[PATCH RFC v4 18/25] af_unix: use scoped_with_init_fs() for coredump socket lookup

[Christian Brauner <brauner@kernel.org>]

Use scoped_with_init_fs() to temporarily override current->fs for the
coredump unix socket path resolution. This replaces the init_root() +
vfs_path_lookup() pattern with scoped_with_init_fs() + kern_path().

The old code used LOOKUP_BENEATH to confine the lookup beneath init's
root. This is dropped because the coredump socket path is absolute and
resolved from root (where ".." is a no-op), and LOOKUP_NO_SYMLINKS
already blocks any symlink-based escape. LOOKUP_BENEATH was redundant
in this context.

unix_find_bsd(SOCK_COREDUMP) ← coredump_sock_connect() ← do_coredump() —
same crashing userspace process

Same security rationale as coredump.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 net/unix/af_unix.c | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index e2d787ca3e74..af719429f3b1 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1198,17 +1198,12 @@ static struct sock *unix_find_bsd(struct sockaddr_un *sunaddr, int addr_len,
 	unix_mkname_bsd(sunaddr, addr_len);
 
 	if (flags & SOCK_COREDUMP) {
-		struct path root;
-
-		task_lock(&init_task);
-		get_fs_root(init_task.fs, &root);
-		task_unlock(&init_task);
-
-		scoped_with_kernel_creds()
-			err = vfs_path_lookup(root.dentry, root.mnt, sunaddr->sun_path,
-					      LOOKUP_BENEATH | LOOKUP_NO_SYMLINKS |
-					      LOOKUP_NO_MAGICLINKS, &path);
-		path_put(&root);
+		scoped_with_init_fs() {
+			scoped_with_kernel_creds()
+				err = kern_path(sunaddr->sun_path,
+						LOOKUP_NO_SYMLINKS |
+						LOOKUP_NO_MAGICLINKS, &path);
+		}
 		if (err)
 			goto fail;
 	} else {

-- 
2.47.3