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> -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html