Re: [PATCH RFC v15 14/21] dm verity: consume root hash digest and signature data via LSM hook

Mike Snitzer
2024-03-20T02:19:52Z

On Tue, Mar 19 2024 at 7:00P -0400,
Paul Moore <paul@paul-moore.com> wrote:

> On Mar 15, 2024 Fan Wu <wufan@linux.microsoft.com> wrote:
> >
> > dm-verity provides a strong guarantee of a block device's integrity. As
> > a generic way to check the integrity of a block device, it provides
> > those integrity guarantees to its higher layers, including the filesystem
> > level.
> >
> > An LSM that control access to a resource on the system based on the
> > available integrity claims can use this transitive property of
> > dm-verity, by querying the underlying block_device of a particular
> > file.
> >
> > The digest and signature information need to be stored in the block
> > device to fulfill the next requirement of authorization via LSM policy.
> > This will enable the LSM to perform revocation of devices that are still
> > mounted, prohibiting execution of files that are no longer authorized
> > by the LSM in question.
> >
> > This patch adds two security hook calls in dm-verity to save the
> > dm-verity roothash and the roothash signature to the block device's
> > LSM blobs. The hook calls are depended on CONFIG_IPE_PROP_DM_VERITY,
> > which will be introduced in the next commit.
> >
> > Signed-off-by: Deven Bowers <deven.desai@linux.microsoft.com>
> > Signed-off-by: Fan Wu <wufan@linux.microsoft.com>
> > ---
> > v2:
> > + No Changes
> >
> > v3:
> > + No changes
> >
> > v4:
> > + No changes
> >
> > v5:
> > + No changes
> >
> > v6:
> > + Fix an improper cleanup that can result in
> > a leak
> >
> > v7:
> > + Squash patch 08/12, 10/12 to [11/16]
> > + Use part0 for block_device, to retrieve the block_device, when
> > calling security_bdev_setsecurity
> >
> > v8:
> > + Undo squash of 08/12, 10/12 - separating drivers/md/ from
> > security/ & block/
> > + Use common-audit function for dmverity_signature.
> > + Change implementation for storing the dm-verity digest to use the
> > newly introduced dm_verity_digest structure introduced in patch
> > 14/20.
> > + Create new structure, dm_verity_digest, containing digest algorithm,
> > size, and digest itself to pass to the LSM layer. V7 was missing the
> > algorithm.
> > + Create an associated public header containing this new structure and
> > the key values for the LSM hook, specific to dm-verity.
> > + Additional information added to commit, discussing the layering of
> > the changes and how the information passed will be used.
> >
> > v9:
> > + No changes
> >
> > v10:
> > + No changes
> >
> > v11:
> > + Add an optional field to save signature
> > + Move the security hook call to the new finalize hook
> >
> > v12:
> > + No changes
> >
> > v13:
> > + No changes
> >
> > v14:
> > + Correct code format
> > + Remove unnecessary header and switch to dm_disk()
> >
> > v15:
> > + Refactor security_bdev_setsecurity() to security_bdev_setintegrity()
> > + Remove unnecessary headers
> > ---
> > drivers/md/dm-verity-target.c | 73 +++++++++++++++++++++++++++++++++++
> > drivers/md/dm-verity.h | 6 +++
> > include/linux/dm-verity.h | 12 ++++++
> > include/linux/security.h | 2 +
> > 4 files changed, 93 insertions(+)
> > create mode 100644 include/linux/dm-verity.h
> >
> > diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> > index bb5da66da4c1..e94cc6a755d5 100644
> > --- a/drivers/md/dm-verity-target.c
> > +++ b/drivers/md/dm-verity-target.c
> > @@ -22,6 +22,8 @@
> > #include <linux/scatterlist.h>
> > #include <linux/string.h>
> > #include <linux/jump_label.h>
> > +#include <linux/security.h>
> > +#include <linux/dm-verity.h>
> >
> > #define DM_MSG_PREFIX "verity"
> >
> > @@ -1017,6 +1019,38 @@ static void verity_io_hints(struct dm_target *ti, struct queue_limits *limits)
> > blk_limits_io_min(limits, limits->logical_block_size);
> > }
> >
> > +#ifdef CONFIG_IPE_PROP_DM_VERITY
> > +
> > +static int verity_init_sig(struct dm_verity *v, const void *sig,
> > + size_t sig_size)
> > +{
> > + v->sig_size = sig_size;
> > + v->root_digest_sig = kmalloc(v->sig_size, GFP_KERNEL);
> > + if (!v->root_digest)
> > + return -ENOMEM;
>
> Either you meant to copy @sig into @v->root_digest_sig and forgot to
> add the code for that, or we don't need to include @sig as a parameter
> to this function. I'm guessing it is the former as it wouldn't make
> sense to even have dm_verity::root_digest_sig if we weren't stashing
> it here.
>
> I'd also suggest looking at kmemdup() instead of a kmalloc()/memcpy()
> combo.
>
> > + return 0;
> > +}
> > +
> > +static void verity_free_sig(struct dm_verity *v)
> > +{
> > + kfree(v->root_digest_sig);
> > +}
> > +#else
> > +
> > +static inline int verity_init_sig(struct dm_verity *v, const void *sig,
> > + size_t sig_size)
> > +{
> > + return 0;
> > +}
> > +
> > +static inline void verity_free_sig(struct dm_verity *v)
> > +{
> > +}
> > +
> > +#endif /* CONFIG_IPE_PROP_DM_VERITY */
>
> It's been a while since I looked at this patch in the patchset, so
> maybe I'm missing something, but in general we don't want CONFIG_XXX
> checks in the kernel, outside of security/, that are specific to a
> particular LSM (what happens when multiple LSMs need this?). Please
> use CONFIG_SECURITY instead.
>
> > static void verity_dtr(struct dm_target *ti)
> > {
> > struct dm_verity *v = ti->private;
> > @@ -1035,6 +1069,7 @@ static void verity_dtr(struct dm_target *ti)
> > kfree(v->salt);
> > kfree(v->root_digest);
> > kfree(v->zero_digest);
> > + verity_free_sig(v);
> >
> > if (v->tfm)
> > crypto_free_ahash(v->tfm);
> > @@ -1434,6 +1469,13 @@ static int verity_ctr(struct dm_target *ti, unsigned int argc, char **argv)
> > ti->error = "Root hash verification failed";
> > goto bad;
> > }
> > +
> > + r = verity_init_sig(v, verify_args.sig, verify_args.sig_size);
> > + if (r < 0) {
> > + ti->error = "Cannot allocate root digest signature";
> > + goto bad;
> > + }
> > +
> > v->hash_per_block_bits =
> > __fls((1 << v->hash_dev_block_bits) / v->digest_size);
> >
> > @@ -1584,6 +1626,34 @@ int dm_verity_get_root_digest(struct dm_target *ti, u8 **root_digest, unsigned i
> > return 0;
> > }
> >
> > +#ifdef CONFIG_IPE_PROP_DM_VERITY
> > +
> > +static int verity_finalize(struct dm_target *ti)
> > +{
> > + struct block_device *bdev;
> > + struct dm_verity_digest root_digest;
> > + struct dm_verity *v;
> > + int r;
> > +
> > + v = ti->private;
> > + bdev = dm_disk(dm_table_get_md(ti->table))->part0;
> > + root_digest.digest = v->root_digest;
> > + root_digest.digest_len = v->digest_size;
> > + root_digest.alg = v->alg_name;
> > +
> > + r = security_bdev_setintegrity(bdev, LSM_INTGR_DMV_ROOTHASH, &root_digest,
> > + sizeof(root_digest));
> > + if (r)
> > + return r;
> > +
> > + return security_bdev_setintegrity(bdev,
> > + LSM_INTGR_DMV_SIG,
> > + v->root_digest_sig,
> > + v->sig_size);
>
> What happens if the second call fails, should we clear the
> LSM_INTGR_DMV_ROOTHASH state in the LSM?
>
> > +}
> > +
> > +#endif /* CONFIG_IPE_PROP_DM_VERITY */
>
> See my comments about CONFIG_SECURITY above. In fact, I would suggest
> moving this up into that part of the file so you only need one #ifdef
> block relating to CONFIG_SECURITY.
>
> I would also recommend making a dummy function so we can get rid of
> the conditional compilation in @verity_target below. For example:
>
> #ifdef CONFIG_SECURITY
> static int verity_finalize(struct dm_target *ti)
> {
> /* real implementation */
> }
> #else
> static int verity_finalize(struct dm_target *ti)
> {
> return 0;
> }
> #endif /* CONFIG_SECURITY */
>
> > static struct target_type verity_target = {
> > .name = "verity",
> > .features = DM_TARGET_SINGLETON | DM_TARGET_IMMUTABLE,
> > @@ -1596,6 +1666,9 @@ static struct target_type verity_target = {
> > .prepare_ioctl = verity_prepare_ioctl,
> > .iterate_devices = verity_iterate_devices,
> > .io_hints = verity_io_hints,
> > +#ifdef CONFIG_IPE_PROP_DM_VERITY
> > + .finalize = verity_finalize,
> > +#endif /* CONFIG_IPE_PROP_DM_VERITY */
> > };
> > module_dm(verity);
>
> If you create a dummy verity_finalize() function like above you can
> get rid of the #ifdef checks.

Think it is better to leave it as-is, to avoid calling the .finalize
hook if it isn't actually needed.

Mike