Christian Brauner <brauner@xxxxxxxxxx> 于2022年8月25日周四 21:25写道：
>
> On Thu, Aug 25, 2022 at 09:05:52PM +0800, Zhang Tianci wrote:
> > There is a wrong case of link() on overlay:
> > $ mkdir /lower /fuse /merge
> > $ mount -t fuse /fuse
> > $ mkdir /fuse/upper /fuse/work
> > $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,\
> > workdir=work
> > $ touch /merge/file
> > $ chown bin.bin /merge/file // the file's caller becomes "bin"
> > $ ln /merge/file /merge/lnkfile
> >
> > Then we will get an error(EACCES) because fuse daemon checks the link()'s
> > caller is "bin", it denied this request.
> >
> > In the changing history of ovl_link(), there are two key commits:
> >
> > The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> > overrides the cred's fsuid/fsgid using the new inode. The new inode's
> > owner is initialized by inode_init_owner(), and inode->fsuid is
> > assigned to the current user. So the override fsuid becomes the
> > current user. We know link() is actually modifying the directory, so
> > the caller must have the MAY_WRITE permission on the directory. The
> > current caller may should have this permission. This is acceptable
> > to use the caller's fsuid.
> >
> > The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> > which removed the inode creation in ovl_link(). This commit move
> > inode_init_owner() into ovl_create_object(), so the ovl_link() just
> > give the old inode to ovl_create_or_link(). Then the override fsuid
> > becomes the old inode's fsuid, neither the caller nor the overlay's
> > creator! So this is incorrect.
> >
> > Fix this bug by using current fsuid/fsgid to do underlying fs's link().
> >
> > Link: https://lore.kernel.org/all/20220817102951.xnvesg3a7rbv576x@wittgenstein/T
> >
> > Signed-off-by: Zhang Tianci <zhangtianci.1997@xxxxxxxxxxxxx>
> > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@xxxxxxxxxxxxx>
> > Signed-off-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx>
> > ---
>
> (Should probably also use a
> Suggested-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx>, I
> think. But not that important.)
>
> Looks good to me but this really should be tested to survive xfstests,
> Reviewed-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx>
>
> > fs/overlayfs/dir.c | 16 +++++++++++-----
> > fs/overlayfs/overlayfs.h | 2 ++
> > 2 files changed, 13 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > index 6b03457f72bb..dd84e6fc5f6e 100644
> > --- a/fs/overlayfs/dir.c
> > +++ b/fs/overlayfs/dir.c
> > @@ -595,8 +595,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > err = -ENOMEM;
> > override_cred = prepare_creds();
> > if (override_cred) {
> > - override_cred->fsuid = inode->i_uid;
> > - override_cred->fsgid = inode->i_gid;
> > + override_cred->fsuid = attr->fsuid;
> > + override_cred->fsgid = attr->fsgid;
> > if (!attr->hardlink) {
> > err = security_dentry_create_files_as(dentry,
> > attr->mode, &dentry->d_name, old_cred,
> > @@ -646,6 +646,8 @@ static int ovl_create_object(struct dentry *dentry, int mode, dev_t rdev,
> > inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
> > attr.mode = inode->i_mode;
> >
> > + attr.fsuid = inode->i_uid;
> > + attr.fsgid = inode->i_gid;
> > err = ovl_create_or_link(dentry, inode, &attr, false);
> > /* Did we end up using the preallocated inode? */
> > if (inode != d_inode(dentry))
> > @@ -702,6 +704,7 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
> > {
> > int err;
> > struct inode *inode;
> > + struct ovl_cattr attr;
> >
> > err = ovl_want_write(old);
> > if (err)
> > @@ -728,9 +731,12 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
> > inode = d_inode(old);
> > ihold(inode);
> >
> > - err = ovl_create_or_link(new, inode,
> > - &(struct ovl_cattr) {.hardlink = ovl_dentry_upper(old)},
> > - ovl_type_origin(old));
> > + attr = (struct ovl_cattr) {
> > + .hardlink = ovl_dentry_upper(old),
> > + .fsuid = current_fsuid(),
> > + .fsgid = current_fsgid(),
> > + };
> > + err = ovl_create_or_link(new, inode, &attr, ovl_type_origin(old));
> > if (err)
> > iput(inode);
> >
> > diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> > index 87759165d32b..85043123a103 100644
> > --- a/fs/overlayfs/overlayfs.h
> > +++ b/fs/overlayfs/overlayfs.h
> > @@ -655,6 +655,8 @@ struct ovl_cattr {
> > umode_t mode;
> > const char *link;
> > struct dentry *hardlink;
> > + kuid_t fsuid;
> > + kgid_t fsgid;
> > };
> >
> > #define OVL_CATTR(m) (&(struct ovl_cattr) { .mode = (m) })
> > --
> > 2.32.1 (Apple Git-133)
> >
I tested this patch base on linux-6.0.0-rc3, and all were successful.
I tested xfstest using this local config:
export TEST_DEV=/dev/vdb1
export TEST_DIR=/mnt/test
export SCRATCH_DEV=/dev/vdb2
export SCRATCH_MNT=/mnt/scratch
FSTYP=xfs
run ./check -overlay in xfstest.
