On 12/02/17 at 10:22pm, Matt Fleming wrote: > (Cc'ing Dave since this is used for kexec on EFI) > > On Fri, 01 Dec, at 09:54:43AM, Ard Biesheuvel wrote: > > On 1 December 2017 at 09:48, Greg Kroah-Hartman > > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > > 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. > > > > That's not what I said :-) > > > > These are real pointers, and stuff will actually be mapped there > > (although I am not intimately familiar with the way x86 does this, but > > on ARM [which does not have these sysfs nodes in the first place], > > these mappings are only live during the time a UEFI runtime service > > call is in progress, and IIRC, work was underway to do the same for > > x86). So while these values don't correlate with the placement of > > kernel data structures, they could still be useful for an attacker to > > figure out where exploitable firmware memory regions are located, > > especially given that some of these may be mapped RWX. > > These are mappings of the EFI firmware's runtime regions, dynamically > allocated by the kernel starting at EFI_VA_START. Because we only get > one chance to tell the firmware where we placed its regions (via > SetVirtualAddressMap()) we have to guarantee that any subsequent kexec > reboots use the same addresses. > > So that's why they're exported to userspace through sysfs. > > Like Ard said, these mappings are not mapped into the regular process > address space. Instead, they're only used while making EFI runtime > calls. > > But this is still an information leak. And I think _ATTR(..0400) is > the right way to fix it. Dave, could you double-check my logic and > write a patch? Matt, thanks for ccing me, I will look into it. Thanks Dave -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
