Re: [PATCH v2 2/3] x86: fix setup of brk area

From: Juergen Gross <jgross@suse.com>
To: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: xen-devel@lists.xenproject.org, x86@kernel.org,
	linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v2 2/3] x86: fix setup of brk area
Date: Thu, 30 Jun 2022 08:55:19 +0200	[thread overview]
Message-ID: <1df188eb-4a31-75a3-146b-b930002c1e68@suse.com> (raw)
In-Reply-To: <20220629171457.amdsrgaxady55hds@treble>

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On 29.06.22 19:14, Josh Poimboeuf wrote:
> Hi Juergen,
> 
> It helps to actually Cc the person who broke it ;-)
> 
> On Thu, Jun 23, 2022 at 11:46:07AM +0200, Juergen Gross wrote:
>> Commit e32683c6f7d2 ("x86/mm: Fix RESERVE_BRK() for older binutils")
>> put the brk area into the .bss..brk section (placed directly behind
>> .bss),
> 
> Hm? It didn't actually do that.
> 
> For individual translation units, it did rename the section from
> ".brk_reservation" to ".bss..brk".  But then during linking it's still
> placed in .brk in vmlinux, just like before.

Sorry, I misread the patch commit message and was fooled by the fact that
bisection clearly pointed at this patch to have introduced the problem.

I only discovered later that the main issue was the added "NOLOAD"
attribute.

>> causing it not to be cleared initially. As the brk area is used
>> to allocate early page tables, these might contain garbage in not
>> explicitly written entries.
>>
>> This is especially a problem for Xen PV guests, as the hypervisor will
>> validate page tables (check for writable page tables and hypervisor
>> private bits) before accepting them to be used. There have been reports
>> of early crashes of PV guests due to illegal page table contents.
>>
>> Fix that by letting clear_bss() clear the brk area, too.
> 
> While it does make sense to clear the brk area, I don't understand how
> my patch broke this.  How was it getting cleared before?

It seemed to have worked by chance. The Xen hypervisor is clearing all
alloc-only sections when loading a kernel (this will "fix" the dom0
case reliably together with patch 3 of this series).

Grub might do the clearing, too (for the PV domU case), but I haven't
verified that by code inspection.

I'll drop the "Fixes:" tag.

Juergen

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