Date: Wed, 11 Aug 2021 14:24:30 +0200 From: Philipp Rudo <prudo(a)redhat.com&gt; To: lijiang <lijiang(a)redhat.com&gt; Cc: "Discussion list for crash utility usage, maintenance and development" <crash-utility(a)redhat.com&gt; Subject: Re: [Crash-utility] [PATCH] x86_64: Fix check for __per_cpu_offset initialisation Message-ID: <20210811142430.5e3e1a86@rhtmp> Content-Type: text/plain; charset=US-ASCII Hi Lianbo, On Wed, 11 Aug 2021 17:05:26 +0800 lijiang <lijiang(a)redhat.com&gt; wrote: > > > > Date: Thu, 5 Aug 2021 15:19:37 +0200 > > From: Philipp Rudo <prudo(a)redhat.com&gt; > > To: crash-utility(a)redhat.com > > Subject: [Crash-utility] [PATCH] x86_64: Fix check for > > __per_cpu_offset initialisation > > Message-ID: <20210805131937.5051-1-prudo(a)redhat.com&gt; > > > > Since at least kernel v2.6.30 the __per_cpu_offset gets initialized to > > __per_cpu_load. So first check if the __per_cpu_offset was set to a > > proper value before reading any per cpu variable to prevent potential > > bugs. > > > > > Hi, Philipp > > Thank you for the patch. Can you help to describe more details about the > potential risks? and what conditions might trigger the potential bugs? the bug is always triggered during initialization of the per-cpu data on x86_64. At least for kernels not using struct x8664_pda, which AFAIK was also removed with kernel v2.6.30. The risk for crash is low. Right after the superfluous read there is a check if the read cpunumber matches the expected one. if (cpunumber != cpus) break; So the worst case scenario I see is that crash initializes one additional cpu with non-sense data. But given that the bug exists for ~12 years and nobody reported such an bug I assume that the check worked well so far.

> Did you mean that it's related to the crash live analysis issue(1978032)? I > tried to reproduce it, but so far I haven't reproduced it with the upstream > kernel. Yes, this bug is related to bz1978032. For whatever reason the superfluous read triggered the panic. I could reproduce the bug upstream with CONFIG_IO_URING _disabled_. Unfortunately there is a RHEL-only patch [1] that tampers with the Kconfig for IO_URING. So when you copy a kernel-ark config to the upstream repo and run 'make oldconfig' the IO_URING will silently be _enabled_.

BTW, I tried to reproduce the panic yesterday on kernel-5.14.0-0.rc4 but failed. Not sure if the bug was fixed in the meantime or I was simply "lucky"...