Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitraryphysical addresses

From: Ryan Mallon
Date: Sun Jun 19 2011 - 21:02:48 EST

On 20/06/11 10:52, Matthew Wilcox wrote:
On Mon, Jun 20, 2011 at 10:46:08AM +1000, Ryan Mallon wrote:
On 20/06/11 10:42, Matthew Wilcox wrote:
On Mon, Jun 20, 2011 at 09:02:17AM +1000, Ryan Mallon wrote:
There are drivers where this makes sense. For example an FPGA device
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.
Calling a 30 line device driver "overkill" might in itself be overkill?

I mean overkill in the sense of having to write the driver at all. Why
write a 30 line driver just to re-implement some functionality of
Because it pushes the tradeoff in the right direction. Somebody wants
to do something weird is a little inconvenienced vs protecting the vast
majority of users from some security escalation problems.
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.
Besides, if you have a real bus with discoverable regions
(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.
Which is also where devices like FPGAs tend to exist :-).


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