Re: [PATCH ghak109 V1] audit: link integrity evm_write_xattrs record to syscall event

Richard Guy Briggs
2019-03-21T00:50:20Z

On 2019-03-20 19:48, Paul Moore wrote:
> On Sat, Mar 16, 2019 at 8:10 AM Richard Guy Briggs <rgb@redhat.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@redhat.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@redhat.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