Re: [PATCH 1/2] PCI/MSI: Cache the MSIX table size

From: Alexander Shishkin <alexander.shishkin@linux.intel.com>
To: Leon Romanovsky <leon@kernel.org>
Cc: Bjorn Helgaas <bhelgaas@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
	Marc Zyngier <maz@kernel.org>,
	darwi@linutronix.de, elena.reshetova@intel.com,
	kirill.shutemov@linux.intel.com,
	Mika Westerberg <mika.westerberg@linux.intel.com>,
	stable@vger.kernel.org, alexander.shishkin@linux.intel.com
Subject: Re: [PATCH 1/2] PCI/MSI: Cache the MSIX table size
Date: Tue, 24 Jan 2023 14:42:11 +0200	[thread overview]
Message-ID: <87pmb4p0ik.fsf@ubik.fi.intel.com> (raw)
In-Reply-To: <Y8/Kyzh+stow83lQ@unreal>

Leon Romanovsky <leon@kernel.org> writes:

> On Tue, Jan 24, 2023 at 01:52:37PM +0200, Alexander Shishkin wrote:
>> Leon Romanovsky <leon@kernel.org> writes:
>> 
>> > I'm not security expert here, but not sure that this protects from anything.
>> > 1. Kernel relies on working and not-malicious HW. There are gazillion ways
>> > to cause crashes other than changing MSI-X.
>> 
>> This particular bug was preventing our fuzzing from going deeper into
>> the code and reaching some more of the aforementioned gazillion bugs.
>
> Your commit message says nothing about fuzzing, but talks about
> malicious device. 

A malicious device is what the fuzzing is aiming to simulate. The fact
of fuzzing process itself didn't seem relevant to the patch, so I didn't
include it, going instead for the problem statement and proposed
solution. Will the commit message benefit from mentioning fuzzing?

> Do you see "gazillion bugs" for devices which don't change their MSI-X
> table size under the hood, which is main kernel assumption?

Not so far.

> If yes, you should fix these bugs.

That's absolutely the intention.

>> > 2. Device can report large table size, kernel will cache it and
>> > malicious device will reduce it back. It is not handled and will cause
>> > to kernel crash too.
>> 
>> How would that happen? If the device decides to have fewer vectors,
>> they'll all still fit in the ioremapped MSIX table. The worst thing that
>> can happen is 0xffffffff reads from the mmio space, which a device can
>> do anyway. But that shouldn't trigger a page fault or otherwise
>> crash. Or am I missing something?
>
> Like I said, I'm no expert. You should tell me if it safe for all
> callers of pci_msix_vec_count().

Well, since you stated that the reverse will cause a kernel crash, I had
to ask how. I'll include some version of the above paragraph in the
commit message to indicate that we reverse situation has been considered.

Regards,
--
Alex