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From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Sat, 18 Jul 2020 22:34:05 -0400
To:     hpa@zytor.com
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Joerg Roedel <jroedel@suse.de>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/idt: Make sure idt_table takes a whole page
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On Sat, Jul 18, 2020 at 06:15:26PM -0700, hpa@zytor.com wrote:
> On July 18, 2020 12:25:46 PM PDT, Andy Lutomirski <luto@amacapital.net> wrote:
> >
> >> On Jul 18, 2020, at 10:57 AM, hpa@zytor.com wrote:
> >> 
> >> ﻿On July 9, 2020 3:33:55 AM PDT, Joerg Roedel <joro@8bytes.org>
> >wrote:
> >>> From: Joerg Roedel <jroedel@suse.de>
> >>> 
> >>> On x86-32 the idt_table with 256 entries needs only 2048 bytes. It
> >is
> >>> page-aligned, but the end of the .bss..page_aligned section is not
> >>> guaranteed to be page-aligned.
> >>> 
> >>> As a result, symbols from other .bss sections may end up on the same
> >>> 4k page as the idt_table, and will accidentially get mapped
> >read-only
> >>> during boot, causing unexpected page-faults when the kernel writes
> >to
> >>> them.
> >>> 
> >>> Avoid this by making the idt_table 4kb in size even on x86-32. On
> >>> x86-64 the idt_table is already 4kb large, so nothing changes there.
> >>> 
> >>> Fixes: 3e77abda65b1c ("x86/idt: Consolidate idt functionality")
> >>> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> >> 
> >> NAK... this isn't the right way to fix this and just really kicks the
> >can down the road. The reason is that you aren't fixing the module that
> >actually has a problem.
> >> 
> >> The Right Way[TM] is to figure out which module(s) lack the proper
> >alignment for this section. A script using objdump -h or readelf -SW
> >running over the .o files looking for alignment less than 2**12 should
> >spot the modules that are missing the proper .balign directives.
> >
> >I don’t see the problem. If we are going to treat an object as though
> >it’s 4096 bytes, making C think it’s 4096 bytes seems entirely
> >reasonable to me.
> >
> >> -- 
> >
> >> Sent from my Android device with K-9 Mail. Please excuse my brevity.
> 
> It isn't the object, it is its alignment. You can have an object
> page-aligned so it doesn't cross page boundaries.
> 
> The bigger issue, though, is that this means there are other object
> files which don't have the correct alignment directives, which means
> that this error can crop up again at any point. The really important
> thing here is that we fix the real problem.  -- 
> Sent from my Android device with K-9 Mail. Please excuse my brevity.

To repeat the commit message, the problem is not misaligned
bss..page_aligned objects, but symbols in _other_ bss sections, which
can get allocated in the last page of bss..page_aligned, because its end
isn't page-aligned (maybe it should be?)

bss..page_aligned objects are unlikely to be misaligned, because its
used in C via a macro that includes the alignment attribute, and its
only use in x86 assembly is in head_{32,64}.S which have correct
alignment.

Given that this IDT's page is actually going to be mapped with different
page protections, it seems like allocating the full page isn't
unreasonable.