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: sta...@vger.kernel.org
> Signed-off-by: Andy Lutomirski <l...@kernel.org>
> ---
> 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 <jlay...@redhat.com>
