On Thu, Apr 7, 2022 at 11:47 AM Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > > On Wed, Apr 6, 2022 at 12:46 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > > > *thread necromancy* > > It's alive! > > > > > Hi Dan, > > > > I'm doing a KSPP bug scrub and am reviewing > > https://github.com/KSPP/linux/issues/74 again. > > > > Do you have a chance to look at this? I'd love a way to make mmap() > > behave the same way as read() for the first meg of /dev/mem. > > You want 0-reads or SIGBUS when attempting to access the first 1MB? > > Because it sounds like what you want is instead of loudly failing with > -EPERM in drivers/char/mem.c::mmap_mem() you want it to silently > succeed but swap in the zero page, right? Otherwise if it's SIGBUS > then IO_STRICT_DEVMEM=y + marking that span as IORESOURCE_BUSY will > "Do the Right Thing (TM).". In other words, if IO_STRICT_DEVMEM is enabled then the enforcement is already there at least for anything marked IORESOURCE_BUSY. So if tools are ok with that protection today, maybe there is no need to do the zero page dance. I.e. legacy tools the read(2) /dev/mem below 1MB get zeroes, and apparently no tools were mmap'ing below 1MB otherwise they would have complained by now? At least Fedora is shipping IO_STRICT_DEVMEM these days: https://src.fedoraproject.org/rpms/kernel/blob/rawhide/f/kernel-x86_64-fedora.config#_2799
