On 24 November 2015 at 22:38, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > Many things are written to only during __init, and never changed > again. These cannot be made "const" since the compiler will do the wrong > thing (we do actually need to write to them). Instead, move these items > into a memory region that will be made read-only during mark_rodata_ro() > which happens after all kernel __init code has finished. > > This introduces __read_only as a way to mark such memory, and uses it on > the x86 vDSO to kill an extant kernel exploitation method. ...just some random notes on the experience with kernels implementing such a feature for quite a lot of locations, not just the vDSO. While having that annotation makes perfect sense, not only from a security perspective but also from a micro-optimization point of view (much like the already existing __read_mostly annotation), it has its drawbacks. Violating the "r/o after init" rule by writing to such annotated variables from non-init code goes unnoticed as far as it concerns the toolchain. Neither the compiler nor the linker will flag that incorrect use. It'll just trap at runtime and that's bad. I myself had some educating experience seeing my machine triple fault when resuming from a S3 sleep. The root cause was a variable that was annotated __read_only but that was (unnecessarily) modified during CPU bring-up phase. Debugging that kind of problems is sort of a PITA, you could imagine. So, prior extending the usage of the __read_only annotation some toolchain support is needed. Maybe a gcc plugin that'll warn/error on code that writes to such a variable but is not __init itself. The initify and checker plugins from the PaX patch might be worth to look at for that purpose, as they're doing similar things already. Adding such a check to sparse might be worth it, too. A modpost check probably won't work as it's unable to tell if it's a legitimate access (r/o) or a violation (/w access). So the gcc plugin is the way to go, IMHO. Regards, Mathias -- 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
