[PATCH 5/9] anon_inode: raise SB_I_NODEV and SB_I_NOEXEC

[Christian Brauner <brauner@kernel.org>]

It isn't possible to execute anoymous inodes because they cannot be
opened in any way after they have been created. This includes execution:

execveat(fd_anon_inode, "", NULL, NULL, AT_EMPTY_PATH)

Anonymous inodes have inode->f_op set to no_open_fops which sets
no_open() which returns ENXIO. That means any call to do_dentry_open()
which is the endpoint of the do_open_execat() will fail. There's no
chance to execute an anonymous inode. Unless a given subsystem overrides
it ofc.

Howerver, we should still harden this and raise SB_I_NODEV and
SB_I_NOEXEC on the superblock itself so that no one gets any creative
ideas.

Cc: <stable@vger.kernel.org> # all LTS kernels
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/anon_inodes.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index cb51a90bece0..e51e7d88980a 100644
--- a/fs/anon_inodes.c
+++ b/fs/anon_inodes.c
@@ -86,6 +86,8 @@ static int anon_inodefs_init_fs_context(struct fs_context *fc)
 	struct pseudo_fs_context *ctx = init_pseudo(fc, ANON_INODE_FS_MAGIC);
 	if (!ctx)
 		return -ENOMEM;
+	fc->s_iflags |= SB_I_NOEXEC;
+	fc->s_iflags |= SB_I_NODEV;
 	ctx->dops = &anon_inodefs_dentry_operations;
 	return 0;
 }

-- 
2.47.2