dm verity: emit audit events on verification failure and more

dm-verity signals integrity violations by returning I/O errors
to user space. To identify integrity violations by a controlling
instance, the kernel audit subsystem can be used to emit audit
events to user space. Analogous to dm-integrity, we also use the
dm-audit submodule allowing to emit audit events on verification
failures of metadata and data blocks as well as if max corrupted
errors are reached.

The construction and destruction of verity device mappings are
also relevant for auditing a system. Thus, those events are also
logged as audit events.

Tested by starting a container with the container manager (cmld) of
GyroidOS which uses a dm-verity protected rootfs image root.img mapped
to /dev/mapper/<uuid>-root. One block was manipulated in the
underlying image file and repeated reads of the verity device were
performed again until the max corrupted errors is reached, e.g.:

  dd if=/dev/urandom of=root.img bs=512 count=1 seek=1000
  for i in range {1..101}; do \
    dd if=/dev/mapper/<uuid>-root of=/dev/null bs=4096 \
       count=1 skip=1000 \
  done

The resulting audit log looks as follows:

  type=DM_CTRL msg=audit(1677618791.876:962):
    module=verity op=ctr ppid=4876 pid=29102 auid=0 uid=0 gid=0
    euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=44
    comm="cmld" exe="/usr/sbin/cml/cmld" subj=unconfined
    dev=254:3 error_msg='success' res=1

  type=DM_EVENT msg=audit(1677619463.786:1074): module=verity
    op=verify-data dev=7:0 sector=1000 res=0
  ...
  type=DM_EVENT msg=audit(1677619596.727:1162): module=verity
    op=verify-data dev=7:0 sector=1000 res=0

  type=DM_EVENT msg=audit(1677619596.731:1163): module=verity
    op=max-corrupted-errors dev=254:3 sector=? res=0
Signed-off-by: Michael Weiß <michael.weiss@aisec.fraunhofer.de>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-verity-target.c

@@ -16,6 +16,7 @@
 #include "dm-verity.h"
 #include "dm-verity-fec.h"
 #include "dm-verity-verify-sig.h"
+#include "dm-audit.h"
 #include <linux/module.h>
 #include <linux/reboot.h>
 #include <linux/scatterlist.h>
@@ -248,8 +249,10 @@ static int verity_handle_err(struct dm_verity *v, enum verity_block_type type,
 	DMERR_LIMIT("%s: %s block %llu is corrupted", v->data_dev->name,
 		    type_str, block);
 
-	if (v->corrupted_errs == DM_VERITY_MAX_CORRUPTED_ERRS)
+	if (v->corrupted_errs == DM_VERITY_MAX_CORRUPTED_ERRS) {
 		DMERR("%s: reached maximum errors", v->data_dev->name);
+		dm_audit_log_target(DM_MSG_PREFIX, "max-corrupted-errors", v->ti, 0);
+	}
 
 	snprintf(verity_env, DM_VERITY_ENV_LENGTH, "%s=%d,%llu",
 		DM_VERITY_ENV_VAR_NAME, type, block);
@@ -340,6 +343,11 @@ static int verity_verify_level(struct dm_verity *v, struct dm_verity_io *io,
 		else if (verity_handle_err(v,
 					   DM_VERITY_BLOCK_TYPE_METADATA,
 					   hash_block)) {
+			struct bio *bio =
+				dm_bio_from_per_bio_data(io,
+							 v->ti->per_io_data_size);
+			dm_audit_log_bio(DM_MSG_PREFIX, "verify-metadata", bio,
+					 block, 0);
 			r = -EIO;
 			goto release_ret_r;
 		}
@@ -590,10 +598,13 @@ static int verity_verify_io(struct dm_verity_io *io)
 				return -EIO;
 			}
 			if (verity_handle_err(v, DM_VERITY_BLOCK_TYPE_DATA,
-					      cur_block))
+					      cur_block)) {
+				dm_audit_log_bio(DM_MSG_PREFIX, "verify-data",
+						 bio, cur_block, 0);
 				return -EIO;
 			}
 		}
+	}
 
 	return 0;
 }
@@ -975,6 +986,8 @@ static void verity_dtr(struct dm_target *ti)
 		static_branch_dec(&use_tasklet_enabled);
 
 	kfree(v);
+
+	dm_audit_log_dtr(DM_MSG_PREFIX, ti, 1);
 }
 
 static int verity_alloc_most_once(struct dm_verity *v)
@@ -1429,11 +1442,14 @@ static int verity_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 
 	verity_verify_sig_opts_cleanup(&verify_args);
 
+	dm_audit_log_ctr(DM_MSG_PREFIX, ti, 1);
+
 	return 0;
 
 bad:
 
 	verity_verify_sig_opts_cleanup(&verify_args);
+	dm_audit_log_ctr(DM_MSG_PREFIX, ti, 0);
 	verity_dtr(ti);
 
 	return r;