From: Maxime Coquelin <hidden> Date: 2021-05-06 09:19:27
When no-IOMMU mode is enabled, VFIO is as unsafe as accessing
the PCI BARs via the device's sysfs, which is locked down when
the kernel is locked down.
Indeed, it is possible for an attacker to craft DMA requests
to modify kernel's code or leak secrets stored in the kernel,
since the device is not isolated by an IOMMU.
This patch introduces a new integrity lockdown reason for the
unsafe VFIO no-iommu mode.
Signed-off-by: Maxime Coquelin <redacted>
---
drivers/vfio/vfio.c | 13 +++++++++----
include/linux/security.h | 1 +
security/security.c | 1 +
3 files changed, 11 insertions(+), 4 deletions(-)
From: Alex Williamson <hidden> Date: 2021-05-06 21:50:30
On Thu, 6 May 2021 11:18:59 +0200
Maxime Coquelin [off-list ref] wrote:
When no-IOMMU mode is enabled, VFIO is as unsafe as accessing
the PCI BARs via the device's sysfs, which is locked down when
the kernel is locked down.
Indeed, it is possible for an attacker to craft DMA requests
to modify kernel's code or leak secrets stored in the kernel,
since the device is not isolated by an IOMMU.
This patch introduces a new integrity lockdown reason for the
unsafe VFIO no-iommu mode.
I'm hoping security folks will chime in here as I'm not familiar with
the standard practices for new lockdown reasons. The vfio no-iommu
backend is clearly an integrity risk, which is why it's already hidden
behind a separate Kconfig option, requires RAWIO capabilities, and
taints the kernel if it's used, but I agree that preventing it during
lockdown seems like a good additional step.
Is it generally advised to create specific reasons, like done here, or
should we aim to create a more generic reason related to unrestricted
userspace DMA?
I understand we don't want to re-use PCI_ACCESS because the vfio
no-iommu backend is device agnostic, it can be used for both PCI and
non-PCI devices.
In these cases where we're testing RAWIO, the idea is to raise the
barrier of passing file descriptors to unprivileged users. Is lockdown
sufficiently static that we might really only need the test on open?
The latter three cases here only make sense if the user were able to
open a no-iommu context when lockdown is not enabled, then lockdown is
later enabled preventing them from doing anything with that context...
but not preventing ongoing unsafe usage that might already exist. I
suspect for that reason that lockdown is static and we really only need
the test on open. Thanks,
Alex
From: Ondrej Mosnacek <hidden> Date: 2021-05-07 08:37:24
On Thu, May 6, 2021 at 11:50 PM Alex Williamson
[off-list ref] wrote:
On Thu, 6 May 2021 11:18:59 +0200
Maxime Coquelin [off-list ref] wrote:
quoted
When no-IOMMU mode is enabled, VFIO is as unsafe as accessing
the PCI BARs via the device's sysfs, which is locked down when
the kernel is locked down.
Indeed, it is possible for an attacker to craft DMA requests
to modify kernel's code or leak secrets stored in the kernel,
since the device is not isolated by an IOMMU.
This patch introduces a new integrity lockdown reason for the
unsafe VFIO no-iommu mode.
I'm hoping security folks will chime in here as I'm not familiar with
the standard practices for new lockdown reasons. The vfio no-iommu
backend is clearly an integrity risk, which is why it's already hidden
behind a separate Kconfig option, requires RAWIO capabilities, and
taints the kernel if it's used, but I agree that preventing it during
lockdown seems like a good additional step.
Is it generally advised to create specific reasons, like done here, or
should we aim to create a more generic reason related to unrestricted
userspace DMA?
I understand we don't want to re-use PCI_ACCESS because the vfio
no-iommu backend is device agnostic, it can be used for both PCI and
non-PCI devices.
In these cases where we're testing RAWIO, the idea is to raise the
barrier of passing file descriptors to unprivileged users. Is lockdown
sufficiently static that we might really only need the test on open?
The latter three cases here only make sense if the user were able to
open a no-iommu context when lockdown is not enabled, then lockdown is
later enabled preventing them from doing anything with that context...
but not preventing ongoing unsafe usage that might already exist. I
suspect for that reason that lockdown is static and we really only need
the test on open. Thanks,
Note that SELinux now also implements the locked_down hook and that
implementation is not static like the Lockdown LSM's. It checks
whether the current task's SELinux domain has either integrity or
confidentiality permission granted by the policy, so for SELinux it
makes sense to have the lockdown hook called in these other places as
well.
--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.
From: Maxime Coquelin <hidden> Date: 2021-05-07 09:16:12
On 5/7/21 10:37 AM, Ondrej Mosnacek wrote:
On Thu, May 6, 2021 at 11:50 PM Alex Williamson
[off-list ref] wrote:
quoted
On Thu, 6 May 2021 11:18:59 +0200
Maxime Coquelin [off-list ref] wrote:
quoted
When no-IOMMU mode is enabled, VFIO is as unsafe as accessing
the PCI BARs via the device's sysfs, which is locked down when
the kernel is locked down.
Indeed, it is possible for an attacker to craft DMA requests
to modify kernel's code or leak secrets stored in the kernel,
since the device is not isolated by an IOMMU.
This patch introduces a new integrity lockdown reason for the
unsafe VFIO no-iommu mode.
I'm hoping security folks will chime in here as I'm not familiar with
the standard practices for new lockdown reasons. The vfio no-iommu
backend is clearly an integrity risk, which is why it's already hidden
behind a separate Kconfig option, requires RAWIO capabilities, and
taints the kernel if it's used, but I agree that preventing it during
lockdown seems like a good additional step.
Is it generally advised to create specific reasons, like done here, or
should we aim to create a more generic reason related to unrestricted
userspace DMA?
I am fine with a more generic reason. I'm also not sure what is the best
thing to do in term of granularity.
quoted
I understand we don't want to re-use PCI_ACCESS because the vfio
no-iommu backend is device agnostic, it can be used for both PCI and
non-PCI devices.
In these cases where we're testing RAWIO, the idea is to raise the
barrier of passing file descriptors to unprivileged users. Is lockdown
sufficiently static that we might really only need the test on open?
The latter three cases here only make sense if the user were able to
open a no-iommu context when lockdown is not enabled, then lockdown is
later enabled preventing them from doing anything with that context...
but not preventing ongoing unsafe usage that might already exist. I
suspect for that reason that lockdown is static and we really only need
the test on open. Thanks,
Note that SELinux now also implements the locked_down hook and that
implementation is not static like the Lockdown LSM's. It checks
whether the current task's SELinux domain has either integrity or
confidentiality permission granted by the policy, so for SELinux it
makes sense to have the lockdown hook called in these other places as
well.
Thanks Ondrej for the insights, is there any plan for selinux to support
finer granularity than integrity and confidentiality? I'm not sure it
makes sense, but it might help to understand if we should choose between
a "VFIO no-iommu mode" reason and a more generic userspace DMA one.
Maxime
From: Ondrej Mosnacek <hidden> Date: 2021-05-07 12:31:49
(Adding the SELinux list, Paul, and Stephen to Cc)
On Fri, May 7, 2021 at 11:11 AM Maxime Coquelin
[off-list ref] wrote:
On 5/7/21 10:37 AM, Ondrej Mosnacek wrote:
quoted
On Thu, May 6, 2021 at 11:50 PM Alex Williamson
[off-list ref] wrote:
quoted
On Thu, 6 May 2021 11:18:59 +0200
Maxime Coquelin [off-list ref] wrote:
quoted
When no-IOMMU mode is enabled, VFIO is as unsafe as accessing
the PCI BARs via the device's sysfs, which is locked down when
the kernel is locked down.
Indeed, it is possible for an attacker to craft DMA requests
to modify kernel's code or leak secrets stored in the kernel,
since the device is not isolated by an IOMMU.
This patch introduces a new integrity lockdown reason for the
unsafe VFIO no-iommu mode.
I'm hoping security folks will chime in here as I'm not familiar with
the standard practices for new lockdown reasons. The vfio no-iommu
backend is clearly an integrity risk, which is why it's already hidden
behind a separate Kconfig option, requires RAWIO capabilities, and
taints the kernel if it's used, but I agree that preventing it during
lockdown seems like a good additional step.
Is it generally advised to create specific reasons, like done here, or
should we aim to create a more generic reason related to unrestricted
userspace DMA?
I am fine with a more generic reason. I'm also not sure what is the best
thing to do in term of granularity.
quoted
quoted
I understand we don't want to re-use PCI_ACCESS because the vfio
no-iommu backend is device agnostic, it can be used for both PCI and
non-PCI devices.
In these cases where we're testing RAWIO, the idea is to raise the
barrier of passing file descriptors to unprivileged users. Is lockdown
sufficiently static that we might really only need the test on open?
The latter three cases here only make sense if the user were able to
open a no-iommu context when lockdown is not enabled, then lockdown is
later enabled preventing them from doing anything with that context...
but not preventing ongoing unsafe usage that might already exist. I
suspect for that reason that lockdown is static and we really only need
the test on open. Thanks,
Note that SELinux now also implements the locked_down hook and that
implementation is not static like the Lockdown LSM's. It checks
whether the current task's SELinux domain has either integrity or
confidentiality permission granted by the policy, so for SELinux it
makes sense to have the lockdown hook called in these other places as
well.
Thanks Ondrej for the insights, is there any plan for selinux to support
finer granularity than integrity and confidentiality? I'm not sure it
makes sense, but it might help to understand if we should choose between
a "VFIO no-iommu mode" reason and a more generic userspace DMA one.
Looking at the ML discussion under the original patch posting [1],
having a finer granularity was preferred, but there was a concern that
the list of reasons would change too much, making the kernel <->
policy interface too unstable. Looking at the git log, the list of
lockdown reasons hasn't changed much since the original SELinux
implementation was added (there was just one addition, not counting
this patch), so it looks like switching SELinux to finer granularity
would be viable now.
I'm not sure whether the less or more generic variant would be better
here... Perhaps Stephen or Paul will have an opinion.
[1] https://lore.kernel.org/selinux/365ca063-6efd-8051-8d4b-5c8aef0d2e12@tycho.nsa.gov/T/
--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.
On Thu, May 06, 2021 at 11:18:59AM +0200, Maxime Coquelin wrote:
quoted hunk
When no-IOMMU mode is enabled, VFIO is as unsafe as accessing
the PCI BARs via the device's sysfs, which is locked down when
the kernel is locked down.
Indeed, it is possible for an attacker to craft DMA requests
to modify kernel's code or leak secrets stored in the kernel,
since the device is not isolated by an IOMMU.
This patch introduces a new integrity lockdown reason for the
unsafe VFIO no-iommu mode.
Signed-off-by: Maxime Coquelin <redacted>
---
drivers/vfio/vfio.c | 13 +++++++++----
include/linux/security.h | 1 +
security/security.c | 1 +
3 files changed, 11 insertions(+), 4 deletions(-)
Is the security threat specific to VFIO? (i.e. could other interfaces
want a similar thing, such that naming this VFIO doesn't make sense?
--
Kees Cook
From: Maxime Coquelin <hidden> Date: 2021-05-20 08:39:47
On 5/11/21 4:58 AM, Kees Cook wrote:
On Thu, May 06, 2021 at 11:18:59AM +0200, Maxime Coquelin wrote:
quoted
When no-IOMMU mode is enabled, VFIO is as unsafe as accessing
the PCI BARs via the device's sysfs, which is locked down when
the kernel is locked down.
Indeed, it is possible for an attacker to craft DMA requests
to modify kernel's code or leak secrets stored in the kernel,
since the device is not isolated by an IOMMU.
This patch introduces a new integrity lockdown reason for the
unsafe VFIO no-iommu mode.
Signed-off-by: Maxime Coquelin <redacted>
---
drivers/vfio/vfio.c | 13 +++++++++----
include/linux/security.h | 1 +
security/security.c | 1 +
3 files changed, 11 insertions(+), 4 deletions(-)
@@ -165,7 +166,8 @@ static void *vfio_noiommu_open(unsigned long arg){if(arg!=VFIO_NOIOMMU_IOMMU)returnERR_PTR(-EINVAL);-if(!capable(CAP_SYS_RAWIO))+if(!capable(CAP_SYS_RAWIO)||+security_locked_down(LOCKDOWN_VFIO_NOIOMMU))
The LSM hook check should come before the capable() check to avoid
setting PF_SUPERPRIV if capable() passes and the LSM doesn't.
OK, good to know, I'll swap in next revision.
BTW, it seems other places are doing the capable check before the LSM
hook check, for example in ioport [0].