on 2022/4/21 16:35, Christian Brauner wrote:
> On Thu, Apr 21, 2022 at 03:54:17PM +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 filesystem 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
>> "
>> arch/powerpc/platforms/cell/spufs/inode.c: inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
>> arch/powerpc/platforms/cell/spufs/inode.c: inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
>> fs/9p/vfs_inode.c: inode_init_owner(&init_user_ns, inode, NULL, mode);
>> fs/bfs/dir.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/btrfs/inode.c: inode_init_owner(mnt_userns, inode, dir, mode);
>> fs/btrfs/tests/btrfs-tests.c: inode_init_owner(&init_user_ns, inode, NULL, S_IFREG);
>> fs/ext2/ialloc.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/ext4/ialloc.c: inode_init_owner(mnt_userns, inode, dir, mode);
>> fs/f2fs/namei.c: inode_init_owner(mnt_userns, inode, dir, mode);
>> fs/hfsplus/inode.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/hugetlbfs/inode.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/jfs/jfs_inode.c: inode_init_owner(&init_user_ns, inode, parent, mode);
>> fs/minix/bitmap.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/nilfs2/inode.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/ntfs3/inode.c: inode_init_owner(mnt_userns, inode, dir, mode);
>> fs/ocfs2/dlmfs/dlmfs.c: inode_init_owner(&init_user_ns, inode, NULL, mode);
>> fs/ocfs2/dlmfs/dlmfs.c: inode_init_owner(&init_user_ns, inode, parent, mode);
>> fs/ocfs2/namei.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/omfs/inode.c: inode_init_owner(&init_user_ns, inode, NULL, mode);
>> fs/overlayfs/dir.c: inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
>> fs/ramfs/inode.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/reiserfs/namei.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/sysv/ialloc.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/ubifs/dir.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/udf/ialloc.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/ufs/ialloc.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/xfs/xfs_inode.c: inode_init_owner(mnt_userns, inode, dir, mode);
>> fs/zonefs/super.c: inode_init_owner(&init_user_ns, inode, parent, S_IFDIR | 0555);
>> kernel/bpf/inode.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> mm/shmem.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> "
>>
>> They are used in filesystem to 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 prepare_mode() 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 new helper 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, four 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
>> 3) spufs which doesn't really go hrough the regular VFS callpath because it has
>> separate system call spu_create, but it t only allows the creation of
>> directories and only allows bits in 0777 and can ignore
>> 4) bpf use vfs_mkobj in bpf_obj_do_pin with
>> "S_IFREG | ((S_IRUSR | S_IWUSR)& ~current_umask()) mode and
>> use bpf_mkobj_ops in bpf_iter_link_pin_kernel with S_IFREG | S_IRUSR mode,
>> so bpf is also not affected
>>
>> 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@xxxxxxxxxxxxx>
>> Signed-off-by: Yang Xu<xuyang2018.jy@xxxxxxxxxxx>
>> ---
>> v4->v5:
>> put inode_sgid_strip before the inode_init_owner in ocfs2 filesystem
>> because the inode->i_mode's assignment is in inode_init_owner
>> fs/inode.c | 2 --
>> fs/namei.c | 22 +++++++++-------------
>> fs/ocfs2/namei.c | 1 +
>> include/linux/fs.h | 11 +++++++++++
>> 4 files changed, 21 insertions(+), 15 deletions(-)
>>
>> diff --git a/fs/inode.c b/fs/inode.c
>> index 57130e4ef8b4..95667e634bd4 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
>> - mode = 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 73646e28fae0..5b8e6288d503 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();
>> + mode = 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();
>> + mode = prepare_mode(mnt_userns, dir, mode);
>> error = dir->i_op->tmpfile(mnt_userns, dir, child, mode);
>> if (error)
>> goto out_err;
>> @@ -3850,13 +3848,12 @@ 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);
>> + mode = 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,
>> @@ -3943,6 +3940,7 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
>> struct path path;
>> int error;
>> unsigned int lookup_flags = LOOKUP_DIRECTORY;
>> + struct user_namespace *mnt_userns;
>>
>> retry:
>> dentry = filename_create(dfd, name,&path, lookup_flags);
>> @@ -3950,15 +3948,13 @@ 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();
>> + mnt_userns = mnt_user_ns(path.mnt);
>> + mode = prepare_mode(mnt_userns, path.dentry->d_inode, mode);
>> error = security_path_mkdir(&path, dentry, mode);
>> - if (!error) {
>> - struct user_namespace *mnt_userns;
>> - mnt_userns = mnt_user_ns(path.mnt);
>> + if (!error)
>> error = vfs_mkdir(mnt_userns, path.dentry->d_inode, dentry,
>> mode);
>> - }
>> +
>> done_path_create(&path, dentry);
>> if (retry_estale(error, lookup_flags)) {
>> lookup_flags |= LOOKUP_REVAL;
>> diff --git a/fs/ocfs2/namei.c b/fs/ocfs2/namei.c
>> index c75fd54b9185..21f3da2e66c9 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 = inode_sgid_strip(&init_user_ns, dir, mode);
>> inode_init_owner(&init_user_ns, inode, dir, mode);
>
> For the record, I'm not too fond of this separate invocation of
> inode_sgid_strip() but since it's only one location this might be fine.
> If there's more than one location a separate helper should exist for
> this that abstracts this away for the filesystem.
Agree. This case only be found when using OCFS2_IOC_REFLINK ioctl. And
other support reflink filesystem(xfs, btrfs) they use FICLONE or
FICLONERANGE ioctl.
Since ocfs2 has supported reflink by using it remap_file_range, should
we still need this ioctl?
commit bd50873dc725a9fa72592ecc986c58805e823051
Author: Tao Ma <tao.ma@xxxxxxxxxx>
Date: Mon Sep 21 11:25:14 2009 +0800
ocfs2: Add ioctl for reflink.
The ioctl will take 3 parameters: old_path, new_path and
preserve and call vfs_reflink. It is useful when we backport
reflink features to old kernels.
Signed-off-by: Tao Ma <tao.ma@xxxxxxxxxx>
Of course, this is a problem doesn't belong to this series.
>
> Two questions:
> - Sould this call prepare_mode(), i.e. should we honor umasks here too?
IMO, it desn't need to honor umask. Because reflink only will update
inode_imode by setattr to strip S_ISGID and S_ISUID instead of creating
a file.
> - How is the sgid bit handled when creating reflinks on other reflink
> supporting filesystems such as xfs and btrfs?
xfstests has a test case generic/673 for this, so btrfs and xfs should
have the same behavior.
I look into xfs code.
Firstly
If we don't have CAP_FSETID and it is a regulre file,also have sgid bit,
then should_remove_suid will give attr a ATTR_KILL_SGID mask.
ATTR_KILL_SGID -> clear S_ISGID if executable
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;
}
Then it will call notify_change to modify inode mode by using setattr
hook as below:
fs/attr.c notfiy_change
{
...
if (ia_valid & ATTR_KILL_SUID) {
if (mode & S_ISUID) {
ia_valid = attr->ia_valid |= ATTR_MODE;
attr->ia_mode = (inode->i_mode & ~S_ISUID);
}
}
if (ia_valid & ATTR_KILL_SGID) {
if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
if (!(ia_valid & ATTR_MODE)) {
ia_valid = attr->ia_valid |= ATTR_MODE;
attr->ia_mode = inode->i_mode;
}
attr->ia_mode &= ~S_ISGID;
}
}
...
Here it will strip attr->ia_mode by check inode->i_mode whether have
sgid bit and group-execute bit.
then setattr_prepare and setattr_copy has the remaining sgid stripping
rule just neither are in the group of the current file nor have
CAP_FSETID in their user namespace.
if (ia_valid & ATTR_MODE) {
umode_t mode = attr->ia_mode;
kgid_t kgid = i_gid_into_mnt(mnt_userns, inode); //
this code seems unnecessary can be used directly in in_group_p
if (!in_group_p(kgid) &&
!capable_wrt_inode_uidgid(mnt_userns, inode,
CAP_FSETID))
mode &= ~S_ISGID;
inode->i_mode = mode;
It seems this way has similar logic but not include umask.
>
>> status = dquot_initialize(inode);
>> if (status)
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index 532de76c9b91..0bf81ab71619 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -3459,6 +3459,17 @@ static inline bool dir_relax_shared(struct inode *inode)
>> return !IS_DEADDIR(inode);
>> }
>>
>> +static inline umode_t prepare_mode(struct user_namespace *mnt_userns,
>> + const struct inode *dir, umode_t mode)
>> +{
>> + mode = inode_sgid_strip(mnt_userns, dir, mode);
>> +
>> + if (!IS_POSIXACL(dir))
>> + mode&= current_umask();
>
> You're missing a "~". I assume you meant:
>
> mode&= ~current_umask();
Yes, sorry for this.
