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Date: Wed, 28 Feb 2024 10:20:13 +0100 (CET)
From: Jiri Kosina <jikos@kernel.org>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
cc: Bjorn Helgaas <helgaas@kernel.org>, 
    =?ISO-8859-15?Q?Carlos_L=F3pez?= <clopez@suse.de>, cve@kernel.org, 
    linux-kernel@vger.kernel.org, Bjorn Helgaas <bhelgaas@google.com>, 
    Jonas Gorski <jonas.gorski@gmail.com>, 
    Andy Shevchenko <andriy.shevchenko@linux.intel.com>, 
    Mika Westerberg <mika.westerberg@linux.intel.com>
Subject: Re: CVE-2023-52466: PCI: Avoid potential out-of-bounds read in
 pci_dev_for_each_resource()
In-Reply-To: <2024022706-kindle-tapering-5f66@gregkh>
Message-ID: <nycvar.YFH.7.76.2402281011470.21798@cbobk.fhfr.pm>
References: <2024022709-magazine-handshake-50da@gregkh> <20240227150744.GA235367@bhelgaas> <2024022706-kindle-tapering-5f66@gregkh>
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On Tue, 27 Feb 2024, Greg Kroah-Hartman wrote:

> > 09cc90063240 ("PCI: Introduce pci_dev_for_each_resource()") added
> > pci_dev_for_each_resource(), which expands to:
> > 
> >   for (...; res = (&(dev)->resource[(bar)]), bar < PCI_NUM_RESOURCES; ...)
> > 
> > We compute "res" before the bounds-check of "bar", so the pointer may
> > be out-of-bounds, but the body of the pci_dev_for_each_resource() loop
> > is never executed with that out-of-bounds value.
> > 
> > So I don't think this is a security issue, no matter how
> > pci_dev_for_each_resource() is used, unless the mere presence of an
> > invalid address in a register is an issue.
> 
> Ah, yeah, now I remember, stuff like this was fixed up in other loops as
> just reading off into the wild can be a speculation issue and so we had
> to fix up a bunch of places in the kernel where we did have "invalid
> data" in a register.  The code didn't use that, but the processor would
> fetch from there, and boom, speculation mess.  There's a whole research
> paper published on this type of thing somewhere...

Greg, could you please elaborate on this?

Where in this whole construct do you see a potential for *_uncached_* (!) 
memory access that'd cause CPU to speculate into the wild? I just don't 
see it.

Thanks,

-- 
Jiri Kosina
SUSE Labs