Fix up non-directory creation in SGID directories

sgid directories have special semantics, making newly created files in
the directory belong to the group of the directory, and newly created
subdirectories will also become sgid.  This is historically used for
group-shared directories.

But group directories writable by non-group members should not imply
that such non-group members can magically join the group, so make sure
to clear the sgid bit on non-directories for non-members (but remember
that sgid without group execute means "mandatory locking", just to
confuse things even more).

Reported-by: Jann Horn <jannh@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/inode.c b/fs/inode.c
index 2c300e98179607ea0062a2c1dbcee17e9bc926c4..8c86c809ca17b30e003913e169626aa42df2e908 100644 (file)
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1999,8 +1999,14 @@ void inode_init_owner(struct inode *inode, const struct inode *dir,
        inode->i_uid = current_fsuid();
        if (dir && dir->i_mode & S_ISGID) {
                inode->i_gid = dir->i_gid;
+
+               /* Directories are special, and always inherit S_ISGID */
                if (S_ISDIR(mode))
                        mode |= S_ISGID;
+               else if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP) &&
+                        !in_group_p(inode->i_gid) &&
+                        !capable_wrt_inode_uidgid(dir, CAP_FSETID))
+                       mode &= ~S_ISGID;
        } else
                inode->i_gid = current_fsgid();
        inode->i_mode = mode;