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

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Message-ID: <2006016.NXIvICiRTL@x2>
Date:   Tue, 26 Mar 2019 11:22:07 -0400
From:   Steve Grubb <sgrubb@...hat.com>
To:     Richard Guy Briggs <rgb@...hat.com>
Cc:     Paul Moore <paul@...l-moore.com>, 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>, 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 Wednesday, March 20, 2019 8:50:08 PM EDT Richard Guy Briggs wrote:
> 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.

All fields with the same name must be represented the same way. If one 
instance is untrusted, every instance of the same keyword must be untrusted.

-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