Re: [PATCH v2] x86/tls: Fix possible spectre-v1 in do_get_thread_area()

[Thomas Gleixner]

On Wed, 26 Jun 2019, Dianzhang Chen wrote:

> The index to access the threads tls array is controlled by userspace
> via syscall: sys_ptrace(), hence leading to a potential exploitation
> of the Spectre variant 1 vulnerability.
> The idx can be controlled from:
>         ptrace -> arch_ptrace -> do_get_thread_area.
> 
> Fix this by sanitizing idx before using it to index p->thread.tls_array.

Just that I can't find a place which sanitizes the value....

> +#include <linux/nospec.h>

and nothing which uses anything from this header file.

>  #include <linux/uaccess.h>
>  #include <asm/desc.h>
> @@ -220,6 +221,7 @@ int do_get_thread_area(struct task_struct *p, int idx,
>  		       struct user_desc __user *u_info)
>  {
>  	struct user_desc info;
> +	int index;
>  
>  	if (idx == -1 && get_user(idx, &u_info->entry_number))
>  		return -EFAULT;
> @@ -227,8 +229,9 @@ int do_get_thread_area(struct task_struct *p, int idx,
>  	if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
>  		return -EINVAL;
>  
> -	fill_user_desc(&info, idx,
> -		       &p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
> +	index = idx - GDT_ENTRY_TLS_MIN;
> +
> +	fill_user_desc(&info, idx, &p->thread.tls_array[index]);

So this is just a cosmetic change and the compiler will create probably
exactly the same binary.

Thanks,

	tglx