Re: [PATCH 3/3] init: use do_mount() instead of ksys_mount()

[Linus Torvalds <torvalds@linux-foundation.org>]

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On Sun, Dec 15, 2019 at 5:35 PM Ondřej Jirman <megi@xff.cz> wrote:
>
> Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000

Duh. So much for the trivial obvious conversion.

It didn't take "data might be NULL" into account.

A patch like this, perhaps? Untested..

               Linus

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 init/do_mounts.c | 23 +++++++++++++----------
 1 file changed, 13 insertions(+), 10 deletions(-)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index f55cbd9cb818..d204f605dbce 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -391,17 +391,19 @@ static int __init do_mount_root(const char *name, const char *fs,
 				 const int flags, const void *data)
 {
 	struct super_block *s;
-	char *data_page;
-	struct page *p;
+	struct page *p = NULL;
+	char *data_page = NULL;
 	int ret;
 
-	/* do_mount() requires a full page as fifth argument */
-	p = alloc_page(GFP_KERNEL);
-	if (!p)
-		return -ENOMEM;
-
-	data_page = page_address(p);
-	strncpy(data_page, data, PAGE_SIZE - 1);
+	if (data) {
+		/* do_mount() requires a full page as fifth argument */
+		p = alloc_page(GFP_KERNEL);
+		if (!p)
+			return -ENOMEM;
+		data_page = page_address(p);
+		strncpy(data_page, data, PAGE_SIZE - 1);
+		data_page[PAGE_SIZE - 1] = '\0';
+	}
 
 	ret = do_mount(name, "/root", fs, flags, data_page);
 	if (ret)
@@ -417,7 +419,8 @@ static int __init do_mount_root(const char *name, const char *fs,
 	       MAJOR(ROOT_DEV), MINOR(ROOT_DEV));
 
 out:
-	put_page(p);
+	if (p)
+		put_page(p);
 	return ret;
 }