Re: [PATCH v2 1/2] fs: Check f_cred as well as of current's creds in should_remove_suid()

[Jeff Layton]

On Fri, 2017-01-27 at 18:49 -0800, Andy Lutomirski wrote:
> If an unprivileged program opens a setgid file for write and passes
> the fd to a privileged program and the privileged program writes to
> it, we currently fail to clear the setgid bit.  Fix it by checking
> f_cred in addition to current's creds whenever a struct file is
> involved.
> 
> I'm checking both because I'm nervous about preserving the SUID and
> SGID bits in any situation in which they're not currently preserved
> and because Ben Hutchings suggested doing it this way.
> 
> I don't know why we check capabilities at all, and we could probably
> get away with clearing the setgid bit regardless of capabilities,
> but this change should be less likely to break some weird program.
> 
> This mitigates exploits that take advantage of world-writable setgid
> files or directories.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
>  fs/inode.c         | 37 +++++++++++++++++++++++++++++++------
>  fs/internal.h      |  2 +-
>  fs/ocfs2/file.c    |  4 ++--
>  fs/open.c          |  2 +-
>  include/linux/fs.h |  2 +-
>  5 files changed, 36 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index 88110fd0b282..0e1e141b094c 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -1733,8 +1733,12 @@ EXPORT_SYMBOL(touch_atime);
>   *
>   *	if suid or (sgid and xgrp)
>   *		remove privs
> + *
> + * If a file is provided, we assume that this is write(), ftruncate() or
> + * similar on that file.  If a file is not provided, we assume that no
> + * file descriptor is involved (e.g. truncate()).
>   */
> -int should_remove_suid(struct dentry *dentry)
> +int should_remove_suid(struct dentry *dentry, struct file *file)
>  {
>  	umode_t mode = d_inode(dentry)->i_mode;
>  	int kill = 0;
> @@ -1750,8 +1754,29 @@ int should_remove_suid(struct dentry *dentry)
>  	if (unlikely((mode & S_ISGID) && (mode & S_IXGRP)))
>  		kill |= ATTR_KILL_SGID;
>  
> -	if (unlikely(kill && !capable(CAP_FSETID) && S_ISREG(mode)))
> -		return kill;
> +	if (unlikely(kill && S_ISREG(mode))) {
> +		/*
> +		 * To minimize the degree to which this code works differently
> +		 * from Linux 4.9 and below, we kill SUID/SGID if the writer
> +		 * is unprivileged even if the file was opened by a privileged
> +		 * process.  Yes, this is a hack and is a technical violation
> +		 * of the "write(2) doesn't check current_cred()" rule.
> +		 *
> +		 * Ideally we would just kill the SUID bit regardless
> +		 * of capabilities.
> +		 */
> +		if (!capable(CAP_FSETID))
> +			return kill;
> +
> +		/*
> +		 * To avoid abuse of stdout/stderr redirection, we need to
> +		 * kill SUID/SGID if the file was opened by an unprivileged
> +		 * task.
> +		 */
> +		if (file && file->f_cred != current_cred() &&
> +		    !file_ns_capable(file, &init_user_ns, CAP_FSETID))
> +			return kill;
> +	}
>  
>  	return 0;
>  }
> @@ -1762,7 +1787,7 @@ EXPORT_SYMBOL(should_remove_suid);
>   * response to write or truncate. Return 0 if nothing has to be changed.
>   * Negative value on error (change should be denied).
>   */
> -int dentry_needs_remove_privs(struct dentry *dentry)
> +int dentry_needs_remove_privs(struct dentry *dentry, struct file *file)
>  {
>  	struct inode *inode = d_inode(dentry);
>  	int mask = 0;
> @@ -1771,7 +1796,7 @@ int dentry_needs_remove_privs(struct dentry *dentry)
>  	if (IS_NOSEC(inode))
>  		return 0;
>  
> -	mask = should_remove_suid(dentry);
> +	mask = should_remove_suid(dentry, file);
>  	ret = security_inode_need_killpriv(dentry);
>  	if (ret < 0)
>  		return ret;
> @@ -1807,7 +1832,7 @@ int file_remove_privs(struct file *file)
>  	if (IS_NOSEC(inode))
>  		return 0;
>  
> -	kill = dentry_needs_remove_privs(dentry);
> +	kill = dentry_needs_remove_privs(dentry, file);
>  	if (kill < 0)
>  		return kill;
>  	if (kill)
> diff --git a/fs/internal.h b/fs/internal.h
> index b63cf3af2dc2..c467ad502cac 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -119,7 +119,7 @@ extern struct file *filp_clone_open(struct file *);
>   */
>  extern long prune_icache_sb(struct super_block *sb, struct shrink_control *sc);
>  extern void inode_add_lru(struct inode *inode);
> -extern int dentry_needs_remove_privs(struct dentry *dentry);
> +extern int dentry_needs_remove_privs(struct dentry *dentry, struct file *file);
>  
>  extern bool __atime_needs_update(const struct path *, struct inode *, bool);
>  static inline bool atime_needs_update_rcu(const struct path *path,
> diff --git a/fs/ocfs2/file.c b/fs/ocfs2/file.c
> index c4889655d32b..db6efd940ac0 100644
> --- a/fs/ocfs2/file.c
> +++ b/fs/ocfs2/file.c
> @@ -1903,7 +1903,7 @@ static int __ocfs2_change_file_space(struct file *file, struct inode *inode,
>  		}
>  	}
>  
> -	if (file && should_remove_suid(file->f_path.dentry)) {
> +	if (file && should_remove_suid(file->f_path.dentry, file)) {
>  		ret = __ocfs2_write_remove_suid(inode, di_bh);
>  		if (ret) {
>  			mlog_errno(ret);
> @@ -2132,7 +2132,7 @@ static int ocfs2_prepare_inode_for_write(struct file *file,
>  		 * inode. There's also the dinode i_size state which
>  		 * can be lost via setattr during extending writes (we
>  		 * set inode->i_size at the end of a write. */
> -		if (should_remove_suid(dentry)) {
> +		if (should_remove_suid(dentry, file)) {
>  			if (meta_level == 0) {
>  				ocfs2_inode_unlock(inode, meta_level);
>  				meta_level = 1;
> diff --git a/fs/open.c b/fs/open.c
> index d3ed8171e8e0..8f54f34d1e3e 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -52,7 +52,7 @@ int do_truncate(struct dentry *dentry, loff_t length, unsigned int time_attrs,
>  	}
>  
>  	/* Remove suid, sgid, and file capabilities on truncate too */
> -	ret = dentry_needs_remove_privs(dentry);
> +	ret = dentry_needs_remove_privs(dentry, filp);
>  	if (ret < 0)
>  		return ret;
>  	if (ret)
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 2ba074328894..87654fb21158 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2718,7 +2718,7 @@ extern void __destroy_inode(struct inode *);
>  extern struct inode *new_inode_pseudo(struct super_block *sb);
>  extern struct inode *new_inode(struct super_block *sb);
>  extern void free_inode_nonrcu(struct inode *inode);
> -extern int should_remove_suid(struct dentry *);
> +extern int should_remove_suid(struct dentry *, struct file *);
>  extern int file_remove_privs(struct file *);
>  
>  extern void __insert_inode_hash(struct inode *, unsigned long hashval);

Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>