anon_inode: rework assertions

Making anonymous inodes regular files comes with a lot of risk and
regression potential as evidenced by a recent hickup in io_uring. We're
better of continuing to not have them be regular files. Since we have
S_ANON_INODE we can port all of our assertions easily.

Link: https://lore.kernel.org/[email protected]
Fixes: cfd86ef ("anon_inode: use a proper mode internally")
Acked-by: Jens Axboe <[email protected]>
Cc: [email protected]
Reported-by: Jens Axboe <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: brauner

fs/exec.c

@@ -114,6 +114,9 @@ static inline void put_binfmt(struct linux_binfmt * fmt)
 
 bool path_noexec(const struct path *path)
 {
+	/* If it's an anonymous inode make sure that we catch any shenanigans. */
+	VFS_WARN_ON_ONCE(IS_ANON_FILE(d_inode(path->dentry)) &&
+			 !(path->mnt->mnt_sb->s_iflags & SB_I_NOEXEC));
 	return (path->mnt->mnt_flags & MNT_NOEXEC) ||
 	       (path->mnt->mnt_sb->s_iflags & SB_I_NOEXEC);
 }
@@ -781,13 +784,15 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
 	if (IS_ERR(file))
 		return file;
 
+	if (path_noexec(&file->f_path))
+		return ERR_PTR(-EACCES);
+
 	/*
 	 * In the past the regular type check was here. It moved to may_open() in
 	 * 633fb6ac3980 ("exec: move S_ISREG() check earlier"). Since then it is
 	 * an invariant that all non-regular files error out before we get here.
 	 */
-	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)) ||
-	    path_noexec(&file->f_path))
+	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
 		return ERR_PTR(-EACCES);
 
 	err = exe_file_deny_write_access(file);

fs/libfs.c

@@ -1649,12 +1649,10 @@ struct inode *alloc_anon_inode(struct super_block *s)
 	 */
 	inode->i_state = I_DIRTY;
 	/*
-	 * Historically anonymous inodes didn't have a type at all and
-	 * userspace has come to rely on this. Internally they're just
-	 * regular files but S_IFREG is masked off when reporting
-	 * information to userspace.
+	 * Historically anonymous inodes don't have a type at all and
+	 * userspace has come to rely on this.
 	 */
-	inode->i_mode = S_IFREG | S_IRUSR | S_IWUSR;
+	inode->i_mode = S_IRUSR | S_IWUSR;
 	inode->i_uid = current_fsuid();
 	inode->i_gid = current_fsgid();
 	inode->i_flags |= S_PRIVATE | S_ANON_INODE;

fs/namei.c

@@ -3471,7 +3471,7 @@ static int may_open(struct mnt_idmap *idmap, const struct path *path,
 			return -EACCES;
 		break;
 	default:
-		VFS_BUG_ON_INODE(1, inode);
+		VFS_BUG_ON_INODE(!IS_ANON_FILE(inode), inode);
 	}
 
 	error = inode_permission(idmap, inode, MAY_OPEN | acc_mode);