Re: [PATCH v17 13/21] dm verity: consume root hash digest and expose signature data via LSM hook

Fan Wu
2024-04-25T20:23:59Z

On 4/24/2024 8:56 PM, Eric Biggers wrote:
> On Fri, Apr 12, 2024 at 05:55:56PM -0700, Fan Wu wrote:
>> dm verity: consume root hash digest and expose signature data via LSM hook
>
> As in the fsverity patch, nothing is being "consumed" here. This patch adds a
> supplier, not a consumer. I think you mean something like: expose root digest
> and signature to LSMs.
>
Thanks for the suggestion.

>> diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
>> index bb5da66da4c1..fbb83c6fd99c 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_SECURITY
>> +
>> +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 = kmemdup(sig, v->sig_size, GFP_KERNEL);
>> + if (!v->root_digest)
>> + return -ENOMEM;
>
> root_digest_sig, not root_digest
>
Thanks for pointing out!

>> +#ifdef CONFIG_SECURITY
>> +
>> +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_INT_DMVERITY_ROOTHASH, &root_digest,
>> + sizeof(root_digest));
>> + if (r)
>> + return r;
>> +
>> + r = security_bdev_setintegrity(bdev,
>> + LSM_INT_DMVERITY_SIG_VALID,
>> + v->root_digest_sig,
>> + v->sig_size);
>
> The signature is only checked if CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG=y, whereas
> this code is built whenever CONFIG_SECURITY=y.
>
> So this seems like the same issue that has turned up elsewhere in the IPE
> patchset, where IPE is (apparently) happy with any signature, even one that
> hasn't been checked...
>

Yes I do agree the second hook call should better depend on
CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG=y.

However, the current implementation does not happy with any signature.

In case of CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG=y, any signature
provided to dm-verity will be checked against the configured keyring,
the hook call won't be reached if the check failed. In case of no
signature is provided and !DM_VERITY_IS_SIG_FORCE_ENABLED(), the hook
will be called with signature value NULL.

In case of CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG=n, signature won't be
accepted by dm-verity. In addition, the whole support of dm-verity will
be disabled for IPE because CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG=n.

>> diff --git a/drivers/md/dm-verity.h b/drivers/md/dm-verity.h
>> index 20b1bcf03474..89e862f0cdf6 100644
>> --- a/drivers/md/dm-verity.h
>> +++ b/drivers/md/dm-verity.h
>> @@ -43,6 +43,9 @@ struct dm_verity {
>> u8 *root_digest; /* digest of the root block */
>> u8 *salt; /* salt: its size is salt_size */
>> u8 *zero_digest; /* digest for a zero block */
>> +#ifdef CONFIG_SECURITY
>> + u8 *root_digest_sig; /* digest signature of the root block */
>> +#endif /* CONFIG_SECURITY */
>
> No, it's not a signature of the root block, at least not directly. It's a
> signature of the root digest (the digest of the root block).
>
>> diff --git a/include/linux/dm-verity.h b/include/linux/dm-verity.h
>> new file mode 100644
>> index 000000000000..a799a8043d85
>> --- /dev/null
>> +++ b/include/linux/dm-verity.h
>> @@ -0,0 +1,12 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +
>> +#ifndef _LINUX_DM_VERITY_H
>> +#define _LINUX_DM_VERITY_H
>> +
>> +struct dm_verity_digest {
>> + const char *alg;
>> + const u8 *digest;
>> + size_t digest_len;
>> +};
>> +
>> +#endif /* _LINUX_DM_VERITY_H */
>> diff --git a/include/linux/security.h b/include/linux/security.h
>> index ac0985641611..9e46b13a356c 100644
>> --- a/include/linux/security.h
>> +++ b/include/linux/security.h
>> @@ -84,7 +84,8 @@ enum lsm_event {
>> };
>>
>> enum lsm_integrity_type {
>> - __LSM_INT_MAX
>> + LSM_INT_DMVERITY_SIG_VALID,
>> + LSM_INT_DMVERITY_ROOTHASH,
>> };
>
> Shouldn't struct dm_verity_digest be defined next to LSM_INT_DMVERITY_ROOTHASH?
> It's the struct that's associated with it.
>
> It seems weird to create a brand new header <linux/dm-verity.h> that just
> contains this one LSM related definition, when there's already a header for the
> LSM definitions that even includes the related value LSM_INT_DMVERITY_ROOTHASH.
>
> - Eric

Yes they can just be in the same header. Thanks for the suggestion.

-Fan