fsverity: improve documentation for builtin signature support

fsverity builtin signatures (CONFIG_FS_VERITY_BUILTIN_SIGNATURES) aren't
the only way to do signatures with fsverity, and they have some major
limitations.  Yet, more users have tried to use them, e.g. recently by
https://github.com/ostreedev/ostree/pull/2640.  In most cases this seems
to be because users aren't sufficiently familiar with the limitations of
this feature and what the alternatives are.

Therefore, make some updates to the documentation to try to clarify the
properties of this feature and nudge users in the right direction.

Note that the Integrity Policy Enforcement (IPE) LSM, which is not yet
upstream, is planned to use the builtin signatures.  (This differs from
IMA, which uses its own signature mechanism.)  For that reason, my
earlier patch "fsverity: mark builtin signatures as deprecated"
(https://lore.kernel.org/r/20221208033548.122704-1-ebiggers@kernel.org),
which marked builtin signatures as "deprecated", was controversial.

This patch therefore stops short of marking the feature as deprecated.
I've also revised the language to focus on better explaining the feature
and what its alternatives are.

Link: https://lore.kernel.org/r/20230620041937.5809-1-ebiggers@kernel.org
Reviewed-by: Colin Walters <walters@verbum.org>
Reviewed-by: Luca Boccassi <bluca@debian.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 8 insertions, 8 deletions

diff --git a/fs/verity/Kconfig b/fs/verity/Kconfig
index a7ffd718f171..e1036e535352 100644
--- a/fs/verity/Kconfig
+++ b/fs/verity/Kconfig
@@ -39,14 +39,14 @@ config FS_VERITY_BUILTIN_SIGNATURES
 	depends on FS_VERITY
 	select SYSTEM_DATA_VERIFICATION
 	help
-	  Support verifying signatures of verity files against the X.509
-	  certificates that have been loaded into the ".fs-verity"
-	  kernel keyring.
+	  This option adds support for in-kernel verification of
+	  fs-verity builtin signatures.
 
-	  This is meant as a relatively simple mechanism that can be
-	  used to provide an authenticity guarantee for verity files, as
-	  an alternative to IMA appraisal.  Userspace programs still
-	  need to check that the verity bit is set in order to get an
-	  authenticity guarantee.
+	  Please take great care before using this feature.  It is not
+	  the only way to do signatures with fs-verity, and the
+	  alternatives (such as userspace signature verification, and
+	  IMA appraisal) can be much better.  For details about the
+	  limitations of this feature, see
+	  Documentation/filesystems/fsverity.rst.
 
 	  If unsure, say N.