* PaX Team <pageexec@xxxxxxxxxxx> wrote: > On 26 Nov 2015 at 9:54, Ingo Molnar wrote: > > > * PaX Team <pageexec@xxxxxxxxxxx> wrote: > > > > > actually the kernel could silently recover from this given how the page fault > > > handler could easily determine that the fault address fell into the > > > data..read_only section and just silently undo the read-only property, log the > > > event to dmesg and retry the faulting access. > > > > So a safer method would be to decode the faulting instruction, to skip it by > > fixing up the return RIP and to log the event. It would be mostly equivalent > > to trying to write to ROM (which get ignored as well), so it's a recoverable > > (and debuggable) event. > > if by skipping you mean ignoring the write attempt then it's not a good idea as > it has a good chance to cause unexpected behaviour down the line. > > e.g., imagine that the write was to a function pointer (even an entire ops > structure) or a boolean that controls some important feature for after-init > code. ignoring/dropping such writes could cause all kinds of logic bugs (if not > worse). Well, the typical case is that it's a logic bug to _do_ the write: the structure was marked readonly for a reason but some init code re-runs during suspend or so. But yes, logic bugs might trigger - but that is true in the opposite case as well, if we do the write despite it being marked readonly: > my somewhat related war story is that i once tried to constify machine_ops (both > the struct and the variable of the same name) directly and just forced the > writes in kvm/xen/etc via type casts. now i knew it was all undefined behaviour > but i didn't expect gcc to take advantage of it but it did (const propagated the > *initial* fptr values into the indirect calls by turning them into direct calls) > and which in turn prevented proper reboots for guests (an event which obviously > happens much later after init/boot to the great puzzlement of end users and > myself). > > misusing __read_only and ignoring write attempts would effectively produce the > same misbehaviour as above so i strongly advise against it. No, the difference to the GCC related aliasing bug is that with my technique the kernel would immediately produce a very visible kernel warning, which is a very clear sign that is wrong - and with a very clear backtrace in the warning that points right to the problematic code - which signature shows us (and users) what is wrong. So your example is not comparable at all. Plus the truly paranoid might panic/halt the system on such warnings, so for highly secure systems there's a way to not even allow the possibility of logic bugs. (at the cost of stopping the system when a bug triggers.) Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
