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Date:   Sat, 28 Nov 2020 16:34:02 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org,
        John Johansen <john.johansen@...onical.com>,
        James Morris <jmorris@...ei.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Geoffrey Thomas <geofft@...reload.com>,
        Mrunal Patel <mpatel@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Andy Lutomirski <luto@...nel.org>,
        Theodore Tso <tytso@....edu>, Alban Crequy <alban@...volk.io>,
        Tycho Andersen <tycho@...ho.ws>,
        David Howells <dhowells@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Seth Forshee <seth.forshee@...onical.com>,
        St├ęphane Graber <stgraber@...ntu.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Lennart Poettering <lennart@...ttering.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>, smbarber@...omium.org,
        Phil Estes <estesp@...il.com>, Serge Hallyn <serge@...lyn.com>,
        Kees Cook <keescook@...omium.org>,
        Todd Kjos <tkjos@...gle.com>, Paul Moore <paul@...l-moore.com>,
        Jonathan Corbet <corbet@....net>,
        containers@...ts.linux-foundation.org, fstests@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-integrity@...r.kernel.org,
        selinux@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v3 08/38] capability: handle idmapped mounts

On Sat, Nov 28, 2020 at 10:34:57PM +0100, Christian Brauner wrote:
> In order to determine whether a caller holds privilege over a given inode the
> capability framework exposes the two helpers privileged_wrt_inode_uidgid() and
> capable_wrt_inode_uidgid(). The former verifies that the inode has a mapping in
> the caller's user namespace and the latter additionally verifies that the caller
> has the requested capability in their current user namespace.
> If the inode is accessed through an idmapped mount we simply need to map it
> according to the mount's user namespace. Afterwards the checks are identical to
> non-idmapped inodes. If the initial user namespace is passed all operations are
> a nop so non-idmapped mounts will not see a change in behavior and will also not
> see any performance impact.
> 
> Cc: Christoph Hellwig <hch@....de>
> Cc: David Howells <dhowells@...hat.com>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Cc: linux-fsdevel@...r.kernel.org
> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>

Acked-by: Serge Hallyn <serge@...lyn.com>

> ---
> /* v2 */
> - Christoph Hellwig <hch@....de>:
>   - Don't pollute the vfs with additional helpers simply extend the existing
>     helpers with an additional argument and switch all callers.
> 
> /* v3 */
> unchanged
> ---
>  fs/attr.c                  |  8 ++++----
>  fs/exec.c                  |  2 +-
>  fs/inode.c                 |  2 +-
>  fs/namei.c                 | 13 ++++++++-----
>  fs/overlayfs/super.c       |  2 +-
>  fs/posix_acl.c             |  2 +-
>  fs/xfs/xfs_ioctl.c         |  2 +-
>  include/linux/capability.h |  7 +++++--
>  kernel/capability.c        | 14 +++++++++-----
>  security/commoncap.c       |  5 +++--
>  10 files changed, 34 insertions(+), 23 deletions(-)
> 
> diff --git a/fs/attr.c b/fs/attr.c
> index b4bbdbd4c8ca..d270f640a192 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -23,7 +23,7 @@ static bool chown_ok(const struct inode *inode, kuid_t uid)
>  	if (uid_eq(current_fsuid(), inode->i_uid) &&
>  	    uid_eq(uid, inode->i_uid))
>  		return true;
> -	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +	if (capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_CHOWN))
>  		return true;
>  	if (uid_eq(inode->i_uid, INVALID_UID) &&
>  	    ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> @@ -36,7 +36,7 @@ static bool chgrp_ok(const struct inode *inode, kgid_t gid)
>  	if (uid_eq(current_fsuid(), inode->i_uid) &&
>  	    (in_group_p(gid) || gid_eq(gid, inode->i_gid)))
>  		return true;
> -	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +	if (capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_CHOWN))
>  		return true;
>  	if (gid_eq(inode->i_gid, INVALID_GID) &&
>  	    ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> @@ -92,7 +92,7 @@ int setattr_prepare(struct dentry *dentry, struct iattr *attr)
>  		/* Also check the setgid bit! */
>  		if (!in_group_p((ia_valid & ATTR_GID) ? attr->ia_gid :
>  				inode->i_gid) &&
> -		    !capable_wrt_inode_uidgid(inode, CAP_FSETID))
> +		    !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID))
>  			attr->ia_mode &= ~S_ISGID;
>  	}
>  
> @@ -193,7 +193,7 @@ void setattr_copy(struct inode *inode, const struct iattr *attr)
>  		umode_t mode = attr->ia_mode;
>  
>  		if (!in_group_p(inode->i_gid) &&
> -		    !capable_wrt_inode_uidgid(inode, CAP_FSETID))
> +		    !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID))
>  			mode &= ~S_ISGID;
>  		inode->i_mode = mode;
>  	}
> diff --git a/fs/exec.c b/fs/exec.c
> index 547a2390baf5..8e75d7a33514 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1398,7 +1398,7 @@ void would_dump(struct linux_binprm *bprm, struct file *file)
>  		/* Ensure mm->user_ns contains the executable */
>  		user_ns = old = bprm->mm->user_ns;
>  		while ((user_ns != &init_user_ns) &&
> -		       !privileged_wrt_inode_uidgid(user_ns, inode))
> +		       !privileged_wrt_inode_uidgid(user_ns, &init_user_ns, inode))
>  			user_ns = user_ns->parent;
>  
>  		if (old != user_ns) {
> diff --git a/fs/inode.c b/fs/inode.c
> index 9d78c37b00b8..7a15372d9c2d 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2147,7 +2147,7 @@ void inode_init_owner(struct inode *inode, const struct inode *dir,
>  			mode |= S_ISGID;
>  		else if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP) &&
>  			 !in_group_p(inode->i_gid) &&
> -			 !capable_wrt_inode_uidgid(dir, CAP_FSETID))
> +			 !capable_wrt_inode_uidgid(&init_user_ns, dir, CAP_FSETID))
>  			mode &= ~S_ISGID;
>  	} else
>  		inode->i_gid = current_fsgid();
> diff --git a/fs/namei.c b/fs/namei.c
> index d4a6dd772303..3f52730af6c5 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -357,10 +357,11 @@ int generic_permission(struct inode *inode, int mask)
>  	if (S_ISDIR(inode->i_mode)) {
>  		/* DACs are overridable for directories */
>  		if (!(mask & MAY_WRITE))
> -			if (capable_wrt_inode_uidgid(inode,
> +			if (capable_wrt_inode_uidgid(&init_user_ns, inode,
>  						     CAP_DAC_READ_SEARCH))
>  				return 0;
> -		if (capable_wrt_inode_uidgid(inode, CAP_DAC_OVERRIDE))
> +		if (capable_wrt_inode_uidgid(&init_user_ns, inode,
> +					     CAP_DAC_OVERRIDE))
>  			return 0;
>  		return -EACCES;
>  	}
> @@ -370,7 +371,8 @@ int generic_permission(struct inode *inode, int mask)
>  	 */
>  	mask &= MAY_READ | MAY_WRITE | MAY_EXEC;
>  	if (mask == MAY_READ)
> -		if (capable_wrt_inode_uidgid(inode, CAP_DAC_READ_SEARCH))
> +		if (capable_wrt_inode_uidgid(&init_user_ns, inode,
> +					     CAP_DAC_READ_SEARCH))
>  			return 0;
>  	/*
>  	 * Read/write DACs are always overridable.
> @@ -378,7 +380,8 @@ int generic_permission(struct inode *inode, int mask)
>  	 * at least one exec bit set.
>  	 */
>  	if (!(mask & MAY_EXEC) || (inode->i_mode & S_IXUGO))
> -		if (capable_wrt_inode_uidgid(inode, CAP_DAC_OVERRIDE))
> +		if (capable_wrt_inode_uidgid(&init_user_ns, inode,
> +					     CAP_DAC_OVERRIDE))
>  			return 0;
>  
>  	return -EACCES;
> @@ -2657,7 +2660,7 @@ int __check_sticky(struct inode *dir, struct inode *inode)
>  		return 0;
>  	if (uid_eq(dir->i_uid, fsuid))
>  		return 0;
> -	return !capable_wrt_inode_uidgid(inode, CAP_FOWNER);
> +	return !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FOWNER);
>  }
>  EXPORT_SYMBOL(__check_sticky);
>  
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 290983bcfbb3..196fe3e3f02b 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -972,7 +972,7 @@ ovl_posix_acl_xattr_set(const struct xattr_handler *handler,
>  	if (unlikely(inode->i_mode & S_ISGID) &&
>  	    handler->flags == ACL_TYPE_ACCESS &&
>  	    !in_group_p(inode->i_gid) &&
> -	    !capable_wrt_inode_uidgid(inode, CAP_FSETID)) {
> +	    !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID)) {
>  		struct iattr iattr = { .ia_valid = ATTR_KILL_SGID };
>  
>  		err = ovl_setattr(dentry, &iattr);
> diff --git a/fs/posix_acl.c b/fs/posix_acl.c
> index 95882b3f5f62..4ca6d53c6f0a 100644
> --- a/fs/posix_acl.c
> +++ b/fs/posix_acl.c
> @@ -656,7 +656,7 @@ int posix_acl_update_mode(struct inode *inode, umode_t *mode_p,
>  	if (error == 0)
>  		*acl = NULL;
>  	if (!in_group_p(inode->i_gid) &&
> -	    !capable_wrt_inode_uidgid(inode, CAP_FSETID))
> +	    !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID))
>  		mode &= ~S_ISGID;
>  	*mode_p = mode;
>  	return 0;
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index 3fbd98f61ea5..97bd29fc8c43 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -1502,7 +1502,7 @@ xfs_ioctl_setattr(
>  	 */
>  
>  	if ((VFS_I(ip)->i_mode & (S_ISUID|S_ISGID)) &&
> -	    !capable_wrt_inode_uidgid(VFS_I(ip), CAP_FSETID))
> +	    !capable_wrt_inode_uidgid(&init_user_ns, VFS_I(ip), CAP_FSETID))
>  		VFS_I(ip)->i_mode &= ~(S_ISUID|S_ISGID);
>  
>  	/* Change the ownerships and register project quota modifications */
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index 1e7fe311cabe..041e336f3369 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -247,8 +247,11 @@ static inline bool ns_capable_setid(struct user_namespace *ns, int cap)
>  	return true;
>  }
>  #endif /* CONFIG_MULTIUSER */
> -extern bool privileged_wrt_inode_uidgid(struct user_namespace *ns, const struct inode *inode);
> -extern bool capable_wrt_inode_uidgid(const struct inode *inode, int cap);
> +extern bool privileged_wrt_inode_uidgid(struct user_namespace *ns,
> +					struct user_namespace *mnt_user_ns,
> +					const struct inode *inode);
> +extern bool capable_wrt_inode_uidgid(struct user_namespace *mnt_user_ns,
> +				     const struct inode *inode, int cap);
>  extern bool file_ns_capable(const struct file *file, struct user_namespace *ns, int cap);
>  extern bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns);
>  static inline bool perfmon_capable(void)
> diff --git a/kernel/capability.c b/kernel/capability.c
> index de7eac903a2a..28e3a599ff7a 100644
> --- a/kernel/capability.c
> +++ b/kernel/capability.c
> @@ -484,10 +484,12 @@ EXPORT_SYMBOL(file_ns_capable);
>   *
>   * Return true if the inode uid and gid are within the namespace.
>   */
> -bool privileged_wrt_inode_uidgid(struct user_namespace *ns, const struct inode *inode)
> +bool privileged_wrt_inode_uidgid(struct user_namespace *ns,
> +				 struct user_namespace *mnt_user_ns,
> +				 const struct inode *inode)
>  {
> -	return kuid_has_mapping(ns, inode->i_uid) &&
> -		kgid_has_mapping(ns, inode->i_gid);
> +	return kuid_has_mapping(ns, i_uid_into_mnt(mnt_user_ns, inode)) &&
> +	       kgid_has_mapping(ns, i_gid_into_mnt(mnt_user_ns, inode));
>  }
>  
>  /**
> @@ -499,11 +501,13 @@ bool privileged_wrt_inode_uidgid(struct user_namespace *ns, const struct inode *
>   * its own user namespace and that the given inode's uid and gid are
>   * mapped into the current user namespace.
>   */
> -bool capable_wrt_inode_uidgid(const struct inode *inode, int cap)
> +bool capable_wrt_inode_uidgid(struct user_namespace *mnt_user_ns,
> +			      const struct inode *inode, int cap)
>  {
>  	struct user_namespace *ns = current_user_ns();
>  
> -	return ns_capable(ns, cap) && privileged_wrt_inode_uidgid(ns, inode);
> +	return ns_capable(ns, cap) &&
> +	       privileged_wrt_inode_uidgid(ns, mnt_user_ns, inode);
>  }
>  EXPORT_SYMBOL(capable_wrt_inode_uidgid);
>  
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 59bf3c1674c8..4cd2bdfd0a8b 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -489,7 +489,7 @@ int cap_convert_nscap(struct dentry *dentry, void **ivalue, size_t size)
>  		return -EINVAL;
>  	if (!validheader(size, cap))
>  		return -EINVAL;
> -	if (!capable_wrt_inode_uidgid(inode, CAP_SETFCAP))
> +	if (!capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_SETFCAP))
>  		return -EPERM;
>  	if (size == XATTR_CAPS_SZ_2)
>  		if (ns_capable(inode->i_sb->s_user_ns, CAP_SETFCAP))
> @@ -957,7 +957,8 @@ int cap_inode_removexattr(struct dentry *dentry, const char *name)
>  		struct inode *inode = d_backing_inode(dentry);
>  		if (!inode)
>  			return -EINVAL;
> -		if (!capable_wrt_inode_uidgid(inode, CAP_SETFCAP))
> +		if (!capable_wrt_inode_uidgid(&init_user_ns, inode,
> +					      CAP_SETFCAP))
>  			return -EPERM;
>  		return 0;
>  	}
> -- 
> 2.29.2

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