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Message-ID: <20210128165852.GA20974@mail.hallyn.com>
Date: Thu, 28 Jan 2021 10:58:52 -0600
From: "Serge E. Hallyn" <serge@...lyn.com>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Miklos Szeredi <mszeredi@...hat.com>,
linux-fsdevel@...r.kernel.org, linux-unionfs@...r.kernel.org,
linux-security-module@...r.kernel.org,
linux-kernel@...r.kernel.org,
"Serge E . Hallyn" <serge@...lyn.com>,
Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [PATCH 2/2] security.capability: fix conversions on getxattr
On Tue, Jan 19, 2021 at 07:34:49PM -0600, Eric W. Biederman wrote:
> Miklos Szeredi <mszeredi@...hat.com> writes:
>
> > If a capability is stored on disk in v2 format cap_inode_getsecurity() will
> > currently return in v2 format unconditionally.
> >
> > This is wrong: v2 cap should be equivalent to a v3 cap with zero rootid,
> > and so the same conversions performed on it.
> >
> > If the rootid cannot be mapped v3 is returned unconverted. Fix this so
> > that both v2 and v3 return -EOVERFLOW if the rootid (or the owner of the fs
> > user namespace in case of v2) cannot be mapped in the current user
> > namespace.
>
> This looks like a good cleanup.
Sorry, I'm not following. Why is this a good cleanup? Why should
the xattr be shown as faked v3 in this case?
A separate question below.
> I do wonder how well this works with stacking. In particular
> ovl_xattr_set appears to call vfs_getxattr without overriding the creds.
> What the purpose of that is I haven't quite figured out. It looks like
> it is just a probe to see if an xattr is present so maybe it is ok.
>
> Acked-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>
> >
> > Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
> > ---
> > security/commoncap.c | 67 ++++++++++++++++++++++++++++----------------
> > 1 file changed, 43 insertions(+), 24 deletions(-)
> >
> > diff --git a/security/commoncap.c b/security/commoncap.c
> > index bacc1111d871..c9d99f8f4c82 100644
> > --- a/security/commoncap.c
> > +++ b/security/commoncap.c
> > @@ -371,10 +371,11 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
> > {
> > int size, ret;
> > kuid_t kroot;
> > + __le32 nsmagic, magic;
> > uid_t root, mappedroot;
> > char *tmpbuf = NULL;
> > struct vfs_cap_data *cap;
> > - struct vfs_ns_cap_data *nscap;
> > + struct vfs_ns_cap_data *nscap = NULL;
> > struct dentry *dentry;
> > struct user_namespace *fs_ns;
> >
> > @@ -396,46 +397,61 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
> > fs_ns = inode->i_sb->s_user_ns;
> > cap = (struct vfs_cap_data *) tmpbuf;
> > if (is_v2header((size_t) ret, cap)) {
> > - /* If this is sizeof(vfs_cap_data) then we're ok with the
> > - * on-disk value, so return that. */
> > - if (alloc)
> > - *buffer = tmpbuf;
> > - else
> > - kfree(tmpbuf);
> > - return ret;
> > - } else if (!is_v3header((size_t) ret, cap)) {
> > - kfree(tmpbuf);
> > - return -EINVAL;
> > + root = 0;
> > + } else if (is_v3header((size_t) ret, cap)) {
> > + nscap = (struct vfs_ns_cap_data *) tmpbuf;
> > + root = le32_to_cpu(nscap->rootid);
> > + } else {
> > + size = -EINVAL;
> > + goto out_free;
> > }
> >
> > - nscap = (struct vfs_ns_cap_data *) tmpbuf;
> > - root = le32_to_cpu(nscap->rootid);
> > kroot = make_kuid(fs_ns, root);
> >
> > /* If the root kuid maps to a valid uid in current ns, then return
> > * this as a nscap. */
> > mappedroot = from_kuid(current_user_ns(), kroot);
> > if (mappedroot != (uid_t)-1 && mappedroot != (uid_t)0) {
> > + size = sizeof(struct vfs_ns_cap_data);
> > if (alloc) {
> > - *buffer = tmpbuf;
> > + if (!nscap) {
> > + /* v2 -> v3 conversion */
> > + nscap = kzalloc(size, GFP_ATOMIC);
> > + if (!nscap) {
> > + size = -ENOMEM;
> > + goto out_free;
> > + }
> > + nsmagic = VFS_CAP_REVISION_3;
> > + magic = le32_to_cpu(cap->magic_etc);
> > + if (magic & VFS_CAP_FLAGS_EFFECTIVE)
> > + nsmagic |= VFS_CAP_FLAGS_EFFECTIVE;
> > + memcpy(&nscap->data, &cap->data, sizeof(__le32) * 2 * VFS_CAP_U32);
> > + nscap->magic_etc = cpu_to_le32(nsmagic);
> > + } else {
> > + /* use allocated v3 buffer */
> > + tmpbuf = NULL;
> > + }
> > nscap->rootid = cpu_to_le32(mappedroot);
> > - } else
> > - kfree(tmpbuf);
> > - return size;
> > + *buffer = nscap;
> > + }
> > + goto out_free;
> > }
> >
> > if (!rootid_owns_currentns(kroot)) {
> > - kfree(tmpbuf);
> > - return -EOPNOTSUPP;
> > + size = -EOVERFLOW;
Why this change? Christian (cc:d) noticed that this is a user visible change.
Without this change, if you are in a userns which has different rootid, the
EOVERFLOW tells vfs_getxattr to vall back to __vfs_getxattr() and so you can
see the v3 capability with its rootid.
With this change, you instead just get EOVERFLOW.
> > + goto out_free;
> > }
> >
> > /* This comes from a parent namespace. Return as a v2 capability */
> > size = sizeof(struct vfs_cap_data);
> > if (alloc) {
> > - *buffer = kmalloc(size, GFP_ATOMIC);
> > - if (*buffer) {
> > - struct vfs_cap_data *cap = *buffer;
> > - __le32 nsmagic, magic;
> > + if (nscap) {
> > + /* v3 -> v2 conversion */
> > + cap = kzalloc(size, GFP_ATOMIC);
> > + if (!cap) {
> > + size = -ENOMEM;
> > + goto out_free;
> > + }
> > magic = VFS_CAP_REVISION_2;
> > nsmagic = le32_to_cpu(nscap->magic_etc);
> > if (nsmagic & VFS_CAP_FLAGS_EFFECTIVE)
> > @@ -443,9 +459,12 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
> > memcpy(&cap->data, &nscap->data, sizeof(__le32) * 2 * VFS_CAP_U32);
> > cap->magic_etc = cpu_to_le32(magic);
> > } else {
> > - size = -ENOMEM;
> > + /* use unconverted v2 */
> > + tmpbuf = NULL;
> > }
> > + *buffer = cap;
> > }
> > +out_free:
> > kfree(tmpbuf);
> > return size;
> > }
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