d_real() is about to be able to fail for D_REAL_DATA. So switch
ipe_build_eval_ctx() from d_real_inode() to d_real_data_inode() and deal
with the error.

Currently, if overlayfs fails to resolve the data of a file it hands
back the overlayfs inode and IPE evaluates the policy against that. The
overlayfs inode has neither a block device nor any fs-verity state so
none of the properties of the file hosting the data are visible.

Remember the error in the evaluation context and if a policy is loaded
refuse the operation with -EACCES without looking at the rules. That's
what IPE returns for every denial and what IMA returns when it can't
appraise a file. Permissive mode allows it like any other denial and
without a policy IPE still allows everything.

Note that the operation wouldn't necessarily fail on its own. overlayfs
doesn't verify the data again on mmap() so a file whose data can't be
resolved could otherwise be mapped executable.

This is a no-op until d_real() can actually fail.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 security/ipe/eval.c | 15 ++++++++++++++-
 security/ipe/eval.h |  1 +
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/security/ipe/eval.c b/security/ipe/eval.c
index 4a7be96c84f03..3751963509df4 100644
--- a/security/ipe/eval.c
+++ b/security/ipe/eval.c
@@ -102,7 +102,11 @@ void ipe_build_eval_ctx(struct ipe_eval_ctx *ctx,
 
 	if (file) {
 		build_ipe_sb_ctx(ctx, file);
-		ino = d_real_inode(file->f_path.dentry);
+		ino = d_real_data_inode(file->f_path.dentry);
+		if (IS_ERR(ino)) {
+			ctx->file_err = PTR_ERR(ino);
+			return;
+		}
 		build_ipe_bdev_ctx(ctx, ino);
 		build_ipe_inode_ctx(ctx, ino);
 	}
@@ -335,6 +339,15 @@ int ipe_evaluate_event(const struct ipe_eval_ctx *const ctx)
 		return 0;
 	}
 
+	/*
+	 * Without the inode hosting the data none of the properties of the
+	 * file can be evaluated. Refuse it unless IPE is permissive.
+	 */
+	if (ctx->file_err) {
+		rcu_read_unlock();
+		return READ_ONCE(enforce) ? -EACCES : 0;
+	}
+
 	if (ctx->op == IPE_OP_INVALID) {
 		if (pol->parsed->global_default_action == IPE_ACTION_INVALID) {
 			WARN(1, "no default rule set for unknown op, ALLOW it");
diff --git a/security/ipe/eval.h b/security/ipe/eval.h
index b26d1147d0708..58ca50987b2b5 100644
--- a/security/ipe/eval.h
+++ b/security/ipe/eval.h
@@ -42,6 +42,7 @@ struct ipe_eval_ctx {
 	enum ipe_hook_type hook;
 
 	const struct file *file;
+	int file_err;
 	bool initramfs;
 #ifdef CONFIG_IPE_PROP_DM_VERITY
 	const struct ipe_bdev *ipe_bdev;

-- 
2.53.0