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

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Date:   Wed, 9 Feb 2022 10:57:15 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Jeff Mahoney <jeffm@...e.com>, Richard Guy Briggs <rgb@...hat.com>
Cc:     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 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.

-- 
paul-moore.com