linux-kernel - Re: [PATCH v4 2/3] audit: add support for the openat2 syscall

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Date:   Wed, 9 Feb 2022 16:40:48 -0500
From:   Richard Guy Briggs <rgb@...hat.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Jeff Mahoney <jeffm@...e.com>,
        Linux-Audit Mailing List <linux-audit@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Eric Paris <eparis@...isplace.org>,
        Steve Grubb <sgrubb@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Eric Paris <eparis@...hat.com>, Tony Jones <tonyj@...e.de>
Subject: Re: [PATCH v4 2/3] audit: add support for the openat2 syscall

On 2022-02-09 10:57, Paul Moore wrote:
> On Tue, Feb 8, 2022 at 10:44 PM Jeff Mahoney <jeffm@...e.com> wrote:
> >
> > Hi Richard -
> >
> > On 5/19/21 16:00, Richard Guy Briggs wrote:
> > > The openat2(2) syscall was added in kernel v5.6 with commit fddb5d430ad9
> > > ("open: introduce openat2(2) syscall")
> > >
> > > Add the openat2(2) syscall to the audit syscall classifier.
> > >
> > > Link: https://github.com/linux-audit/audit-kernel/issues/67
> > > Signed-off-by: Richard Guy Briggs <rgb@...hat.com>
> > > Link: https://lore.kernel.org/r/f5f1a4d8699613f8c02ce762807228c841c2e26f.1621363275.git.rgb@redhat.com
> > > ---
> >
> > [...]
> >
> > > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> > > index d775ea16505b..3f59ab209dfd 100644
> > > --- a/kernel/auditsc.c
> > > +++ b/kernel/auditsc.c
> > > @@ -76,6 +76,7 @@
> > >  #include <linux/fsnotify_backend.h>
> > >  #include <uapi/linux/limits.h>
> > >  #include <uapi/linux/netfilter/nf_tables.h>
> > > +#include <uapi/linux/openat2.h>
> > >
> > >  #include "audit.h"
> > >
> > > @@ -196,6 +197,8 @@ static int audit_match_perm(struct audit_context *ctx, int mask)
> > >               return ((mask & AUDIT_PERM_WRITE) && ctx->argv[0] == SYS_BIND);
> > >       case AUDITSC_EXECVE:
> > >               return mask & AUDIT_PERM_EXEC;
> > > +     case AUDITSC_OPENAT2:
> > > +             return mask & ACC_MODE((u32)((struct open_how *)ctx->argv[2])->flags);
> > >       default:
> > >               return 0;
> > >       }
> >
> > ctx->argv[2] holds a userspace pointer and can't be dereferenced like this.
> >
> > I'm getting oopses, like so:
> > BUG: unable to handle page fault for address: 00007fff961bbe70
> 
> Thanks Jeff.
> 
> Yes, this is obviously the wrong thing to being doing; I remember
> checking to make sure we placed the audit_openat2_how() hook after the
> open_how was copied from userspace, but I missed the argv dereference
> in the syscall exit path when reviewing the code.
> 
> Richard, as we are already copying the open_how info into
> audit_context::openat2 safely, the obvious fix is to convert
> audit_match_perm() to use the previously copied value instead of argv.
> If you can't submit a patch for this today please let me know.

Agreed.  It would have been more awkward with the original order of the
patches.

The syscalls_file test in the audit-testsuite should have caught this.
https://github.com/rgbriggs/audit-testsuite/commit/1c99021ae27ea23eccce2bb1861df4c9c665cd5b
The test provided does essentially the same thing.

I should have a tested patch posted today.

> paul-moore.com

- 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