linux-kernel - Re: [PATCH] ACPI/PPTT: fix off-by-one error

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Message-ID: <20250507-argue-rant-9f48ba38@mheyne-amazon>
Date: Wed, 7 May 2025 11:53:38 +0000
From: "Heyne, Maximilian" <mheyne@...zon.de>
To: Jeremy Linton <jeremy.linton@....com>
CC: Sudeep Holla <sudeep.holla@....com>, "stable@...r.kernel.org"
	<stable@...r.kernel.org>, "Rafael J. Wysocki" <rafael@...nel.org>, Len Brown
	<lenb@...nel.org>, Ard Biesheuvel <ardb@...nel.org>, Catalin Marinas
	<catalin.marinas@....com>, "linux-acpi@...r.kernel.org"
	<linux-acpi@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ACPI/PPTT: fix off-by-one error

On Tue, May 06, 2025 at 03:11:20PM -0500, Jeremy Linton wrote:
> Hi,
> 
> On 5/6/25 8:43 AM, Sudeep Holla wrote:
> > On Tue, May 06, 2025 at 01:13:02PM +0000, Heyne, Maximilian wrote:
> > > Commit 7ab4f0e37a0f ("ACPI PPTT: Fix coding mistakes in a couple of
> > > sizeof() calls") corrects the processer entry size but unmasked a longer
> > > standing bug where the last entry in the structure can get skipped due
> > > to an off-by-one mistake if the last entry ends exactly at the end of
> > > the ACPI subtable.
> > > 
> > 
> > Unless the firmware has populated an incorrect value for the header length, I
> > don't see how this is possible. The table_end should point to the address
> > immediately following the last byte of the table. None of the headers are only
> > one byte long, so what am I missing that could explain this apparent
> > off-by-one issue?.
> 
> More likely its because the sizeof() fix was merged without proper review
> and is wrong because the type isn't actually known on the object until the
> header is checked.

I agree that the type might not be known at this point but the condition

  	proc_sz = sizeof(struct acpi_pptt_processor);
	  while((unsigned long)entry + proc_sz <= table_end)

would make sure that there could potentially be a node of type
acpi_pptt_processor because there is at least space for it. If the entry
can't be of that size because it would go over table_end then it can't
be an acpi_pptt_processor.

Therefore, I don't think the sizeof() fix is that wrong but we just need
to adjust the while condition.

Alternatively, we could at least make sure that we can safely access
(without crossing table_end) the acpi_subtable_header to check the type.
But the current approach seems cleaner to me.



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