On Fri, Apr 07, 2017 at 04:51:16PM +0100, Ard Biesheuvel wrote:
> On 7 April 2017 at 16:47, James Morse <james.morse@xxxxxxx> wrote:
> > On 24/03/17 13:24, Ard Biesheuvel wrote:
> >> Update the allocation logic for the virtual mapping of the UEFI runtime
> >> services to start from a randomized base address if KASLR is in effect,
> >> and if the UEFI firmware exposes an implementation of EFI_RNG_PROTOCOL.
> >>
> >> This makes it more difficult to predict the location of exploitable
> >> data structures in the runtime UEFI firmware, which increases robustness
> >> against attacks. Note that these regions are only mapped during the
> >> time a runtime service call is in progress, and only on a single CPU
> >> at a time, bit give the lack of a downside, let's enable it nonetheless.
> >
> > With next-20170407 on Seattle Overdrive, I get an SError[0] on boot:
> > * Revert "ef/libstub/arm/arm64: Randomize the base of the UEFI rt services region"
> >
> > At which point the machine start booting to a prompt again, (its noisier than
> > usual but looks like double-printing).
> That is quite interesting, to be honest, because that patch should
> effectively be a NOP on systems that do not implement
> EFI_RNG_PROTOCOL.
FWIW, I'm also seeing a crash as a result of this patch, on a Juno R1
with an EFI_RNG_PROTOCOL implementation, with next-20170410 defconfig.
I'm using the Linaro 15.08 toolchain.
----
EFI stub: Booting Linux Kernel...
EFI stub: Using DTB from configuration table
WARNING: this system is using an unsafe pseudo-random implementation of
RngLib!
Synchronous Exception at 0x00000000F80FC7E8
----
If I replace the !nokaslr() in efi_entry() with false, my kernel boots.
If I replace !nokaslr() with true, it fails to boot, even with the call
to efi_get_random_bytes() replaced with status = EFI_ERROR.
The problem appears to be the use of TASK_SIZE, since on arm64 that's:
#define TASK_SIZE (test_thread_flag(TIF_32BIT) ? \
TASK_SIZE_32 : TASK_SIZE_64)
... which would mean that the stub would be trying to dereference whatever it
found in sp_el0. Looking at objdump:
0000000000000160 <efi_entry>:
...
4b4: 94000000 bl 0 <nokaslr>
4b8: 35000280 cbnz w0, 508 <efi_entry+0x3a8>
4bc: d5384100 mrs x0, sp_el0
4c0: f9400004 ldr x4, [x0]
...
So I think we should revert this for now.
Thanks,
Mark.
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