linux-kernel - Re: [PATCH ghak109 V1] audit: link integrity evm_write

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Message-ID: <20190321005008.wfz3bk7q262km5fz@madcap2.tricolour.ca>
Date:   Wed, 20 Mar 2019 20:50:08 -0400
From:   Richard Guy Briggs <rgb@...hat.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Linux-Audit Mailing List <linux-audit@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, sgrubb@...hat.com,
        omosnace@...hat.com, Eric Paris <eparis@...isplace.org>,
        Serge Hallyn <serge@...lyn.com>, zohar@...ux.ibm.com,
        mjg59@...gle.com
Subject: Re: [PATCH ghak109 V1] audit: link integrity evm_write_xattrs record
 to syscall event

On 2019-03-20 19:48, Paul Moore wrote:
> On Sat, Mar 16, 2019 at 8:10 AM Richard Guy Briggs <rgb@...hat.com> wrote:
> > In commit fa516b66a1bf ("EVM: Allow runtime modification of the set of
> > verified xattrs"), the call to audit_log_start() is missing a context to
> > link it to an audit event. Since this event is in user context, add
> > the process' syscall context to the record.
> >
> > In addition, the orphaned keyword "locked" appears in the record.
> > Normalize this by changing it to "xattr=(locked)".
> >
> > Please see the github issue
> > https://github.com/linux-audit/audit-kernel/issues/109
> >
> > Signed-off-by: Richard Guy Briggs <rgb@...hat.com>
> > ---
> >  security/integrity/evm/evm_secfs.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c
> > index 015aea8fdf1e..4171d174e9da 100644
> > --- a/security/integrity/evm/evm_secfs.c
> > +++ b/security/integrity/evm/evm_secfs.c
> > @@ -192,7 +192,8 @@ static ssize_t evm_write_xattrs(struct file *file, const char __user *buf,
> >         if (count > XATTR_NAME_MAX)
> >                 return -E2BIG;
> >
> > -       ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_INTEGRITY_EVM_XATTR);
> > +       ab = audit_log_start(audit_context(), GFP_KERNEL,
> > +                            AUDIT_INTEGRITY_EVM_XATTR);
> 
> This part is fine.
> 
> >         if (!ab)
> >                 return -ENOMEM;
> >
> > @@ -222,7 +223,7 @@ static ssize_t evm_write_xattrs(struct file *file, const char __user *buf,
> >                 inode_lock(inode);
> >                 err = simple_setattr(evm_xattrs, &newattrs);
> >                 inode_unlock(inode);
> > -               audit_log_format(ab, "locked");
> > +               audit_log_format(ab, "xattr=(locked)");
> 
> Two things come to mind:
> 
> * While we can clearly trust the string above, should we be logging
> the xattr field value as an untrusted string so it is consistent with
> how we record other xattr names?

That would be a question for Steve.

> * I'm not sure you can ever have parens in a xattr (I would hope not),
> but if we are going to use the xattr field, perhaps we should simply
> stick with the name as provided (".") so we don't ever run afoul of
> xattr names?  I'm curious to hear what the IMA/EVM folks think of
> this.

The legal xaddr names start with XATTR_SECURITY_PREFIX which is
"security." so there is no danger of collision with legal names, but I
suppose someone could try to use "(locked)" as a name which would look
identical but fail with a different res= number.  I think I prefer your
idea of printing the given value verbatim.

> paul moore

- RGB

--
Richard Guy Briggs <rgb@...hat.com>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635