2016-09-05 18:57 GMT+02:00 <Valdis.Kletnieks@xxxxxx>:
On Mon, 05 Sep 2016 12:59:46 +0200, Oscar Salvador said:
> I'm writing a module to read/write kernel memory, and for this I'd like to
> check if a page is marked as read-only
Actually, you almost certainly want to do a *much* stricter check than
that. If your module is doing unrestricted writes, there's almost certainly
a major design failure. Modules should *only* access memory that belongs
to them - for instance, a driver for some new widget shouldn't be doing
anything with memory that isn't either I/O buffer space allocated for
that device, or the various struct * that the driver core sets up for a device.
If you're trying to scribble *anywhere*, you're either trying to write a
rootkit, or you're mis-designing something that will almost certainly be
abused by somebody to backdoor in a rootkit.
And I don't have much sympathy for "it's just a toy module" - if you can't
be bothered to write modules with proper design, you shouldn't be coding
in kernelspace. Learn to do it right from the beginning and don't learn
sloppy habits.
I guess I explained it wrong. I'm not writing neither a rootkit nor a module which is messing with kernel memory. I'm writing a module to be able to r/w kernel/ user linear memory. It's for a forensic tool.
Actually it's nothing that you couldn't do with
# gdb /bin/ls /proc/kcore
# gdb /bin/ls /proc/kcore
# x/4w [kernel_address]
I wanted to check which kind of perms does it have a kernel page because I didn't want to be bothered by the "unable to handle kernel page request" if a user tries to write to a page which is read-only, so I wanted to check if that was the case, and just return an error in such case.
Or also if a page is not resident in memory, either because is swapped or because was never mapped (checking the _PAGE_PRESENT bit).
So far I'm using "lookup_address" to get the pte related to a kernel address in order to perform those checks, and that's working.
The only doubt I have is:
Can kernel pages be swapped? and if that's the case, there is a way to get check if a page is missing because was never mapped or because was swapped?
Thanks
So what actual problem are you trying to solve by scribbling all over kernel
space? There's probably a better way to do it.
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