Re: [GIT PULL] hash addresses printed with %p

[Greg Kroah-Hartman]

On Thu, Nov 30, 2017 at 05:18:42PM +0000, Ard Biesheuvel wrote:
> On 30 November 2017 at 17:10, Greg Kroah-Hartman
> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Thu, Nov 30, 2017 at 04:32:35PM +0000, Greg Kroah-Hartman wrote:
> >> On Wed, Nov 29, 2017 at 01:36:25PM -0800, Linus Torvalds wrote:
> >> > On Wed, Nov 29, 2017 at 1:14 PM, Linus Torvalds
> >> > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >> > >
> >> > > Not because %pK itself changed, but because the semantics of %p did.
> >> > > The baseline moved, and the "safe" version did not.
> >> >
> >> > Btw, that baseline for me is now that I can do
> >> >
> >> >   ./scripts/leaking_addresses.pl | wc -l
> >> >   18
> >> >
> >> > and of those 18 hits, six are false positives (looks like bitmaps in
> >> > the uevent keys).
> >> >
> >> > The remaining 12 are from the EFI runtime map files
> >> > (/sys/firmware/efi/runtime-map/*). They should presumably not be
> >> > world-readable, but sadly the kset_create_and_add() helper seems to do
> >> > that by default.
> >> >
> >> > I think the sysfs code makes it insanely too easy to make things
> >> > world-readable. You try to be careful, and mark things read-only etc,
> >> > but __ATTR_RO() jkust means S_IRUGO, which means world-readable.
> >> >
> >> > There seems to be no convenient model for kobjects having better
> >> > permissions. Greg?
> >>
> >> They can just use __ATTR() which lets you set the exact mode settings
> >> that are wanted.
> >>
> >> Something like the patch below, which breaks the build as the
> >> map_attributes are "odd", but you get the idea.  The EFI developers can
> >> fix this up properly :)
> >>
> >> Note, this only accounts for 5 attributes, what is the whole list?
> >
> > Ah, it's the virt_addr file 12 times, I just ran it on my laptop:
> >
> > /sys/firmware/efi/runtime-map/7/virt_addr: 0xfffffffeea6ea000
> > /sys/firmware/efi/runtime-map/5/virt_addr: 0xfffffffeee88b000
> > /sys/firmware/efi/runtime-map/3/virt_addr: 0xfffffffefea00000
> > /sys/firmware/efi/runtime-map/11/virt_addr: 0xfffffffed9c00000
> > /sys/firmware/efi/runtime-map/1/virt_addr: 0xfffffffefee00000
> > /sys/firmware/efi/runtime-map/8/virt_addr: 0xfffffffedba4e000
> > /sys/firmware/efi/runtime-map/6/virt_addr: 0xfffffffeee2de000
> > /sys/firmware/efi/runtime-map/4/virt_addr: 0xfffffffeeea00000
> > /sys/firmware/efi/runtime-map/2/virt_addr: 0xfffffffefec00000
> > /sys/firmware/efi/runtime-map/10/virt_addr: 0xfffffffed9c60000
> > /sys/firmware/efi/runtime-map/0/virt_addr: 0xfffffffeff000000
> > /sys/firmware/efi/runtime-map/9/virt_addr: 0xfffffffedb9c9000
> >
> > So changing it to use __ATTR() should fix this remaning leakage up.
> > That is if we even really need to export these values at all.  What does
> > userspace do with them?  Shouldn't they just be in debugfs instead?
> >
> 
> These are the virtual mappings UEFI firmware regions, which must
> remain in the same place across kexec reboots. So kexec tooling
> consumes this information and passes it on to the incoming kernel in
> some way.
> 
> Note that these are not kernel addresses, so while I agree they should
> not be world readable, they won't give you any clue as to where the
> kernel itself is mapped.
> 
> So the recommendation is to switch to __ATTR( ... 0400 ... ) instead?
> If so, I'll code up a patch.

If these pointers are not "real", I recommend just leaving them as-is.
But perhaps put a comment in the file saying that, so the next time we
run across them in a few years, we don't freak out and worry :)

thanks,

greg k-h