Re: [PATCH v2 1/3] dm: introduce audit event module for device mapper

On Sat, Aug 14, 2021 at 2:34 PM Michael Weiß <michael.weiss(a)aisec.fraunhofer.de&gt; wrote: > To be able to send auditing events to user space, we introduce > a generic dm-audit module. It provides helper functions to emit > audit events through the kernel audit subsystem. We claim the > AUDIT_DM type=1336 out of the audit event messages range in the > corresponding userspace api in 'include/uapi/linux/audit.h' for > those events. > > Following commits to device mapper targets actually will make > use of this to emit those events in relevant cases. > > Signed-off-by: Michael Weiß <michael.weiss(a)aisec.fraunhofer.de&gt; Hi Michael, You went into more detail in your patchset cover letter, e.g. example audit records, which I think would be helpful here in the commit description so we have it as part of the git log. I don't want to discourage you from writing cover letters, but don't forget that the cover letters can be lost to the ether after a couple of years whereas the git log has a much longer lifetime (we hope!) and a tighter binding to the related code.

> --- > drivers/md/Kconfig | 10 +++++++ > drivers/md/Makefile | 4 +++ > drivers/md/dm-audit.c | 59 ++++++++++++++++++++++++++++++++++++++ > drivers/md/dm-audit.h | 33 +++++++++++++++++++++ > include/uapi/linux/audit.h | 1 + > 5 files changed, 107 insertions(+) > create mode 100644 drivers/md/dm-audit.c > create mode 100644 drivers/md/dm-audit.h > > diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig > index 0602e82a9516..48adbec12148 100644 > --- a/drivers/md/Kconfig > +++ b/drivers/md/Kconfig > @@ -608,6 +608,7 @@ config DM_INTEGRITY > select CRYPTO > select CRYPTO_SKCIPHER > select ASYNC_XOR > + select DM_AUDIT if AUDIT > help > This device-mapper target emulates a block device that has > additional per-sector tags that can be used for storing > @@ -640,4 +641,13 @@ config DM_ZONED > > If unsure, say N. > > +config DM_AUDIT > + bool "DM audit events" > + depends on AUDIT > + help > + Generate audit events for device-mapper. > + > + Enables audit logging of several security relevant events in the > + particular device-mapper targets, especially the integrity target. > + > endif # MD > diff --git a/drivers/md/Makefile b/drivers/md/Makefile > index a74aaf8b1445..4cd47623c742 100644 > --- a/drivers/md/Makefile > +++ b/drivers/md/Makefile > @@ -103,3 +103,7 @@ endif > ifeq ($(CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG),y) > dm-verity-objs += dm-verity-verify-sig.o > endif > + > +ifeq ($(CONFIG_DM_AUDIT),y) > +dm-mod-objs += dm-audit.o > +endif > diff --git a/drivers/md/dm-audit.c b/drivers/md/dm-audit.c > new file mode 100644 > index 000000000000..c7e5824821bb > --- /dev/null > +++ b/drivers/md/dm-audit.c > @@ -0,0 +1,59 @@ > +// SPDX-License-Identifier: GPL-2.0 > +/* > + * Creating audit records for mapped devices. > + * > + * Copyright (C) 2021 Fraunhofer AISEC. All rights reserved. > + * > + * Authors: Michael Weiß <michael.weiss(a)aisec.fraunhofer.de&gt; > + */ > + > +#include <linux/audit.h> > +#include <linux/module.h> > +#include <linux/device-mapper.h> > +#include <linux/bio.h> > +#include <linux/blkdev.h> > + > +#include "dm-audit.h" > +#include "dm-core.h" > + > +void dm_audit_log_bio(const char *dm_msg_prefix, const char *op, > + struct bio *bio, sector_t sector, int result) > +{ > + struct audit_buffer *ab; > + > + if (audit_enabled == AUDIT_OFF) > + return; > + > + ab = audit_log_start(audit_context(), GFP_KERNEL, AUDIT_DM); > + if (unlikely(!ab)) > + return; > + > + audit_log_format(ab, "module=%s dev=%d:%d op=%s sector=%llu res=%d", > + dm_msg_prefix, MAJOR(bio->bi_bdev->bd_dev), > + MINOR(bio->bi_bdev->bd_dev), op, sector, result); > + audit_log_end(ab); > +} > +EXPORT_SYMBOL_GPL(dm_audit_log_bio); > + > +void dm_audit_log_target(const char *dm_msg_prefix, const char *op, > + struct dm_target *ti, int result) > +{ > + struct audit_buffer *ab; > + struct mapped_device *md = dm_table_get_md(ti->table); > + > + if (audit_enabled == AUDIT_OFF) > + return; > + > + ab = audit_log_start(audit_context(), GFP_KERNEL, AUDIT_DM); > + if (unlikely(!ab)) > + return; > + > + audit_log_format(ab, "module=%s dev=%s op=%s", > + dm_msg_prefix, dm_device_name(md), op); > + > + if (!result && !strcmp("ctr", op)) > + audit_log_format(ab, " error_msg='%s'", ti->error); > + audit_log_format(ab, " res=%d", result); > + audit_log_end(ab); > +} Generally speaking we try to keep a consistent format and ordering within a given audit record type. However you appear to have at least three different formats for the AUDIT_DM record in this patch: "... module=%s dev=%d:%d op=%s sector=%llu res=%d" "... module=%s dev=%s op=%s error_msg='%s' res=%d" "... module=%s dev=%s op=%s res=%d" The first thing that jumps out is that some fields, e.g. "sector", are not always present in the record; we typically handle this by using a "?" for the field value in those cases where you would otherwise drop the field from the record, for example the following record: "... module=%s dev=%s op=%s res=%d" ... would be rewritten like this: "... module=%s dev=%s op=%s sector=? res=%d"

The second thing that I noticed is that the "dev" field changes from a "major:minor" number representation to an arbitrary string value, e.g. "dev=%s". This generally isn't something we do with audit records, please stick to a single representation for a given audit record-type/field combination.