Re: [PATCH v3 6/7] fs: strip file's S_ISGID mode on vfs instead of on underlying filesystem

Christian Brauner
2022-04-15T14:24:26Z

On Fri, Apr 15, 2022 at 07:02:22PM +0800, Yang Xu wrote:
> Currently, vfs only passes mode argument to filesystem, then use inode_init_owner()
> to strip S_ISGID. Some filesystem(ie ext4/btrfs) will call inode_init_owner
> firstly, then posxi acl setup, but xfs uses the contrary order. It will affect
> S_ISGID clear especially we filter S_IXGRP by umask or acl.
>
> Regardless of which filesystem is in use, failure to strip the SGID correctly is
> considered a security failure that needs to be fixed. The current VFS infrastructure
> requires the filesystem to do everything right and not step on any landmines to
> strip the SGID bit, when in fact it can easily be done at the VFS and the filesystems
> then don't even need to be aware that the SGID needs to be (or has been stripped) by
> the operation the user asked to be done.
>
> Vfs has all the info it needs - it doesn't need the filesystems to do everything
> correctly with the mode and ensuring that they order things like posix acl setup
> functions correctly with inode_init_owner() to strip the SGID bit.
>
> Just strip the SGID bit at the VFS, and then the filesystems can't get it wrong.
>
> Also, the inode_sgid_strip() api should be used before IS_POSIXACL() because
> this api may change mode.
>
> Only the following places use inode_init_owner
> "hugetlbfs/inode.c:846: inode_init_owner(&init_user_ns, inode, dir, mode);
> nilfs2/inode.c:354: inode_init_owner(&init_user_ns, inode, dir, mode);
> zonefs/super.c:1289: inode_init_owner(&init_user_ns, inode, parent, S_IFDIR | 0555);
> reiserfs/namei.c:619: inode_init_owner(&init_user_ns, inode, dir, mode);
> jfs/jfs_inode.c:67: inode_init_owner(&init_user_ns, inode, parent, mode);
> f2fs/namei.c:50: inode_init_owner(mnt_userns, inode, dir, mode);
> ext2/ialloc.c:549: inode_init_owner(&init_user_ns, inode, dir, mode);
> overlayfs/dir.c:643: inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
> ufs/ialloc.c:292: inode_init_owner(&init_user_ns, inode, dir, mode);
> ntfs3/inode.c:1283: inode_init_owner(mnt_userns, inode, dir, mode);
> ramfs/inode.c:64: inode_init_owner(&init_user_ns, inode, dir, mode);
> 9p/vfs_inode.c:263: inode_init_owner(&init_user_ns, inode, NULL, mode);
> btrfs/tests/btrfs-tests.c:65: inode_init_owner(&init_user_ns, inode, NULL, S_IFREG);
> btrfs/inode.c:6215: inode_init_owner(mnt_userns, inode, dir, mode);
> sysv/ialloc.c:166: inode_init_owner(&init_user_ns, inode, dir, mode);
> omfs/inode.c:51: inode_init_owner(&init_user_ns, inode, NULL, mode);
> ubifs/dir.c:97: inode_init_owner(&init_user_ns, inode, dir, mode);
> udf/ialloc.c:108: inode_init_owner(&init_user_ns, inode, dir, mode);
> ext4/ialloc.c:979: inode_init_owner(mnt_userns, inode, dir, mode);
> hfsplus/inode.c:393: inode_init_owner(&init_user_ns, inode, dir, mode);
> xfs/xfs_inode.c:840: inode_init_owner(mnt_userns, inode, dir, mode);
> ocfs2/dlmfs/dlmfs.c:331: inode_init_owner(&init_user_ns, inode, NULL, mode);
> ocfs2/dlmfs/dlmfs.c:354: inode_init_owner(&init_user_ns, inode, parent, mode);
> ocfs2/namei.c:200: inode_init_owner(&init_user_ns, inode, dir, mode);
> minix/bitmap.c:255: inode_init_owner(&init_user_ns, inode, dir, mode);
> bfs/dir.c:99: inode_init_owner(&init_user_ns, inode, dir, mode);
> "
>
> They are used in filesystem init new inode function and these init inode functions are used
> by following operations:
> mkdir
> symlink
> mknod
> create
> tmpfile
> rename
>
> We don't care about mkdir because we don't strip SGID bit for directory except fs.xfs.irix_sgid_inherit.
> But we even call it in do_mkdirat() since inode_sgid_strip() will skip directories anyway. This will
> enforce the same ordering for all relevant operations and it will make the code more uniform and
> easier to understand by using prepare_mode().
>
> symlink and rename only use valid mode that doesn't have SGID bit.
>
> We have added inode_sgid_strip api for the remaining operations.
>
> In addition to the above six operations, two filesystems has a little difference
> 1) btrfs has btrfs_create_subvol_root to create new inode but used non SGID bit mode and can ignore
> 2) ocfs2 reflink function should add inode_sgid_strip api manually because we don't add it in vfs
>
> This patch also changed grpid behaviour for ext4/xfs because the mode passed to them may been
> changed by inode_sgid_strip.
>
> Also as Christian Brauner said"
> The patch itself is useful as it would move a security sensitive operation that is currently burried in
> individual filesystems into the vfs layer. But it has a decent regression potential since it might strip
> filesystems that have so far relied on getting the S_ISGID bit with a mode argument. So this needs a lot
> of testing and long exposure in -next for at least one full kernel cycle."
>
> Suggested-by: Dave Chinner <david@fromorbit.com>
> Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
> ---
> v2->v3:
> 1.use new helper prepare_mode to do inode sgid strip and umask strip
> 2.also use prepare_mode() for mkdirat
> fs/inode.c | 2 --
> fs/namei.c | 14 +++++---------
> fs/ocfs2/namei.c | 1 +
> 3 files changed, 6 insertions(+), 11 deletions(-)
>
> diff --git a/fs/inode.c b/fs/inode.c
> index 1b569ad882ce..a250aa01d3c3 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2246,8 +2246,6 @@ 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
> - inode_sgid_strip(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 bbc7c950bbdc..0fadc884af7f 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3287,8 +3287,7 @@ 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();
> + prepare_mode(mnt_userns, dir->d_inode, &mode);
> if (likely(got_write))
> create_error = may_o_create(mnt_userns, &nd->path,
> dentry, mode);
> @@ -3521,8 +3520,7 @@ 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();
> + prepare_mode(mnt_userns, dir, &mode);
> error = dir->i_op->tmpfile(mnt_userns, dir, child, mode);
> if (error)
> goto out_err;
> @@ -3852,13 +3850,12 @@ 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();
> + mnt_userns = mnt_user_ns(path.mnt);
> + prepare_mode(mnt_userns, path.dentry->d_inode, &mode);
> error = security_path_mknod(&path, dentry, mode, dev);
> if (error)
> goto out2;
>
> - mnt_userns = mnt_user_ns(path.mnt);
> switch (mode & S_IFMT) {
> case 0: case S_IFREG:
> error = vfs_create(mnt_userns, path.dentry->d_inode,
> @@ -3952,12 +3949,11 @@ 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);

Your changes causes the security and the filesystem layer to potentially
see different values for mode.

You need to change the patch so prepare_mode() is called before the mode
is passed to the security layer. This will ensure that both the security
layer and the vfs see the same mode.

> if (!error) {
> struct user_namespace *mnt_userns;
> mnt_userns = mnt_user_ns(path.mnt);
> + prepare_mode(mnt_userns, path.dentry->d_inode, &mode);
> error = vfs_mkdir(mnt_userns, path.dentry->d_inode, dentry,
> mode);
> }
> diff --git a/fs/ocfs2/namei.c b/fs/ocfs2/namei.c
> index c75fd54b9185..c81b8e0847aa 100644
> --- a/fs/ocfs2/namei.c
> +++ b/fs/ocfs2/namei.c
> @@ -198,6 +198,7 @@ static struct inode *ocfs2_get_init_inode(struct inode *dir, umode_t mode)
> if (S_ISDIR(mode))
> set_nlink(inode, 2);
> inode_init_owner(&init_user_ns, inode, dir, mode);
> + inode_sgid_strip(&init_user_ns, dir, &mode);
> status = dquot_initialize(inode);
> if (status)
> return ERR_PTR(status);
> --
> 2.27.0
>