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

[Ren Qiaowei]

On 10/31/2014 06:38 AM, Dave Hansen wrote:
> > +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'?

This should check the return value of do_mpx_bounds and should be fixed.

Thanks,
Qiaowei

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