linux-kernel - Re: [x86/entry_32] aa93e2ad74: BUG:soft_lockup-CPU##stuck

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Date:   Wed, 12 Jan 2022 16:14:19 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        lkp@...ts.01.org, lkp@...el.com
Subject: Re: [x86/entry_32]  aa93e2ad74:
 BUG:soft_lockup-CPU##stuck_for#s![systemd-logind:#]

On Wed, Jan 12, 2022, Peter Zijlstra wrote:
> On Wed, Jan 12, 2022 at 01:28:58AM +0000, Sean Christopherson wrote:
> > The issue is that PARANOID_EXIT_TO_KERNEL_MODE in the handle_exception_return
> > path overwrites the entry stack data with the task stack data, restoring the "bad"
> > segment value.
> 
> Oh gawd... that's terrible, and yes, that now makes perfect sense.
> 
> However did you find that?

printf and running under QEMU, which has a interactive "monitor" that lets you
read/write guest memory and can also do VA=>PA translations.  Code inspection
once I realized the value on the stack was being restored between the exception
fixup and the POP.