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

Jonathan Cameron
2023-01-23T10:22:47Z

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