| Message ID | 20220922152847.3670513-1-Jason@zx2c4.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | x86: re-initialize RNG seed when selecting kernel | expand |
On Thu, Sep 22, 2022 at 5:28 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > > We don't want it to be possible to re-read the RNG seed after ingesting > it, because this ruins forward secrecy. Currently, however, the setup > data section can just be re-read. Since the kernel is always read after > the setup data, use the selection of the kernel as a trigger to > re-initialize the RNG seed, just like we do on reboot, to preserve > forward secrecy. > > Cc: Paolo Bonzini <pbonzini@redhat.com> > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> > --- > Paolo- this applies on top of the 4 you merged this morning. -Jason Just bumping this, in hopes that this can go out with the same PULL for the other 4 you merged last week. Jason
On Mon, Sep 26, 2022 at 3:45 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > On Thu, Sep 22, 2022 at 5:28 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > > We don't want it to be possible to re-read the RNG seed after ingesting > > it, because this ruins forward secrecy. Currently, however, the setup > > data section can just be re-read. Since the kernel is always read after > > the setup data, use the selection of the kernel as a trigger to > > re-initialize the RNG seed, just like we do on reboot, to preserve > > forward secrecy. > > > > Cc: Paolo Bonzini <pbonzini@redhat.com> > > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> > > --- > > Paolo- this applies on top of the 4 you merged this morning. -Jason > > Just bumping this, in hopes that this can go out with the same PULL > for the other 4 you merged last week. Thanks, queued but I have a question. If I understand correctly, this protects against rereading the seed while the OS is running. If so, does that mean that the device tree-based seed initialization does not have forward secrecy at all? Paolo
Hi Paolo, On Mon, Sep 26, 2022 at 06:07:43PM +0200, Paolo Bonzini wrote: > On Mon, Sep 26, 2022 at 3:45 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > > On Thu, Sep 22, 2022 at 5:28 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > > > We don't want it to be possible to re-read the RNG seed after ingesting > > > it, because this ruins forward secrecy. Currently, however, the setup > > > data section can just be re-read. Since the kernel is always read after > > > the setup data, use the selection of the kernel as a trigger to > > > re-initialize the RNG seed, just like we do on reboot, to preserve > > > forward secrecy. > > > > > > Cc: Paolo Bonzini <pbonzini@redhat.com> > > > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> > > > --- > > > Paolo- this applies on top of the 4 you merged this morning. -Jason > > > > Just bumping this, in hopes that this can go out with the same PULL > > for the other 4 you merged last week. > > Thanks, queued but I have a question. Thank you. Also hope to see pc-bios/qboot.rom rebuilt based on that new submodule commit. QEMU uses the prebuilt in its tree, rather than rebuilding from the submodule. (I sent a patch earlier rebuilding qboot.rom in the tree, but you should probably do this yourself because it's pretty darn hard to verify binary diffs like that on the mailing list.) > If I understand correctly, this protects against rereading the seed while the > OS is running. If so, does that mean that the device tree-based seed > initialization does not have forward secrecy at all? On both x86 and dtb-based archs, the seed in memory is zeroed out by the kernel after reading. So, as far as the guest is concerned, there's forward secrecy. Except! Except if the guest has someway of re-requesting that seed from the host. This patch prevents that from happening through fw_cfg on x86. Somebody told me last week that device tree archs don't use fw_cfg, so this won't be a problem there. I haven't yet looked to verify that yet, though, or looked if there are other mechanisms. Jason
On Mon, 26 Sept 2022 at 17:53, Jason A. Donenfeld <Jason@zx2c4.com> wrote: > On both x86 and dtb-based archs, the seed in memory is zeroed out by the > kernel after reading. So, as far as the guest is concerned, there's > forward secrecy. Except! Except if the guest has someway of > re-requesting that seed from the host. This patch prevents that from > happening through fw_cfg on x86. Somebody told me last week that device > tree archs don't use fw_cfg, so this won't be a problem there. I haven't > yet looked to verify that yet, though, or looked if there are other > mechanisms. I am leaping in here with no context, so I may well have the wrong end of the stick, but: "does this system have a fw_cfg device" and "does this system pass a device tree to the kernel" are orthogonal questions: fw_cfg, no device tree: classic x86 pc; arm virt board when booting UEFI firmware in the guest fw_cfg, device tree: arm virt board booting a kernel directly no fw_cfg, device tree: arm vexpress-a15 board (or any other just-emulating-hardware machine type) no fw_cfg, no device tree: arm sbsa-ref board (and probably lots of non-arm architecture machines too) PS: if we're going to give FW_CFG_KERNEL_DATA magic side effect behaviour, is there somewhere we can document that magic? thanks -- PMM
Hi Peter, On Mon, Sep 26, 2022 at 7:05 PM Peter Maydell <peter.maydell@linaro.org> wrote: > > On Mon, 26 Sept 2022 at 17:53, Jason A. Donenfeld <Jason@zx2c4.com> wrote: > > On both x86 and dtb-based archs, the seed in memory is zeroed out by the > > kernel after reading. So, as far as the guest is concerned, there's > > forward secrecy. Except! Except if the guest has someway of > > re-requesting that seed from the host. This patch prevents that from > > happening through fw_cfg on x86. Somebody told me last week that device > > tree archs don't use fw_cfg, so this won't be a problem there. I haven't > > yet looked to verify that yet, though, or looked if there are other > > mechanisms. > > I am leaping in here with no context, so I may well have > the wrong end of the stick, but: > > "does this system have a fw_cfg device" and "does this system > pass a device tree to the kernel" are orthogonal questions: > > fw_cfg, no device tree: classic x86 pc; arm virt board when > booting UEFI firmware in the guest > fw_cfg, device tree: arm virt board booting a kernel directly > no fw_cfg, device tree: arm vexpress-a15 board (or any other > just-emulating-hardware machine type) > no fw_cfg, no device tree: arm sbsa-ref board (and probably > lots of non-arm architecture machines too) Okay it sounds like I've got to look into this indeed (as my "yet" in the previous message suggested). Specifically, the case relevant to this discussion is device tree that goes through fw_cfg. I've got a few other investigations I'd like to do over there anyway (looking into how reboots work), so I'll send a series for that when I've got things worked out. For the time being, though, this x86 work here is independent of that. But I suppose you can expect to hear from me not before long about device tree things. Jason
diff --git a/hw/i386/x86.c b/hw/i386/x86.c index f9a4ddaa4a..1148f70c03 100644 --- a/hw/i386/x86.c +++ b/hw/i386/x86.c @@ -1112,11 +1112,14 @@ void x86_load_linux(X86MachineState *x86ms, setup_data->len = cpu_to_le32(RNG_SEED_LENGTH); qemu_guest_getrandom_nofail(setup_data->data, RNG_SEED_LENGTH); qemu_register_reset(reset_rng_seed, setup_data); + fw_cfg_add_bytes_callback(fw_cfg, FW_CFG_KERNEL_DATA, reset_rng_seed, NULL, + setup_data, kernel, kernel_size, true); + } else { + fw_cfg_add_bytes(fw_cfg, FW_CFG_KERNEL_DATA, kernel, kernel_size); } fw_cfg_add_i32(fw_cfg, FW_CFG_KERNEL_ADDR, prot_addr); fw_cfg_add_i32(fw_cfg, FW_CFG_KERNEL_SIZE, kernel_size); - fw_cfg_add_bytes(fw_cfg, FW_CFG_KERNEL_DATA, kernel, kernel_size); sev_load_ctx.kernel_data = (char *)kernel; sev_load_ctx.kernel_size = kernel_size;
We don't want it to be possible to re-read the RNG seed after ingesting it, because this ruins forward secrecy. Currently, however, the setup data section can just be re-read. Since the kernel is always read after the setup data, use the selection of the kernel as a trigger to re-initialize the RNG seed, just like we do on reboot, to preserve forward secrecy. Cc: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> --- Paolo- this applies on top of the 4 you merged this morning. -Jason hw/i386/x86.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)