[PATCH] Fix msr check in compat_sys_swapcontext

[Paul Mackerras]

Andreas Schwab writes:

> The new context may not be 16-byte aligned, so the real address of the
> mcontext structure should be read from the uc_regs pointer instead of
> directly using the (unaligned) uc_mcontext field.

Good catch, but...

> @@ -941,9 +941,17 @@ long sys_swapcontext(struct ucontext __user *old_ctx,
>  #ifdef CONFIG_PPC64
>  	unsigned long new_msr = 0;
>  
> -	if (new_ctx &&
> -	    get_user(new_msr, &new_ctx->uc_mcontext.mc_gregs[PT_MSR]))
> -		return -EFAULT;
> +	if (new_ctx) {
> +		struct mcontext __user *mcp;
> +		u32 cmcp;
> +
> +		/* Get pointer to the real mcontext. */
> +		if (__get_user(cmcp, &new_ctx->uc_regs))

we need to use get_user, not __get_user, since we haven't done an
access_ok() check on the address.

> +			return -EFAULT;
> +		mcp = (struct mcontext __user *)(u64)cmcp;
> +		if (__get_user(new_msr, &mcp->mc_gregs[PT_MSR]))

ditto here.

Paul.