On Thu, Apr 07, 2022 at 04:43:10PM -0700, Dan Williams wrote: > 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 When I try to mmap a RAM area <1MiB, mmap succeeds (range_is_allowed() is non-zero), so I don't think IO_STRICT_DEVMEM would trip anything using mmap on /dev/mem there. I am only reading 0s from there, though, but I don't see what's all happening. I thought maybe it was just literally unused, but even with CONFIG_PAGE_POISONING=y booted with page_poison=1, I still read 0s (not 0xaa), but I'd like to understand _why_ (i.e. I can't tell if it is accidentally safe, intentionally safe, or my test is bad.) For example: # cat /proc/iomem 00000000-00000fff : Reserved 00001000-0009fbff : System RAM 0009fc00-0009ffff : Reserved 000a0000-000bffff : PCI Bus 0000:00 000c0000-000c99ff : Video ROM ... If I mmap page 0, it's rejected (non-RAM). If I mmap page 1, it works, but it's all 0s. (Which is what I'd like, but I don't see where this is happening.) Hmmm. -- Kees Cook
