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

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Date:   Mon, 23 Jan 2023 10:22:37 +0000
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     Leon Romanovsky <leon@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        <linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Marc Zyngier <maz@...nel.org>, <darwi@...utronix.de>,
        <elena.reshetova@...el.com>, <kirill.shutemov@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        <stable@...r.kernel.org>, Lukas Wunner <lukas@...ner.de>
Subject: Re: [PATCH 1/2] PCI/MSI: Cache the MSIX table size

On Sun, 22 Jan 2023 11:57:58 +0100
Greg KH <gregkh@...uxfoundation.org> wrote:

> On Sun, Jan 22, 2023 at 11:00:04AM +0200, Leon Romanovsky wrote:
> > 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@...ux.intel.com>
> > > Reviewed-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> > > Cc: stable@...r.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.  
> 
> Linux does NOT protect from malicious PCIe devices at this point in
> time, you are correct.  If we wish to change that model, then we can
> work on that with the explict understanding that most all drivers will
> need to change as will the bus logic for the busses involved.
> 
> To do piece-meal patches like this for no good reason is not a good idea
> as it achieves nothing in the end :(
> 
> thanks,
> 
> greg k-h

If you care enough about potential malicious PCIe devices, do device
attestation and reject any devices that don't support it (which means
rejecting pretty much everything today ;).
Or potentially limit what non attested devices are allowed to do.

+CC Lukas who is working on this.

Jonathan