Re: [PATCH v17 07/15] fs, arm64: untag user pointers in copy_mount_options
From: Vincenzo Frascino
Date: Wed Jun 12 2019 - 10:40:12 EST
On 12/06/2019 12:43, 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
> Untag the address before subtracting.
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> 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;