linux-kernel - Re: [PATCH] x86/alternatives: check int3 breakpoint physical addresses

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Message-ID: <alpine.DEB.2.21.1902111012130.1728@nanos.tec.linutronix.de>
Date:   Mon, 11 Feb 2019 10:15:03 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86/alternatives: check int3 breakpoint physical
 addresses

On Mon, 11 Feb 2019, Alexandre Chartre wrote:
> On 02/10/2019 10:23 PM, Thomas Gleixner wrote:
> > On Fri, 25 Jan 2019, Alexandre Chartre wrote:
> > > Note that this issue has been observed and reproduced with a custom kernel
> > > with some code mapped to different virtual addresses and using jump labels
> > > As jump labels use text_poke_bp(), crashes were sometimes observed when
> > > updating jump labels.
> > 
> > Rightfully so. text_poke_bp() pokes at the kernels text mapping which is
> > very well defined and unique. Why would you map the same text to different
> > virtual addresses and then use a randomly chosen one to poke at it?
> > 
> 
> As an example, we used to have per-CPU SYSCALL entry trampoline [1] where the
> entry_SYSCALL_64_trampoline function was mapped to a different virtual address
> for each CPU. So, a syscall would execute entry_SYSCALL_64_trampoline() from a
> different virtual address depending on the CPU being used. With that code,
> adding a jump label in entry_SYSCALL_64_trampoline() causes the described
> issue.

Right, but we knew that and there was no reason to put a jump label into
that. AFAICT we don't have anything like this in the kernel.

That said, I'm not opposed to apply the patch as is, I just wanted to get a
better understanding about the background.

Thanks,

	tglx