[PATCH 2/2] fs: Harden against open(..., O_CREAT, 02777) in a setgid directory

[Andy Lutomirski <luto@kernel.org>]

Currently, if you open("foo", O_WRONLY | O_CREAT | ..., 02777) in a
directory that is setgid and owned by a different gid than current's
fsgid, you end up with an SGID executable that is owned by the
directory's GID.  This is a Bad Thing (tm).  Exploiting this is
nontrivial because most ways of creating a new file create an empty
file and empty executables aren't particularly interesting, but this
is nevertheless quite dangerous.

Harden against this type of attack by detecting this particular
corner case (unprivileged program creates SGID executable inode in
SGID directory owned by a different GID) and clearing the new
inode's SGID bit.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 fs/inode.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index f7029c40cfbd..d7e4b80470dd 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2007,11 +2007,28 @@ void inode_init_owner(struct inode *inode, const struct inode *dir,
 {
 	inode->i_uid = current_fsuid();
 	if (dir && dir->i_mode & S_ISGID) {
+		bool changing_gid = !gid_eq(inode->i_gid, dir->i_gid);
+
 		inode->i_gid = dir->i_gid;
-		if (S_ISDIR(mode))
+		if (S_ISDIR(mode)) {
 			mode |= S_ISGID;
-	} else
+		} else if (((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP))
+			   && S_ISREG(mode) && changing_gid
+			   && !capable(CAP_FSETID)) {
+			/*
+			 * Whoa there!  An unprivileged program just
+			 * tried to create a new executable with SGID
+			 * set in a directory with SGID set that belongs
+			 * to a different group.  Don't let this program
+			 * create a SGID executable that ends up owned
+			 * by the wrong group.
+			 */
+			mode &= ~S_ISGID;
+		}
+
+	} else {
 		inode->i_gid = current_fsgid();
+	}
 	inode->i_mode = mode;
 }
 EXPORT_SYMBOL(inode_init_owner);
-- 
2.9.3