

[PATCH 5.15 05/11] fs: add mode_strip_sgid() helper

[Leah Rumancik]

From: Yang Xu <xuyang2018.jy@fujitsu.com>

commit 2b3416ceff5e6bd4922f6d1c61fb68113dd82302 upsream.

Add a dedicated helper to handle the setgid bit when creating a new file
in a setgid directory. This is a preparatory patch for moving setgid
stripping into the vfs. The patch contains no functional changes.

Currently the setgid stripping logic is open-coded directly in
inode_init_owner() and the individual filesystems are responsible for
handling setgid inheritance. Since this has proven to be brittle as
evidenced by old issues we uncovered over the last months (see [1] to
[3] below) we will try to move this logic into the vfs.

Link: e014f37db1a2 ("xfs: use setattr_copy to set vfs inode attributes") [1]
Link: 01ea173e103e ("xfs: fix up non-directory creation in SGID directories") [2]
Link: fd84bfdddd16 ("ceph: fix up non-directory creation in SGID directories") [3]
Link: https://lore.kernel.org/r/1657779088-2242-1-git-send-email-xuyang2018.jy@fujitsu.com
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Reviewed-and-Tested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Tested-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/inode.c         | 36 ++++++++++++++++++++++++++++++++----
 include/linux/fs.h |  2 ++
 2 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 8279c700a2b7..3740102c9bd5 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2165,10 +2165,8 @@ void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
 		/* Directories are special, and always inherit S_ISGID */
 		if (S_ISDIR(mode))
 			mode |= S_ISGID;
-		else if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP) &&
-			 !in_group_p(i_gid_into_mnt(mnt_userns, dir)) &&
-			 !capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
-			mode &= ~S_ISGID;
+		else
+			mode = mode_strip_sgid(mnt_userns, dir, mode);
 	} else
 		inode_fsgid_set(inode, mnt_userns);
 	inode->i_mode = mode;
@@ -2324,3 +2322,33 @@ struct timespec64 current_time(struct inode *inode)
 	return timestamp_truncate(now, inode);
 }
 EXPORT_SYMBOL(current_time);
+
+/**
+ * mode_strip_sgid - handle the sgid bit for non-directories
+ * @mnt_userns: User namespace of the mount the inode was created from
+ * @dir: parent directory inode
+ * @mode: mode of the file to be created in @dir
+ *
+ * If the @mode of the new file has both the S_ISGID and S_IXGRP bit
+ * raised and @dir has the S_ISGID bit raised ensure that the caller is
+ * either in the group of the parent directory or they have CAP_FSETID
+ * in their user namespace and are privileged over the parent directory.
+ * In all other cases, strip the S_ISGID bit from @mode.
+ *
+ * Return: the new mode to use for the file
+ */
+umode_t mode_strip_sgid(struct user_namespace *mnt_userns,
+			const struct inode *dir, umode_t mode)
+{
+	if ((mode & (S_ISGID | S_IXGRP)) != (S_ISGID | S_IXGRP))
+		return mode;
+	if (S_ISDIR(mode) || !dir || !(dir->i_mode & S_ISGID))
+		return mode;
+	if (in_group_p(i_gid_into_mnt(mnt_userns, dir)))
+		return mode;
+	if (capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
+		return mode;
+
+	return mode & ~S_ISGID;
+}
+EXPORT_SYMBOL(mode_strip_sgid);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 1e1ac116dd13..be9be4a7216c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1941,6 +1941,8 @@ extern long compat_ptr_ioctl(struct file *file, unsigned int cmd,
 void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
 		      const struct inode *dir, umode_t mode);
 extern bool may_open_dev(const struct path *path);
+umode_t mode_strip_sgid(struct user_namespace *mnt_userns,
+			const struct inode *dir, umode_t mode);
 
 /*
  * This is the "filldir" function type, used by readdir() to let
-- 
2.40.0.rc0.216.gc4246ad0f0-goog

[PATCH 5.15 06/11] fs: move S_ISGID stripping into the vfs_*() helpers

[Leah Rumancik]

From: Yang Xu <xuyang2018.jy@fujitsu.com>

commit 1639a49ccdce58ea248841ed9b23babcce6dbb0b upsream.

Move setgid handling out of individual filesystems and into the VFS
itself to stop the proliferation of setgid inheritance bugs.

Creating files that have both the S_IXGRP and S_ISGID bit raised in
directories that themselves have the S_ISGID bit set requires additional
privileges to avoid security issues.

When a filesystem creates a new inode it needs to take care that the
caller is either in the group of the newly created inode or they have
CAP_FSETID in their current user namespace and are privileged over the
parent directory of the new inode. If any of these two conditions is
true then the S_ISGID bit can be raised for an S_IXGRP file and if not
it needs to be stripped.

However, there are several key issues with the current implementation:

* S_ISGID stripping logic is entangled with umask stripping.

  If a filesystem doesn't support or enable POSIX ACLs then umask
  stripping is done directly in the vfs before calling into the
  filesystem.
  If the filesystem does support POSIX ACLs then unmask stripping may be
  done in the filesystem itself when calling posix_acl_create().

  Since umask stripping has an effect on S_ISGID inheritance, e.g., by
  stripping the S_IXGRP bit from the file to be created and all relevant
  filesystems have to call posix_acl_create() before inode_init_owner()
  where we currently take care of S_ISGID handling S_ISGID handling is
  order dependent. IOW, whether or not you get a setgid bit depends on
  POSIX ACLs and umask and in what order they are called.

  Note that technically filesystems are free to impose their own
  ordering between posix_acl_create() and inode_init_owner() meaning
  that there's additional ordering issues that influence S_SIGID
  inheritance.

* Filesystems that don't rely on inode_init_owner() don't get S_ISGID
  stripping logic.

  While that may be intentional (e.g. network filesystems might just
  defer setgid stripping to a server) it is often just a security issue.

This is not just ugly it's unsustainably messy especially since we do
still have bugs in this area years after the initial round of setgid
bugfixes.

So the current state is quite messy and while we won't be able to make
it completely clean as posix_acl_create() is still a filesystem specific
call we can improve the S_SIGD stripping situation quite a bit by
hoisting it out of inode_init_owner() and into the vfs creation
operations. This means we alleviate the burden for filesystems to handle
S_ISGID stripping correctly and can standardize the ordering between
S_ISGID and umask stripping in the vfs.

We add a new helper vfs_prepare_mode() so S_ISGID handling is now done
in the VFS before umask handling. This has S_ISGID handling is
unaffected unaffected by whether umask stripping is done by the VFS
itself (if no POSIX ACLs are supported or enabled) or in the filesystem
in posix_acl_create() (if POSIX ACLs are supported).

The vfs_prepare_mode() helper is called directly in vfs_*() helpers that
create new filesystem objects. We need to move them into there to make
sure that filesystems like overlayfs hat have callchains like:

sys_mknod()
-> do_mknodat(mode)
   -> .mknod = ovl_mknod(mode)
      -> ovl_create(mode)
         -> vfs_mknod(mode)

get S_ISGID stripping done when calling into lower filesystems via
vfs_*() creation helpers. Moving vfs_prepare_mode() into e.g.
vfs_mknod() takes care of that. This is in any case semantically cleaner
because S_ISGID stripping is VFS security requirement.

Security hooks so far have seen the mode with the umask applied but
without S_ISGID handling done. The relevant hooks are called outside of
vfs_*() creation helpers so by calling vfs_prepare_mode() from vfs_*()
helpers the security hooks would now see the mode without umask
stripping applied. For now we fix this by passing the mode with umask
settings applied to not risk any regressions for LSM hooks. IOW, nothing
changes for LSM hooks. It is worth pointing out that security hooks
never saw the mode that is seen by the filesystem when actually creating
the file. They have always been completely misplaced for that to work.

The following filesystems use inode_init_owner() and thus relied on
S_ISGID stripping: spufs, 9p, bfs, btrfs, ext2, ext4, f2fs, hfsplus,
hugetlbfs, jfs, minix, nilfs2, ntfs3, ocfs2, omfs, overlayfs, ramfs,
reiserfs, sysv, ubifs, udf, ufs, xfs, zonefs, bpf, tmpfs.

All of the above filesystems end up calling inode_init_owner() when new
filesystem objects are created through the ->mkdir(), ->mknod(),
->create(), ->tmpfile(), ->rename() inode operations.

Since directories always inherit the S_ISGID bit with the exception of
xfs when irix_sgid_inherit mode is turned on S_ISGID stripping doesn't
apply. The ->symlink() and ->link() inode operations trivially inherit
the mode from the target and the ->rename() inode operation inherits the
mode from the source inode. All other creation inode operations will get
S_ISGID handling via vfs_prepare_mode() when called from their relevant
vfs_*() helpers.

In addition to this there are filesystems which allow the creation of
filesystem objects through ioctl()s or - in the case of spufs -
circumventing the vfs in other ways. If filesystem objects are created
through ioctl()s the vfs doesn't know about it and can't apply regular
permission checking including S_ISGID logic. Therfore, a filesystem
relying on S_ISGID stripping in inode_init_owner() in their ioctl()
callpath will be affected by moving this logic into the vfs. We audited
those filesystems:

* btrfs allows the creation of filesystem objects through various
  ioctls(). Snapshot creation literally takes a snapshot and so the mode
  is fully preserved and S_ISGID stripping doesn't apply.

  Creating a new subvolum relies on inode_init_owner() in
  btrfs_new_subvol_inode() but only creates directories and doesn't
  raise S_ISGID.

* ocfs2 has a peculiar implementation of reflinks. In contrast to e.g.
  xfs and btrfs FICLONE/FICLONERANGE ioctl() that is only concerned with
  the actual extents ocfs2 uses a separate ioctl() that also creates the
  target file.

  Iow, ocfs2 circumvents the vfs entirely here and did indeed rely on
  inode_init_owner() to strip the S_ISGID bit. This is the only place
  where a filesystem needs to call mode_strip_sgid() directly but this
  is self-inflicted pain.

* spufs doesn't go through the vfs at all and doesn't use ioctl()s
  either. Instead it has a dedicated system call spufs_create() which
  allows the creation of filesystem objects. But spufs only creates
  directories and doesn't allo S_SIGID bits, i.e. it specifically only
  allows 0777 bits.

* bpf uses vfs_mkobj() but also doesn't allow S_ISGID bits to be created.

The patch will have an effect on ext2 when the EXT2_MOUNT_GRPID mount
option is used, on ext4 when the EXT4_MOUNT_GRPID mount option is used,
and on xfs when the XFS_FEAT_GRPID mount option is used. When any of
these filesystems are mounted with their respective GRPID option then
newly created files inherit the parent directories group
unconditionally. In these cases non of the filesystems call
inode_init_owner() and thus did never strip the S_ISGID bit for newly
created files. Moving this logic into the VFS means that they now get
the S_ISGID bit stripped. This is a user visible change. If this leads
to regressions we will either need to figure out a better way or we need
to revert. However, given the various setgid bugs that we found just in
the last two years this is a regression risk we should take.

Associated with this change is a new set of fstests to enforce the
semantics for all new filesystems.

Link: https://lore.kernel.org/ceph-devel/20220427092201.wvsdjbnc7b4dttaw@wittgenstein [1]
Link: e014f37db1a2 ("xfs: use setattr_copy to set vfs inode attributes") [2]
Link: 01ea173e103e ("xfs: fix up non-directory creation in SGID directories") [3]
Link: fd84bfdddd16 ("ceph: fix up non-directory creation in SGID directories") [4]
Link: https://lore.kernel.org/r/1657779088-2242-3-git-send-email-xuyang2018.jy@fujitsu.com
Suggested-by: Dave Chinner <david@fromorbit.com>
Suggested-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-and-Tested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
[<brauner@kernel.org>: rewrote commit message]
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Tested-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/inode.c       |  2 --
 fs/namei.c       | 82 ++++++++++++++++++++++++++++++++++++++++--------
 fs/ocfs2/namei.c |  1 +
 3 files changed, 70 insertions(+), 15 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 3740102c9bd5..957b2d18ec29 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2165,8 +2165,6 @@ void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
 		/* Directories are special, and always inherit S_ISGID */
 		if (S_ISDIR(mode))
 			mode |= S_ISGID;
-		else
-			mode = mode_strip_sgid(mnt_userns, dir, mode);
 	} else
 		inode_fsgid_set(inode, mnt_userns);
 	inode->i_mode = mode;
diff --git a/fs/namei.c b/fs/namei.c
index 81b31d9a063f..02e99606c65b 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3000,6 +3000,65 @@ void unlock_rename(struct dentry *p1, struct dentry *p2)
 }
 EXPORT_SYMBOL(unlock_rename);
 
+/**
+ * mode_strip_umask - handle vfs umask stripping
+ * @dir:	parent directory of the new inode
+ * @mode:	mode of the new inode to be created in @dir
+ *
+ * Umask stripping depends on whether or not the filesystem supports POSIX
+ * ACLs. If the filesystem doesn't support it umask stripping is done directly
+ * in here. If the filesystem does support POSIX ACLs umask stripping is
+ * deferred until the filesystem calls posix_acl_create().
+ *
+ * Returns: mode
+ */
+static inline umode_t mode_strip_umask(const struct inode *dir, umode_t mode)
+{
+	if (!IS_POSIXACL(dir))
+		mode &= ~current_umask();
+	return mode;
+}
+
+/**
+ * vfs_prepare_mode - prepare the mode to be used for a new inode
+ * @mnt_userns:		user namespace of the mount the inode was found from
+ * @dir:	parent directory of the new inode
+ * @mode:	mode of the new inode
+ * @mask_perms:	allowed permission by the vfs
+ * @type:	type of file to be created
+ *
+ * This helper consolidates and enforces vfs restrictions on the @mode of a new
+ * object to be created.
+ *
+ * Umask stripping depends on whether the filesystem supports POSIX ACLs (see
+ * the kernel documentation for mode_strip_umask()). Moving umask stripping
+ * after setgid stripping allows the same ordering for both non-POSIX ACL and
+ * POSIX ACL supporting filesystems.
+ *
+ * Note that it's currently valid for @type to be 0 if a directory is created.
+ * Filesystems raise that flag individually and we need to check whether each
+ * filesystem can deal with receiving S_IFDIR from the vfs before we enforce a
+ * non-zero type.
+ *
+ * Returns: mode to be passed to the filesystem
+ */
+static inline umode_t vfs_prepare_mode(struct user_namespace *mnt_userns,
+				       const struct inode *dir, umode_t mode,
+				       umode_t mask_perms, umode_t type)
+{
+	mode = mode_strip_sgid(mnt_userns, dir, mode);
+	mode = mode_strip_umask(dir, mode);
+
+	/*
+	 * Apply the vfs mandated allowed permission mask and set the type of
+	 * file to be created before we call into the filesystem.
+	 */
+	mode &= (mask_perms & ~S_IFMT);
+	mode |= (type & S_IFMT);
+
+	return mode;
+}
+
 /**
  * vfs_create - create new file
  * @mnt_userns:	user namespace of the mount the inode was found from
@@ -3025,8 +3084,8 @@ int vfs_create(struct user_namespace *mnt_userns, struct inode *dir,
 
 	if (!dir->i_op->create)
 		return -EACCES;	/* shouldn't it be ENOSYS? */
-	mode &= S_IALLUGO;
-	mode |= S_IFREG;
+
+	mode = vfs_prepare_mode(mnt_userns, dir, mode, S_IALLUGO, S_IFREG);
 	error = security_inode_create(dir, dentry, mode);
 	if (error)
 		return error;
@@ -3291,8 +3350,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 	if (open_flag & O_CREAT) {
 		if (open_flag & O_EXCL)
 			open_flag &= ~O_TRUNC;
-		if (!IS_POSIXACL(dir->d_inode))
-			mode &= ~current_umask();
+		mode = vfs_prepare_mode(mnt_userns, dir->d_inode, mode, mode, mode);
 		if (likely(got_write))
 			create_error = may_o_create(mnt_userns, &nd->path,
 						    dentry, mode);
@@ -3525,8 +3583,7 @@ struct dentry *vfs_tmpfile(struct user_namespace *mnt_userns,
 	child = d_alloc(dentry, &slash_name);
 	if (unlikely(!child))
 		goto out_err;
-	if (!IS_POSIXACL(dir))
-		mode &= ~current_umask();
+	mode = vfs_prepare_mode(mnt_userns, dir, mode, mode, mode);
 	error = dir->i_op->tmpfile(mnt_userns, dir, child, mode);
 	if (error)
 		goto out_err;
@@ -3804,6 +3861,7 @@ int vfs_mknod(struct user_namespace *mnt_userns, struct inode *dir,
 	if (!dir->i_op->mknod)
 		return -EPERM;
 
+	mode = vfs_prepare_mode(mnt_userns, dir, mode, mode, mode);
 	error = devcgroup_inode_mknod(mode, dev);
 	if (error)
 		return error;
@@ -3854,9 +3912,8 @@ static int do_mknodat(int dfd, struct filename *name, umode_t mode,
 	if (IS_ERR(dentry))
 		goto out1;
 
-	if (!IS_POSIXACL(path.dentry->d_inode))
-		mode &= ~current_umask();
-	error = security_path_mknod(&path, dentry, mode, dev);
+	error = security_path_mknod(&path, dentry,
+			mode_strip_umask(path.dentry->d_inode, mode), dev);
 	if (error)
 		goto out2;
 
@@ -3926,7 +3983,7 @@ int vfs_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
 	if (!dir->i_op->mkdir)
 		return -EPERM;
 
-	mode &= (S_IRWXUGO|S_ISVTX);
+	mode = vfs_prepare_mode(mnt_userns, dir, mode, S_IRWXUGO | S_ISVTX, 0);
 	error = security_inode_mkdir(dir, dentry, mode);
 	if (error)
 		return error;
@@ -3954,9 +4011,8 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
 	if (IS_ERR(dentry))
 		goto out_putname;
 
-	if (!IS_POSIXACL(path.dentry->d_inode))
-		mode &= ~current_umask();
-	error = security_path_mkdir(&path, dentry, mode);
+	error = security_path_mkdir(&path, dentry,
+			mode_strip_umask(path.dentry->d_inode, mode));
 	if (!error) {
 		struct user_namespace *mnt_userns;
 		mnt_userns = mnt_user_ns(path.mnt);
diff --git a/fs/ocfs2/namei.c b/fs/ocfs2/namei.c
index 11807034dd48..5b8237ceb8cc 100644
--- a/fs/ocfs2/namei.c
+++ b/fs/ocfs2/namei.c
@@ -197,6 +197,7 @@ static struct inode *ocfs2_get_init_inode(struct inode *dir, umode_t mode)
 	 * callers. */
 	if (S_ISDIR(mode))
 		set_nlink(inode, 2);
+	mode = mode_strip_sgid(&init_user_ns, dir, mode);
 	inode_init_owner(&init_user_ns, inode, dir, mode);
 	status = dquot_initialize(inode);
 	if (status)
-- 
2.40.0.rc0.216.gc4246ad0f0-goog

[PATCH 5.15 07/11] attr: add in_group_or_capable()

[Leah Rumancik]

From: Christian Brauner <brauner@kernel.org>

commit 11c2a8700cdcabf9b639b7204a1e38e2a0b6798e upstream.

[backport to 5.15.y, prior to vfsgid_t]

In setattr_{copy,prepare}() we need to perform the same permission
checks to determine whether we need to drop the setgid bit or not.
Instead of open-coding it twice add a simple helper the encapsulates the
logic. We will reuse this helpers to make dropping the setgid bit during
write operations more consistent in a follow up patch.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Tested-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/attr.c     |  8 ++++----
 fs/inode.c    | 28 ++++++++++++++++++++++++----
 fs/internal.h |  2 ++
 3 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/fs/attr.c b/fs/attr.c
index f581c4d00897..686840aa91c8 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -18,6 +18,8 @@
 #include <linux/evm.h>
 #include <linux/ima.h>
 
+#include "internal.h"
+
 /**
  * chown_ok - verify permissions to chown inode
  * @mnt_userns:	user namespace of the mount @inode was found from
@@ -141,8 +143,7 @@ int setattr_prepare(struct user_namespace *mnt_userns, struct dentry *dentry,
 			mapped_gid = i_gid_into_mnt(mnt_userns, inode);
 
 		/* Also check the setgid bit! */
-		if (!in_group_p(mapped_gid) &&
-		    !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+		if (!in_group_or_capable(mnt_userns, inode, mapped_gid))
 			attr->ia_mode &= ~S_ISGID;
 	}
 
@@ -257,8 +258,7 @@ void setattr_copy(struct user_namespace *mnt_userns, struct inode *inode,
 	if (ia_valid & ATTR_MODE) {
 		umode_t mode = attr->ia_mode;
 		kgid_t kgid = i_gid_into_mnt(mnt_userns, inode);
-		if (!in_group_p(kgid) &&
-		    !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+		if (!in_group_or_capable(mnt_userns, inode, kgid))
 			mode &= ~S_ISGID;
 		inode->i_mode = mode;
 	}
diff --git a/fs/inode.c b/fs/inode.c
index 957b2d18ec29..a71fb82279bb 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2321,6 +2321,28 @@ struct timespec64 current_time(struct inode *inode)
 }
 EXPORT_SYMBOL(current_time);
 
+/**
+ * in_group_or_capable - check whether caller is CAP_FSETID privileged
+ * @mnt_userns: user namespace of the mount @inode was found from
+ * @inode:	inode to check
+ * @gid:	the new/current gid of @inode
+ *
+ * Check wether @gid is in the caller's group list or if the caller is
+ * privileged with CAP_FSETID over @inode. This can be used to determine
+ * whether the setgid bit can be kept or must be dropped.
+ *
+ * Return: true if the caller is sufficiently privileged, false if not.
+ */
+bool in_group_or_capable(struct user_namespace *mnt_userns,
+			 const struct inode *inode, kgid_t gid)
+{
+	if (in_group_p(gid))
+		return true;
+	if (capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+		return true;
+	return false;
+}
+
 /**
  * mode_strip_sgid - handle the sgid bit for non-directories
  * @mnt_userns: User namespace of the mount the inode was created from
@@ -2342,11 +2364,9 @@ umode_t mode_strip_sgid(struct user_namespace *mnt_userns,
 		return mode;
 	if (S_ISDIR(mode) || !dir || !(dir->i_mode & S_ISGID))
 		return mode;
-	if (in_group_p(i_gid_into_mnt(mnt_userns, dir)))
-		return mode;
-	if (capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
+	if (in_group_or_capable(mnt_userns, dir,
+				i_gid_into_mnt(mnt_userns, dir)))
 		return mode;
-
 	return mode & ~S_ISGID;
 }
 EXPORT_SYMBOL(mode_strip_sgid);
diff --git a/fs/internal.h b/fs/internal.h
index 9075490f21a6..c89814727281 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -150,6 +150,8 @@ extern int vfs_open(const struct path *, struct file *);
 extern long prune_icache_sb(struct super_block *sb, struct shrink_control *sc);
 extern void inode_add_lru(struct inode *inode);
 extern int dentry_needs_remove_privs(struct dentry *dentry);
+bool in_group_or_capable(struct user_namespace *mnt_userns,
+			 const struct inode *inode, kgid_t gid);
 
 /*
  * fs-writeback.c
-- 
2.40.0.rc0.216.gc4246ad0f0-goog

[PATCH 5.15 08/11] fs: move should_remove_suid()

[Leah Rumancik]

From: Christian Brauner <brauner@kernel.org>

commit e243e3f94c804ecca9a8241b5babe28f35258ef4 upstream.

Move the helper from inode.c to attr.c. This keeps the the core of the
set{g,u}id stripping logic in one place when we add follow-up changes.
It is the better place anyway, since should_remove_suid() returns
ATTR_KILL_S{G,U}ID flags.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Tested-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/attr.c  | 29 +++++++++++++++++++++++++++++
 fs/inode.c | 29 -----------------------------
 2 files changed, 29 insertions(+), 29 deletions(-)

diff --git a/fs/attr.c b/fs/attr.c
index 686840aa91c8..f045431bab1a 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -20,6 +20,35 @@
 
 #include "internal.h"
 
+/*
+ * The logic we want is
+ *
+ *	if suid or (sgid and xgrp)
+ *		remove privs
+ */
+int should_remove_suid(struct dentry *dentry)
+{
+	umode_t mode = d_inode(dentry)->i_mode;
+	int kill = 0;
+
+	/* suid always must be killed */
+	if (unlikely(mode & S_ISUID))
+		kill = ATTR_KILL_SUID;
+
+	/*
+	 * sgid without any exec bits is just a mandatory locking mark; leave
+	 * it alone.  If some exec bits are set, it's a real sgid; kill it.
+	 */
+	if (unlikely((mode & S_ISGID) && (mode & S_IXGRP)))
+		kill |= ATTR_KILL_SGID;
+
+	if (unlikely(kill && !capable(CAP_FSETID) && S_ISREG(mode)))
+		return kill;
+
+	return 0;
+}
+EXPORT_SYMBOL(should_remove_suid);
+
 /**
  * chown_ok - verify permissions to chown inode
  * @mnt_userns:	user namespace of the mount @inode was found from
diff --git a/fs/inode.c b/fs/inode.c
index a71fb82279bb..3811269259e1 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1864,35 +1864,6 @@ void touch_atime(const struct path *path)
 }
 EXPORT_SYMBOL(touch_atime);
 
-/*
- * The logic we want is
- *
- *	if suid or (sgid and xgrp)
- *		remove privs
- */
-int should_remove_suid(struct dentry *dentry)
-{
-	umode_t mode = d_inode(dentry)->i_mode;
-	int kill = 0;
-
-	/* suid always must be killed */
-	if (unlikely(mode & S_ISUID))
-		kill = ATTR_KILL_SUID;
-
-	/*
-	 * sgid without any exec bits is just a mandatory locking mark; leave
-	 * it alone.  If some exec bits are set, it's a real sgid; kill it.
-	 */
-	if (unlikely((mode & S_ISGID) && (mode & S_IXGRP)))
-		kill |= ATTR_KILL_SGID;
-
-	if (unlikely(kill && !capable(CAP_FSETID) && S_ISREG(mode)))
-		return kill;
-
-	return 0;
-}
-EXPORT_SYMBOL(should_remove_suid);
-
 /*
  * Return mask of changes for notify_change() that need to be done as a
  * response to write or truncate. Return 0 if nothing has to be changed.
-- 
2.40.0.rc0.216.gc4246ad0f0-goog

[PATCH 5.15 09/11] attr: add setattr_should_drop_sgid()

[Leah Rumancik]

From: Christian Brauner <brauner@kernel.org>

commit 72ae017c5451860443a16fb2a8c243bff3e396b8 upstream.

[backport to 5.15.y, prior to vfsgid_t]

The current setgid stripping logic during write and ownership change
operations is inconsistent and strewn over multiple places. In order to
consolidate it and make more consistent we'll add a new helper
setattr_should_drop_sgid(). The function retains the old behavior where
we remove the S_ISGID bit unconditionally when S_IXGRP is set but also
when it isn't set and the caller is neither in the group of the inode
nor privileged over the inode.

We will use this helper both in write operation permission removal such
as file_remove_privs() as well as in ownership change operations.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Tested-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/attr.c     | 28 ++++++++++++++++++++++++++++
 fs/internal.h |  6 ++++++
 2 files changed, 34 insertions(+)

diff --git a/fs/attr.c b/fs/attr.c
index f045431bab1a..965be68ed8fa 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -20,6 +20,34 @@
 
 #include "internal.h"
 
+/**
+ * setattr_should_drop_sgid - determine whether the setgid bit needs to be
+ *                            removed
+ * @mnt_userns:	user namespace of the mount @inode was found from
+ * @inode:	inode to check
+ *
+ * This function determines whether the setgid bit needs to be removed.
+ * We retain backwards compatibility and require setgid bit to be removed
+ * unconditionally if S_IXGRP is set. Otherwise we have the exact same
+ * requirements as setattr_prepare() and setattr_copy().
+ *
+ * Return: ATTR_KILL_SGID if setgid bit needs to be removed, 0 otherwise.
+ */
+int setattr_should_drop_sgid(struct user_namespace *mnt_userns,
+			     const struct inode *inode)
+{
+	umode_t mode = inode->i_mode;
+
+	if (!(mode & S_ISGID))
+		return 0;
+	if (mode & S_IXGRP)
+		return ATTR_KILL_SGID;
+	if (!in_group_or_capable(mnt_userns, inode,
+				 i_gid_into_mnt(mnt_userns, inode)))
+		return ATTR_KILL_SGID;
+	return 0;
+}
+
 /*
  * The logic we want is
  *
diff --git a/fs/internal.h b/fs/internal.h
index c89814727281..45cf31d7380b 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -231,3 +231,9 @@ struct xattr_ctx {
 int setxattr_copy(const char __user *name, struct xattr_ctx *ctx);
 int do_setxattr(struct user_namespace *mnt_userns, struct dentry *dentry,
 		struct xattr_ctx *ctx);
+
+/*
+ * fs/attr.c
+ */
+int setattr_should_drop_sgid(struct user_namespace *mnt_userns,
+			     const struct inode *inode);
-- 
2.40.0.rc0.216.gc4246ad0f0-goog

[PATCH 5.15 10/11] attr: use consistent sgid stripping checks

[Leah Rumancik]

From: Christian Brauner <brauner@kernel.org>

commit ed5a7047d2011cb6b2bf84ceb6680124cc6a7d95 upstream.

[backport to 5.15.y, prior to vfsgid_t]

Currently setgid stripping in file_remove_privs()'s should_remove_suid()
helper is inconsistent with other parts of the vfs. Specifically, it only
raises ATTR_KILL_SGID if the inode is S_ISGID and S_IXGRP but not if the
inode isn't in the caller's groups and the caller isn't privileged over the
inode although we require this already in setattr_prepare() and
setattr_copy() and so all filesystem implement this requirement implicitly
because they have to use setattr_{prepare,copy}() anyway.

But the inconsistency shows up in setgid stripping bugs for overlayfs in
xfstests (e.g., generic/673, generic/683, generic/685, generic/686,
generic/687). For example, we test whether suid and setgid stripping works
correctly when performing various write-like operations as an unprivileged
user (fallocate, reflink, write, etc.):

echo "Test 1 - qa_user, non-exec file $verb"
setup_testfile
chmod a+rws $junk_file
commit_and_check "$qa_user" "$verb" 64k 64k

The test basically creates a file with 6666 permissions. While the file has
the S_ISUID and S_ISGID bits set it does not have the S_IXGRP set. On a
regular filesystem like xfs what will happen is:

sys_fallocate()
-> vfs_fallocate()
   -> xfs_file_fallocate()
      -> file_modified()
         -> __file_remove_privs()
            -> dentry_needs_remove_privs()
               -> should_remove_suid()
            -> __remove_privs()
               newattrs.ia_valid = ATTR_FORCE | kill;
               -> notify_change()
                  -> setattr_copy()

In should_remove_suid() we can see that ATTR_KILL_SUID is raised
unconditionally because the file in the test has S_ISUID set.

But we also see that ATTR_KILL_SGID won't be set because while the file
is S_ISGID it is not S_IXGRP (see above) which is a condition for
ATTR_KILL_SGID being raised.

So by the time we call notify_change() we have attr->ia_valid set to
ATTR_KILL_SUID | ATTR_FORCE. Now notify_change() sees that
ATTR_KILL_SUID is set and does:

ia_valid = attr->ia_valid |= ATTR_MODE
attr->ia_mode = (inode->i_mode & ~S_ISUID);

which means that when we call setattr_copy() later we will definitely
update inode->i_mode. Note that attr->ia_mode still contains S_ISGID.

Now we call into the filesystem's ->setattr() inode operation which will
end up calling setattr_copy(). Since ATTR_MODE is set we will hit:

if (ia_valid & ATTR_MODE) {
        umode_t mode = attr->ia_mode;
        vfsgid_t vfsgid = i_gid_into_vfsgid(mnt_userns, inode);
        if (!vfsgid_in_group_p(vfsgid) &&
            !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
                mode &= ~S_ISGID;
        inode->i_mode = mode;
}

and since the caller in the test is neither capable nor in the group of the
inode the S_ISGID bit is stripped.

But assume the file isn't suid then ATTR_KILL_SUID won't be raised which
has the consequence that neither the setgid nor the suid bits are stripped
even though it should be stripped because the inode isn't in the caller's
groups and the caller isn't privileged over the inode.

If overlayfs is in the mix things become a bit more complicated and the bug
shows up more clearly. When e.g., ovl_setattr() is hit from
ovl_fallocate()'s call to file_remove_privs() then ATTR_KILL_SUID and
ATTR_KILL_SGID might be raised but because the check in notify_change() is
questioning the ATTR_KILL_SGID flag again by requiring S_IXGRP for it to be
stripped the S_ISGID bit isn't removed even though it should be stripped:

sys_fallocate()
-> vfs_fallocate()
   -> ovl_fallocate()
      -> file_remove_privs()
         -> dentry_needs_remove_privs()
            -> should_remove_suid()
         -> __remove_privs()
            newattrs.ia_valid = ATTR_FORCE | kill;
            -> notify_change()
               -> ovl_setattr()
                  // TAKE ON MOUNTER'S CREDS
                  -> ovl_do_notify_change()
                     -> notify_change()
                  // GIVE UP MOUNTER'S CREDS
     // TAKE ON MOUNTER'S CREDS
     -> vfs_fallocate()
        -> xfs_file_fallocate()
           -> file_modified()
              -> __file_remove_privs()
                 -> dentry_needs_remove_privs()
                    -> should_remove_suid()
                 -> __remove_privs()
                    newattrs.ia_valid = attr_force | kill;
                    -> notify_change()

The fix for all of this is to make file_remove_privs()'s
should_remove_suid() helper to perform the same checks as we already
require in setattr_prepare() and setattr_copy() and have notify_change()
not pointlessly requiring S_IXGRP again. It doesn't make any sense in the
first place because the caller must calculate the flags via
should_remove_suid() anyway which would raise ATTR_KILL_SGID.

While we're at it we move should_remove_suid() from inode.c to attr.c
where it belongs with the rest of the iattr helpers. Especially since it
returns ATTR_KILL_S{G,U}ID flags. We also rename it to
setattr_should_drop_suidgid() to better reflect that it indicates both
setuid and setgid bit removal and also that it returns attr flags.

Running xfstests with this doesn't report any regressions. We should really
try and use consistent checks.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Tested-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
---
 Documentation/trace/ftrace.rst |  2 +-
 fs/attr.c                      | 33 +++++++++++++++++++--------------
 fs/fuse/file.c                 |  2 +-
 fs/inode.c                     |  7 ++++---
 fs/internal.h                  |  2 +-
 fs/ocfs2/file.c                |  4 ++--
 fs/open.c                      |  8 ++++----
 include/linux/fs.h             |  2 +-
 8 files changed, 33 insertions(+), 27 deletions(-)

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 4e5b26f03d5b..d036946bce7a 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -2929,7 +2929,7 @@ Produces::
               bash-1994  [000] ....  4342.324898: ima_get_action <-process_measurement
               bash-1994  [000] ....  4342.324898: ima_match_policy <-ima_get_action
               bash-1994  [000] ....  4342.324899: do_truncate <-do_last
-              bash-1994  [000] ....  4342.324899: should_remove_suid <-do_truncate
+              bash-1994  [000] ....  4342.324899: setattr_should_drop_suidgid <-do_truncate
               bash-1994  [000] ....  4342.324899: notify_change <-do_truncate
               bash-1994  [000] ....  4342.324900: current_fs_time <-notify_change
               bash-1994  [000] ....  4342.324900: current_kernel_time <-current_fs_time
diff --git a/fs/attr.c b/fs/attr.c
index 965be68ed8fa..0ca14cbd4b8b 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -48,34 +48,39 @@ int setattr_should_drop_sgid(struct user_namespace *mnt_userns,
 	return 0;
 }
 
-/*
- * The logic we want is
+/**
+ * setattr_should_drop_suidgid - determine whether the set{g,u}id bit needs to
+ *                               be dropped
+ * @mnt_userns:	user namespace of the mount @inode was found from
+ * @inode:	inode to check
  *
- *	if suid or (sgid and xgrp)
- *		remove privs
+ * This function determines whether the set{g,u}id bits need to be removed.
+ * If the setuid bit needs to be removed ATTR_KILL_SUID is returned. If the
+ * setgid bit needs to be removed ATTR_KILL_SGID is returned. If both
+ * set{g,u}id bits need to be removed the corresponding mask of both flags is
+ * returned.
+ *
+ * Return: A mask of ATTR_KILL_S{G,U}ID indicating which - if any - setid bits
+ * to remove, 0 otherwise.
  */
-int should_remove_suid(struct dentry *dentry)
+int setattr_should_drop_suidgid(struct user_namespace *mnt_userns,
+				struct inode *inode)
 {
-	umode_t mode = d_inode(dentry)->i_mode;
+	umode_t mode = inode->i_mode;
 	int kill = 0;
 
 	/* suid always must be killed */
 	if (unlikely(mode & S_ISUID))
 		kill = ATTR_KILL_SUID;
 
-	/*
-	 * sgid without any exec bits is just a mandatory locking mark; leave
-	 * it alone.  If some exec bits are set, it's a real sgid; kill it.
-	 */
-	if (unlikely((mode & S_ISGID) && (mode & S_IXGRP)))
-		kill |= ATTR_KILL_SGID;
+	kill |= setattr_should_drop_sgid(mnt_userns, inode);
 
 	if (unlikely(kill && !capable(CAP_FSETID) && S_ISREG(mode)))
 		return kill;
 
 	return 0;
 }
-EXPORT_SYMBOL(should_remove_suid);
+EXPORT_SYMBOL(setattr_should_drop_suidgid);
 
 /**
  * chown_ok - verify permissions to chown inode
@@ -440,7 +445,7 @@ int notify_change(struct user_namespace *mnt_userns, struct dentry *dentry,
 		}
 	}
 	if (ia_valid & ATTR_KILL_SGID) {
-		if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
+		if (mode & S_ISGID) {
 			if (!(ia_valid & ATTR_MODE)) {
 				ia_valid = attr->ia_valid |= ATTR_MODE;
 				attr->ia_mode = inode->i_mode;
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index cc95a1c37644..2b19d281351e 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1295,7 +1295,7 @@ static ssize_t fuse_cache_write_iter(struct kiocb *iocb, struct iov_iter *from)
 			return err;
 
 		if (fc->handle_killpriv_v2 &&
-		    should_remove_suid(file_dentry(file))) {
+		    setattr_should_drop_suidgid(&init_user_ns, file_inode(file))) {
 			goto writethrough;
 		}
 
diff --git a/fs/inode.c b/fs/inode.c
index 3811269259e1..079b64f9b756 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1869,7 +1869,8 @@ EXPORT_SYMBOL(touch_atime);
  * response to write or truncate. Return 0 if nothing has to be changed.
  * Negative value on error (change should be denied).
  */
-int dentry_needs_remove_privs(struct dentry *dentry)
+int dentry_needs_remove_privs(struct user_namespace *mnt_userns,
+			      struct dentry *dentry)
 {
 	struct inode *inode = d_inode(dentry);
 	int mask = 0;
@@ -1878,7 +1879,7 @@ int dentry_needs_remove_privs(struct dentry *dentry)
 	if (IS_NOSEC(inode))
 		return 0;
 
-	mask = should_remove_suid(dentry);
+	mask = setattr_should_drop_suidgid(mnt_userns, inode);
 	ret = security_inode_need_killpriv(dentry);
 	if (ret < 0)
 		return ret;
@@ -1920,7 +1921,7 @@ int file_remove_privs(struct file *file)
 	if (IS_NOSEC(inode) || !S_ISREG(inode->i_mode))
 		return 0;
 
-	kill = dentry_needs_remove_privs(dentry);
+	kill = dentry_needs_remove_privs(file_mnt_user_ns(file), dentry);
 	if (kill < 0)
 		return kill;
 	if (kill)
diff --git a/fs/internal.h b/fs/internal.h
index 45cf31d7380b..46df4ce58e87 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -149,7 +149,7 @@ extern int vfs_open(const struct path *, struct file *);
  */
 extern long prune_icache_sb(struct super_block *sb, struct shrink_control *sc);
 extern void inode_add_lru(struct inode *inode);
-extern int dentry_needs_remove_privs(struct dentry *dentry);
+int dentry_needs_remove_privs(struct user_namespace *, struct dentry *dentry);
 bool in_group_or_capable(struct user_namespace *mnt_userns,
 			 const struct inode *inode, kgid_t gid);
 
diff --git a/fs/ocfs2/file.c b/fs/ocfs2/file.c
index fc5f780fa235..92182d4be247 100644
--- a/fs/ocfs2/file.c
+++ b/fs/ocfs2/file.c
@@ -1994,7 +1994,7 @@ static int __ocfs2_change_file_space(struct file *file, struct inode *inode,
 		}
 	}
 
-	if (file && should_remove_suid(file->f_path.dentry)) {
+	if (file && setattr_should_drop_suidgid(&init_user_ns, file_inode(file))) {
 		ret = __ocfs2_write_remove_suid(inode, di_bh);
 		if (ret) {
 			mlog_errno(ret);
@@ -2282,7 +2282,7 @@ static int ocfs2_prepare_inode_for_write(struct file *file,
 		 * inode. There's also the dinode i_size state which
 		 * can be lost via setattr during extending writes (we
 		 * set inode->i_size at the end of a write. */
-		if (should_remove_suid(dentry)) {
+		if (setattr_should_drop_suidgid(&init_user_ns, inode)) {
 			if (meta_level == 0) {
 				ocfs2_inode_unlock_for_extent_tree(inode,
 								   &di_bh,
diff --git a/fs/open.c b/fs/open.c
index 5e322f188e83..e93c33069055 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -54,7 +54,7 @@ int do_truncate(struct user_namespace *mnt_userns, struct dentry *dentry,
 	}
 
 	/* Remove suid, sgid, and file capabilities on truncate too */
-	ret = dentry_needs_remove_privs(dentry);
+	ret = dentry_needs_remove_privs(mnt_userns, dentry);
 	if (ret < 0)
 		return ret;
 	if (ret)
@@ -671,10 +671,10 @@ int chown_common(const struct path *path, uid_t user, gid_t group)
 		newattrs.ia_valid |= ATTR_GID;
 		newattrs.ia_gid = gid;
 	}
-	if (!S_ISDIR(inode->i_mode))
-		newattrs.ia_valid |=
-			ATTR_KILL_SUID | ATTR_KILL_SGID | ATTR_KILL_PRIV;
 	inode_lock(inode);
+	if (!S_ISDIR(inode->i_mode))
+		newattrs.ia_valid |= ATTR_KILL_SUID | ATTR_KILL_PRIV |
+				     setattr_should_drop_sgid(mnt_userns, inode);
 	error = security_path_chown(path, uid, gid);
 	if (!error)
 		error = notify_change(mnt_userns, path->dentry, &newattrs,
diff --git a/include/linux/fs.h b/include/linux/fs.h
index be9be4a7216c..9601c2d774c8 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3133,7 +3133,7 @@ extern void __destroy_inode(struct inode *);
 extern struct inode *new_inode_pseudo(struct super_block *sb);
 extern struct inode *new_inode(struct super_block *sb);
 extern void free_inode_nonrcu(struct inode *inode);
-extern int should_remove_suid(struct dentry *);
+extern int setattr_should_drop_suidgid(struct user_namespace *, struct inode *);
 extern int file_remove_privs(struct file *);
 
 extern void __insert_inode_hash(struct inode *, unsigned long hashval);
-- 
2.40.0.rc0.216.gc4246ad0f0-goog

[PATCH 5.15 11/11] fs: use consistent setgid checks in is_sxid()

[Leah Rumancik]

From: Christian Brauner <brauner@kernel.org>

commit 8d84e39d76bd83474b26cb44f4b338635676e7e8 upstream.

Now that we made the VFS setgid checking consistent an inode can't be
marked security irrelevant even if the setgid bit is still set. Make
this function consistent with all other helpers.

Note that enforcing consistent setgid stripping checks for file
modification and mode- and ownership changes will cause the setgid bit
to be lost in more cases than useed to be the case. If an unprivileged
user wrote to a non-executable setgid file that they don't have
privilege over the setgid bit will be dropped. This will lead to
temporary failures in some xfstests until they have been updated.

Reported-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Tested-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
---
 include/linux/fs.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 9601c2d774c8..23ecfecdc450 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3571,7 +3571,7 @@ int __init list_bdev_fs_names(char *buf, size_t size);
 
 static inline bool is_sxid(umode_t mode)
 {
-	return (mode & S_ISUID) || ((mode & S_ISGID) && (mode & S_IXGRP));
+	return mode & (S_ISUID | S_ISGID);
 }
 
 static inline int check_sticky(struct user_namespace *mnt_userns,
-- 
2.40.0.rc0.216.gc4246ad0f0-goog

Re: [PATCH 5.15 00/11] sgid fixes for 5.15.y

[Sasha Levin]

On Tue, Mar 07, 2023 at 10:59:11AM -0800, Leah Rumancik wrote:
>Hello,
>
>I finished testing the sgid fixes which Amir graciously backported to
>5.15. This series fixes the previously failing generic/673 and
>generic/68[3-7]. No regressions were seen in the 25 runs of the auto
>group x 8 configs. I also did some extra runs on the perms group and
>no regressions there either. The corresponding fixes are already in
>6.1.y.

Queued up, thanks!

-- 
Thanks,
Sasha