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fs: add mode_strip_sgid() helper

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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: e014f37db1 ("xfs: use setattr_copy to set vfs inode attributes") [1]
Link: 01ea173e10 ("xfs: fix up non-directory creation in SGID directories") [2]
Link: fd84bfdddd ("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>
This commit is contained in:
Yang Xu 2022-07-14 14:11:25 +08:00 • committed by Christian Brauner (Microsoft)
parent ff6992735a
commit 2b3416ceff
No known key found for this signature in database
GPG key ID: 91C61BC06578DCA2
2 changed files with 34 additions and 4 deletions
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36
fs/inode.c
View file

@ -2246,10 +2246,8 @@ 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;
@ -2405,3 +2403,33 @@ 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);

2
include/linux/fs.h
View file

@ -1903,6 +1903,8 @@ 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