> From: Mimi Zohar [mailto:[email protected]]
> Sent: Friday, December 4, 2020 2:05 PM
> On Fri, 2020-12-04 at 08:05 +0000, Roberto Sassu wrote:
> > > From: Mimi Zohar [mailto:[email protected]]
> > > Sent: Thursday, December 3, 2020 9:43 PM
> > > Hi Roberto,
> > >
> > > On Wed, 2020-11-11 at 10:22 +0100, Roberto Sassu wrote:
> > > > When a file is being created, LSMs can set the initial label with the
> > > > inode_init_security hook. If no HMAC key is loaded, the new file will
> have
> > > > LSM xattrs but not the HMAC.
> > > >
> > > > Unfortunately, EVM will deny any further metadata operation on new
> > > files,
> > > > as evm_protect_xattr() will always return the INTEGRITY_NOLABEL
> error.
> > > This
> > > > would limit the usability of EVM when only a public key is loaded, as
> > > > commands such as cp or tar with the option to preserve xattrs won't
> work.
> > > >
> > > > Ignoring this error won't be an issue if no HMAC key is loaded, as the
> > > > inode is locked until the post hook, and EVM won't calculate the HMAC
> on
> > > > metadata that wasn't previously verified. Thus this patch checks if an
> > > > HMAC key is loaded and if not, ignores INTEGRITY_NOLABEL.
> > >
> > > I'm not sure what problem this patch is trying to solve.
> > > evm_protect_xattr() is only called by evm_inode_setxattr() and
> > > evm_inode_removexattr(), which first checks whether
> > > EVM_ALLOW_METADATA_WRITES is enabled.
> >
> > The idea is to also support EVM verification when only a public key
> > is loaded. An advantage to do that is that for example we can prevent
> > accidental metadata changes when the signature is portable.
>
> Right, there are a couple of scenarios. Let's be more specific as to
> which scenario this patch is addressing.
>
> - a public key is loaded and EVM_ALLOW_METADATA_WRITES is enabled,
> - a public key is loaded and EVM_ALLOW_METADATA_WRITES is disabled,
> - an HMAC key is loaded
>
> For the first and last case, this patch shouldn't be necessary. Only
> the second case, with EVM_ALLOW_METADATA_WRITES disabled, probably
> does
> not work. I would claim that is working as designed.

If there is no HMAC key loaded and a file is created, I think EVM should
not expect an HMAC and return an error. If we do metadata verification
only when an HMAC key is loaded, we miss a functionality that could be
useful also when only a public key is loaded.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

2021-03-03 16:13:11

by Mimi Zohar

[permalink] [raw]

Subject: Re: [PATCH v3 02/11] evm: Load EVM key in ima_load_x509() to avoid appraisal

Hi Roberto,

On Wed, 2020-11-11 at 10:22 +0100, Roberto Sassu wrote:
> Public keys do not need to be appraised by IMA as the restriction on the
> IMA/EVM keyrings ensures that a key can be loaded only if it is signed with
> a key in the primary or secondary keyring.
>
> However, when evm_load_x509() is called, appraisal is already enabled and
> a valid IMA signature must be added to the EVM key to pass verification.
>
> Since the restriction is applied on both IMA and EVM keyrings, it is safe
> to disable appraisal also when the EVM key is loaded. This patch calls
> evm_load_x509() inside ima_load_x509() if CONFIG_IMA_LOAD_X509 is defined.
>
> Signed-off-by: Roberto Sassu <[email protected]>
> Reviewed-by: Mimi Zohar <[email protected]>
> ---
> security/integrity/iint.c | 2 ++
> security/integrity/ima/ima_init.c | 4 ++++
> 2 files changed, 6 insertions(+)
>
> diff --git a/security/integrity/iint.c b/security/integrity/iint.c
> index 1d20003243c3..7d08c31c612f 100644
> --- a/security/integrity/iint.c
> +++ b/security/integrity/iint.c
> @@ -200,7 +200,9 @@ int integrity_kernel_read(struct file *file, loff_t offset,
> void __init integrity_load_keys(void)
> {
> ima_load_x509();
> +#ifndef CONFIG_IMA_LOAD_X509
> evm_load_x509();
> +#endif

Please replace the ifdef with the IS_ENABLED() equivalent.

thanks,

Mimi