Re: [PATCH v9 09/12] x86, mpx: decode MPX instruction to get bound violation information

[Dave Hansen]

> +void do_mpx_bounds(struct pt_regs *regs, siginfo_t *info,
> +		struct xsave_struct *xsave_buf)
> +{
> +	struct mpx_insn insn;
> +	uint8_t bndregno;
> +	unsigned long addr_vio;
> +
> +	addr_vio = mpx_insn_decode(&insn, regs);
> +
> +	bndregno = X86_MODRM_REG(insn.modrm.value);
> +	if (bndregno > 3)
> +		return;
> +
> +	/* Note: the upper 32 bits are ignored in 32-bit mode. */
> +	info->si_lower = (void __user *)(unsigned long)
> +		(xsave_buf->bndregs.bndregs[2*bndregno]);
> +	info->si_upper = (void __user *)(unsigned long)
> +		(~xsave_buf->bndregs.bndregs[2*bndregno+1]);
> +	info->si_addr_lsb = 0;
> +	info->si_signo = SIGSEGV;
> +	info->si_errno = 0;
> +	info->si_code = SEGV_BNDERR;
> +	info->si_addr = (void __user *)addr_vio;
> +}
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 611b6ec..b2a916b 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -284,6 +284,7 @@ dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
>  	unsigned long status;
>  	struct xsave_struct *xsave_buf;
>  	struct task_struct *tsk = current;
> +	siginfo_t info;
>  
>  	prev_state = exception_enter();
>  	if (notify_die(DIE_TRAP, "bounds", regs, error_code,
> @@ -316,6 +317,11 @@ dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
>  		break;
>  
>  	case 1: /* Bound violation. */
> +		do_mpx_bounds(regs, &info, xsave_buf);
> +		do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs,
> +				error_code, &info);
> +		break;
> +
>  	case 0: /* No exception caused by Intel MPX operations. */
>  		do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code, NULL);
>  		break;
> 

So, siginfo is stack-allocarted here.  do_mpx_bounds() can error out if
it sees an invalid bndregno.  We still send the signal with the &info
whether or not we filled the 'info' in do_mpx_bounds().

Can't this leak some kernel stack out in the 'info'?
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