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

From: "Reshetova, Elena" <elena.reshetova@intel.com>
To: Greg KH <gregkh@linuxfoundation.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Leon Romanovsky <leon@kernel.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Marc Zyngier <maz@kernel.org>,
	"darwi@linutronix.de" <darwi@linutronix.de>,
	"kirill.shutemov@linux.intel.com"
	<kirill.shutemov@linux.intel.com>,
	"Mika Westerberg" <mika.westerberg@linux.intel.com>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: RE: [PATCH 1/2] PCI/MSI: Cache the MSIX table size
Date: Wed, 25 Jan 2023 12:33:04 +0000	[thread overview]
Message-ID: <DM8PR11MB5750F5873EA182A28ABC7701E7CE9@DM8PR11MB5750.namprd11.prod.outlook.com> (raw)
In-Reply-To: <Y8/6InIxcpwM5u2M@kroah.com>

> On Tue, Jan 24, 2023 at 01:52:37PM +0200, Alexander Shishkin wrote:
> > Leon Romanovsky <leon@kernel.org> writes:
> >
> > > On Thu, Jan 19, 2023 at 07:06:32PM +0200, Alexander Shishkin wrote:
> > >> A malicious device can change its MSIX table size between the table
> > >> ioremap() and subsequent accesses, resulting in a kernel page fault in
> > >> pci_write_msg_msix().
> > >>
> > >> To avoid this, cache the table size observed at the moment of table
> > >> ioremap() and use the cached value. This, however, does not help drivers
> > >> that peek at the PCIE_MSIX_FLAGS register directly.
> > >>
> > >> Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> > >> Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
> > >> Cc: stable@vger.kernel.org
> > >> ---
> > >>  drivers/pci/msi/api.c | 7 ++++++-
> > >>  drivers/pci/msi/msi.c | 2 +-
> > >>  include/linux/pci.h   | 1 +
> > >>  3 files changed, 8 insertions(+), 2 deletions(-)
> > >
> > > 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.
> 
> As per our documentation, if you are "fixing" things based on a tool you
> have, you HAVE TO document that in the changelog.  Otherwise we just get
> to reject the patch flat out (hint, this has caused more than one group
> of developers to just be flat out banned for ignoring...)
> 
> And what kind of tool is now fuzzing PCI config accesses with
> "malicious" devices?  Again, this is NOT something that Linux currently
> even attempts to want to protect itself against.  If you are wanting to
> change that model, wonderful, let's discuss that and work on defining
> the scope of your new security threat model and go from there.  Don't
> throw random patches at us and expect us to think they are even valid.
> 
> Again, Linux trusts PCI devices.  If you don't want to trust PCI
> devices, we already have a framework in place to prevent that which is
> independant of this area of the PCI code.  Use that, or let's discuss
> why that is insufficient.

Sure, I have started a new thread on this in 
https://lore.kernel.org/all/DM8PR11MB57505481B2FE79C3D56C9201E7CE9@DM8PR11MB5750.namprd11.prod.outlook.com/

I think it is much bigger topic to discuss, so better be handled separately. 

Best Regards,
Elena.