Re: [PATCH RFC v11 5/19] ipe: introduce 'boot_verified' as a trust provider

[Paul Moore]

On Thu, Oct 26, 2023 at 5:33 PM Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx> wrote:
> On 10/23/2023 8:52 PM, Paul Moore wrote:
> > On Oct  4, 2023 Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx> wrote:
> >>
> >> IPE is designed to provide system level trust guarantees, this usually
> >> implies that trust starts from bootup with a hardware root of trust,
> >> which validates the bootloader. After this, the bootloader verifies the
> >> kernel and the initramfs.
> >>
> >> As there's no currently supported integrity method for initramfs, and
> >> it's typically already verified by the bootloader, introduce a property
> >> that causes the first superblock to have an execution to be "pinned",
> >> which is typically initramfs.
> >>
> >> When the "pinned" device is unmounted, it will be "unpinned" and
> >> `boot_verified` property will always evaluate to false afterward.
> >>
> >> We use a pointer with a spin_lock to "pin" the device instead of rcu
> >> because rcu synchronization may sleep, which is not allowed when
> >> unmounting a device.
> >>
> >> Signed-off-by: Deven Bowers <deven.desai@xxxxxxxxxxxxxxxxxxx>
> >> Signed-off-by: Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx>
> ...
> >> ---
> >>   security/ipe/eval.c          | 72 +++++++++++++++++++++++++++++++++++-
> >>   security/ipe/eval.h          |  2 +
> >>   security/ipe/hooks.c         | 12 ++++++
> >>   security/ipe/hooks.h         |  2 +
> >>   security/ipe/ipe.c           |  1 +
> >>   security/ipe/policy.h        |  2 +
> >>   security/ipe/policy_parser.c | 35 +++++++++++++++++-
> >>   7 files changed, 124 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/security/ipe/eval.c b/security/ipe/eval.c
> >> index 8a8bcc5c7d7f..bdac4abc0ddb 100644
> >> --- a/security/ipe/eval.c
> >> +++ b/security/ipe/eval.c
> >> @@ -9,6 +9,7 @@
> >>   #include <linux/file.h>
> >>   #include <linux/sched.h>
> >>   #include <linux/rcupdate.h>
> >> +#include <linux/spinlock.h>
> >>
> >>   #include "ipe.h"
> >>   #include "eval.h"
> >> @@ -16,6 +17,44 @@
> >>
> >>   struct ipe_policy __rcu *ipe_active_policy;
> >>
> >> +static const struct super_block *pinned_sb;
> >> +static DEFINE_SPINLOCK(pin_lock);
> >> +#define FILE_SUPERBLOCK(f) ((f)->f_path.mnt->mnt_sb)
> >> +
> >> +/**
> >> + * pin_sb - Pin the underlying superblock of @f, marking it as trusted.
> >> + * @sb: Supplies a super_block structure to be pinned.
> >> + */
> >> +static void pin_sb(const struct super_block *sb)
> >> +{
> >> +    if (!sb)
> >> +            return;
> >> +    spin_lock(&pin_lock);
> >> +    if (!pinned_sb)
> >> +            pinned_sb = sb;
> >> +    spin_unlock(&pin_lock);
> >> +}
> >> +
> >> +/**
> >> + * from_pinned - Determine whether @sb is the pinned super_block.
> >> + * @sb: Supplies a super_block to check against the pinned super_block.
> >> + *
> >> + * Return:
> >> + * * true   - @sb is the pinned super_block
> >> + * * false  - @sb is not the pinned super_block
> >> + */
> >> +static bool from_pinned(const struct super_block *sb)
> >> +{
> >> +    bool rv;
> >> +
> >> +    if (!sb)
> >> +            return false;
> >> +    spin_lock(&pin_lock);
> >> +    rv = !IS_ERR_OR_NULL(pinned_sb) && pinned_sb == sb;
> >> +    spin_unlock(&pin_lock);
> >
> > It's okay for an initial version, but I still think you need to get
> > away from this spinlock in from_pinned() as quickly as possible.
> > Maybe I'm wrong, but this looks like a major source of lock contention.
> >
> > I understand the issue around RCU and the potential for matching on
> > a reused buffer/address, but if you modified IPE to have its own LSM
> > security blob in super_block::security you could mark the superblock
> > when it was mounted and do a lockless lookup here in from_pinned().
>
> Thank you for the suggestion. After some testing, I discovered that
> switching to RCU to pin the super block and using a security blob to
> mark a pinned super block works. This approach do avoid many spinlock
> operations. I'll incorporate these changes in the next version of the patch.

I probably wasn't as clear as I should have been, I was thinking of
doing away with the @pinned_sb global variable entirely, as well as
its associated lock problems and simply marking the initramfs/initrd
superblock when it was mounted.  I will admit that I haven't fully
thought about all the implementation details, but I think you could
leverage the security_sb_mount() hook to set a flag in IPE's
superblock metadata when the initramfs was mounted.

--
paul-moore.com