Extract the initramfs unpacking code into a separate
unpack_initramfs() function and wrap its invocation from
do_populate_rootfs() with scoped_with_init_fs(). This ensures all
file operations during initramfs unpacking (including filp_open()
calls in do_name() and populate_initrd_image()) happen in init's
filesystem context.

Note that security_initramfs_populated() needs the scope as well since
it does use current->fs to derive the initramfs superblock.

do_populate_rootfs() ← async_schedule_domain() ← kworker (async
workqueue)

May also run synchronously from PID 1 in case async workqueue is
considered full. Overriding in that case is fine as well.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 init/initramfs.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/init/initramfs.c b/init/initramfs.c
index 139baed06589..3faa2045b9cf 100644
--- a/init/initramfs.c
+++ b/init/initramfs.c
@@ -3,6 +3,7 @@
 #include <linux/async.h>
 #include <linux/export.h>
 #include <linux/fs.h>
+#include <linux/fs_struct.h>
 #include <linux/slab.h>
 #include <linux/types.h>
 #include <linux/fcntl.h>
@@ -715,7 +716,7 @@ static void __init populate_initrd_image(char *err)
 }
 #endif /* CONFIG_BLK_DEV_RAM */
 
-static void __init do_populate_rootfs(void *unused, async_cookie_t cookie)
+static void __init unpack_initramfs(async_cookie_t cookie)
 {
 	/* Load the built in initramfs */
 	char *err = unpack_to_rootfs(__initramfs_start, __initramfs_size);
@@ -723,7 +724,7 @@ static void __init do_populate_rootfs(void *unused, async_cookie_t cookie)
 		panic_show_mem("%s", err); /* Failed to decompress INTERNAL initramfs */
 
 	if (!initrd_start || IS_ENABLED(CONFIG_INITRAMFS_FORCE))
-		goto done;
+		return;
 
 	if (IS_ENABLED(CONFIG_BLK_DEV_RAM))
 		printk(KERN_INFO "Trying to unpack rootfs image as initramfs...\n");
@@ -738,9 +739,14 @@ static void __init do_populate_rootfs(void *unused, async_cookie_t cookie)
 		printk(KERN_EMERG "Initramfs unpacking failed: %s\n", err);
 #endif
 	}
+}
 
-done:
-	security_initramfs_populated();
+static void __init do_populate_rootfs(void *unused, async_cookie_t cookie)
+{
+	scoped_with_init_fs() {
+		unpack_initramfs(cookie);
+		security_initramfs_populated();
+	}
 
 	/*
 	 * If the initrd region is overlapped with crashkernel reserved region,

-- 
2.47.3