[PATCH 1/2] fs: Check f_cred instead of current's creds in should_remove_suid()

[Andy Lutomirski]

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 instead of current's creds whenever a struct file is
involved.

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         | 16 +++++++++++-----
 fs/internal.h      |  2 +-
 fs/ocfs2/file.c    |  4 ++--
 fs/open.c          |  2 +-
 include/linux/fs.h |  2 +-
 5 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 88110fd0b282..f7029c40cfbd 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 ftruncate() or similar
+ * on that file.  If a file is not provided, we assume that no file
+ * descriptor is involved.
  */
-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;
@@ -1750,7 +1754,9 @@ 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)))
+	if (unlikely(kill && S_ISREG(mode) &&
+		     !(file ? file_ns_capable(file, &init_user_ns, CAP_FSETID) :
+		       capable(CAP_FSETID))))
 		return kill;
 
 	return 0;
@@ -1762,7 +1768,7 @@ 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 +1777,7 @@ 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 +1813,7 @@ 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);
-- 
2.9.3