On Mon, Jun 20, 2011 at 10:46:08AM +1000, Ryan Mallon wrote:How does it protect against security escalation? A process mapping a region either from /dev/mem or from some custom char device can't escape that region right? In either case you need root privileges to make the mapping in the first place.On 20/06/11 10:42, Matthew Wilcox wrote:Because it pushes the tradeoff in the right direction. Somebody wantsOn Mon, Jun 20, 2011 at 09:02:17AM +1000, Ryan Mallon wrote:I mean overkill in the sense of having to write the driver at all. WhyThere are drivers where this makes sense. For example an FPGA deviceCalling a 30 line device driver "overkill" might in itself be overkill?
with a proprietary register layout on the memory bus can be done this
way. The FPGA can simply be mapped in user-space via /dev/mem and
handled there. If the device requires no access other than memory bus
reads and writes then writing a custom char device driver just to get an
mmap function seems a bit overkill.
write a 30 line driver just to re-implement some functionality of
/dev/mem?
to do something weird is a little inconvenienced vs protecting the vast
majority of users from some security escalation problems.
Besides, if you have a real bus with discoverable regionsWhich is also where devices like FPGAs tend to exist :-).
(like PCI BARs), the bus should have sysfs entries like
/sys/bus/pci/devices/0000\:06\:06.0/resource0 that can be mmaped.
Then there's no need for a device driver at all, *and* the privilege
escalation isn't achievable.
Of course, most embedded architectures have crap discoverability.