linux-kernel - Re: [PATCH] x86/bugs: Don't warn when overwriting retbleed_return_thunk with srso_return

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Message-ID: <20250515170633.sn27zil2wie54yhn@desk>
Date: Thu, 15 May 2025 10:06:33 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Suraj Jitindar Singh <surajjs@...zon.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, stable@...r.kernel.org
Subject: Re: [PATCH] x86/bugs: Don't warn when overwriting
 retbleed_return_thunk with srso_return_thunk

On Thu, May 15, 2025 at 11:36:52AM +0200, Borislav Petkov wrote:
> On Wed, May 14, 2025 at 04:30:22PM -0700, Pawan Gupta wrote:
> > This was discussed during the mitigation, and pr_warn() was chosen because
> > it was not obvious that srso mitigation also mitigates retbleed. (On a
> > retrospect, there should have been a comment about it).
> 
> Why is that important?

There are 4 mitigations that currently use return thunks,
retbleed=stuff(Call Depth Tracking), retbleed=unret, SRSO and ITS. They all
set the return thunks they want without checking if return thunks are
already set by another mitigation. I understand the SRSO mitigates
retbleed(BTC), but the same is not true for retbleed(RSB underflow
mitigated by CDT) and ITS. If ITS overrides CDT return thunk, it will make
CDT ineffective.

> We have multiple cases where a mitigation strategy addresses multiple attacks.

Agree, but here we are talking about the opposite case where a mitigation
unintentionally renders a previously set mitigation ineffective.

> > The conclusion was to make the srso and retbleed relationship clear and
> > then take care of the pr_warn().
> 
> So let's ask ourselves: who is really going to see what single-line warning?

Don't know. I guess some do, hence this patch.

> What are we *actually* trying to prevent here?

As I said above, a mitigation unintentionally make another mitigation
ineffective.

> How about a big fat splat at least if we're really trying to prevent something
> nasty which causes a panic on warn...?

Yes, maybe a WARN_ON() conditional to sanity checks for retbleed/SRSO.