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Commit 0cb0093f authored by Yang Xu's avatar Yang Xu Committed by Greg Kroah-Hartman
Browse files

fs: add mode_strip_sgid() helper 

commit 2b3416ceff5e6bd4922f6d1c61fb68113dd82302 upstream.

[remove userns argument of helper for 5.4.y backport]

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: default avatarDarrick J. Wong <djwong@kernel.org>
Reviewed-by: default avatarChristian Brauner (Microsoft) <brauner@kernel.org>
Reviewed-and-Tested-by: default avatarJeff Layton <jlayton@kernel.org>
Signed-off-by: default avatarYang Xu <xuyang2018.jy@fujitsu.com>
Signed-off-by: default avatarChristian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: default avatarAmir Goldstein <amir73il@gmail.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
[commit 347750e1b69cef62966fbc5bd7dc579b4c00688a upstream
	backported from 5.10.y, resolved context conflicts]
Signed-off-by: default avatarMahmoud Adam <mngyadam@amazon.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 635a0039

fs/inode.c

+30 −4
Original line number Diff line number Diff line
@@ -2100,10 +2100,8 @@ void inode_init_owner(struct inode *inode, const struct inode *dir, 
		/* 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(inode->i_gid) &&

			 !capable_wrt_inode_uidgid(dir, CAP_FSETID))

			mode &= ~S_ISGID;

		else

			mode = mode_strip_sgid(dir, mode);

	} else

		inode->i_gid = current_fsgid();

	inode->i_mode = mode;
@@ -2359,3 +2357,31 @@ int vfs_ioc_fssetxattr_check(struct inode *inode, const struct fsxattr *old_fa, 
	return 0;

}

EXPORT_SYMBOL(vfs_ioc_fssetxattr_check);



/**

 * mode_strip_sgid - handle the sgid bit for non-directories

 * @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(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(dir->i_gid))

		return mode;

	if (capable_wrt_inode_uidgid(dir, CAP_FSETID))

		return mode;



	return mode & ~S_ISGID;

}

EXPORT_SYMBOL(mode_strip_sgid);

include/linux/fs.h

+1 −0
Original line number Diff line number Diff line
@@ -1743,6 +1743,7 @@ extern long compat_ptr_ioctl(struct file *file, unsigned int cmd, 
extern void inode_init_owner(struct inode *inode, const struct inode *dir,

			umode_t mode);

extern bool may_open_dev(const struct path *path);

umode_t mode_strip_sgid(const struct inode *dir, umode_t mode);

/*

 * VFS FS_IOC_FIEMAP helper definitions.

 */