Re: [rfc 3/3] prctl: Add PR_SET_MM codes to tune up mm_struct entires

[Kees Cook <keescook@chromium.org>]

On Tue, Nov 29, 2011 at 11:12:55PM +0400, Cyrill Gorcunov wrote:
> At restore time we need a mechanism to restore those values
> back and for this sake PR_SET_MM prctl code is introduced.
> 
> Note at moment this inteface is allowed for CAP_SYS_ADMIN
> only.

NAK from me; this needs more bounds checking. Though, yes, it absolutely
must be a privileged action since this is potentially very dangerous. Can
we invent something stronger than CAP_SYS_ADMIN? ;)

> @@ -1841,6 +1841,58 @@ SYSCALL_DEFINE5(prctl, int, option, unsi
>  			else
>  				error = PR_MCE_KILL_DEFAULT;
>  			break;
> +		case PR_SET_MM: {
> +			struct mm_struct *mm;
> +			struct vm_area_struct *vma;
> +
> +			if (arg4 | arg5)
> +				return -EINVAL;
> +
> +			if (!capable(CAP_SYS_ADMIN))
> +				return -EPERM;
> +
> +			error = -ENOENT;
> +			mm = get_task_mm(current);
> +			if (!mm)
> +				return error;
> +
> +			down_read(&mm->mmap_sem);
> +			vma = find_vma(mm, arg3);
> +			if (!vma)
> +				goto out;

arg3 needs to be significantly more carefully validated. find_vma() doesn't
say that vm_start <= addr, only that vm_end > addr. This effectively
bypasses all the vma checks (mmap_min_addr, max process size, etc), with
some pretty crazy side-effects, I think.

-Kees

-- 
Kees Cook