On Thu, Apr 07, 2022 at 11:47:40AM -0700, Dan Williams wrote: > On Wed, Apr 6, 2022 at 12:46 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > > > *thread necromancy* > > It's alive! *lightning* > > 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).". I'd like it to just return zeros, which is what "read" currently does. (i.e. if page_is_allowed(), really devmem_is_allowed(), returns 2, then the user should see all zeros.) So, yes, "silently succeed but swap in the zero page" is exactly what I'd like. When I last looked, for the first meg, mmap did _not_ fail, but _actually_ reads it. -- Kees Cook
