Subject: Re: [PATCH v16 08/16] fs, arm64: untag user pointers in copy_mount_options

From: Kees Cook <[email-masked]>
Date: Fri, 7 Jun 2019 21:02:25 -0700

On Mon, Jun 03, 2019 at 06:55:10PM +0200, Andrey Konovalov wrote:
> This patch is a part of a series that extends arm64 kernel ABI to allow to
> pass tagged user pointers (with the top byte set to something else other
> than 0x00) as syscall arguments.
> 
> In copy_mount_options a user address is being subtracted from TASK_SIZE.
> If the address is lower than TASK_SIZE, the size is calculated to not
> allow the exact_copy_from_user() call to cross TASK_SIZE boundary.
> However if the address is tagged, then the size will be calculated
> incorrectly.
> 
> Untag the address before subtracting.
> 
> Reviewed-by: Catalin Marinas <[email-masked]>
> Signed-off-by: Andrey Konovalov <[email-masked]>

One thing I just noticed in the commit titles... "arm64" is in the
prefix, but these are arch-indep areas. Should the ", arm64" be left
out?

I would expect, instead:

	fs/namespace: untag user pointers in copy_mount_options

Reviewed-by: Kees Cook <[email-masked]>

-Kees

> ---
>  fs/namespace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index b26778bdc236..2e85712a19ed 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2993,7 +2993,7 @@ void *copy_mount_options(const void __user * data)
>  	 * the remainder of the page.
>  	 */
>  	/* copy_from_user cannot cross TASK_SIZE ! */
> -	size = TASK_SIZE - (unsigned long)data;
> +	size = TASK_SIZE - (unsigned long)untagged_addr(data);
>  	if (size > PAGE_SIZE)
>  		size = PAGE_SIZE;
>  
> -- 
> 2.22.0.rc1.311.g5d7573a151-goog
> 

-- 
Kees Cook

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