Use scoped_with_init_fs() to temporarily override current->fs for
the filp_open() call so the coredump path lookup happens in init's
filesystem context. This replaces the init_root() + file_open_root()
pattern with the simpler scoped override.

coredump_file() ← do_coredump() ← vfs_coredump() ← get_signal() — runs
as the crashing userspace process

Uses init's root to prevent a chrooted/user-namespaced process from
controlling where suid coredumps land. Not a kthread, but intentionally
needs init's fs for security.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/coredump.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 29df8aa19e2e..7428349f10bf 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -919,15 +919,10 @@ static bool coredump_file(struct core_name *cn, struct coredump_params *cprm,
 		 * with a fully qualified path" rule is to control where
 		 * coredumps may be placed using root privileges,
 		 * current->fs->root must not be used. Instead, use the
-		 * root directory of init_task.
+		 * root directory of PID 1.
 		 */
-		struct path root;
-
-		task_lock(&init_task);
-		get_fs_root(init_task.fs, &root);
-		task_unlock(&init_task);
-		file = file_open_root(&root, cn->corename, open_flags, 0600);
-		path_put(&root);
+		scoped_with_init_fs()
+			file = filp_open(cn->corename, open_flags, 0600);
 	} else {
 		file = filp_open(cn->corename, open_flags, 0600);
 	}

-- 
2.47.3