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

[Paul Moore]

On Wed, Feb 9, 2022 at 10:57 AM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
> On Tue, Feb 8, 2022 at 10:44 PM Jeff Mahoney <jeffm@xxxxxxxx> 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@xxxxxxxxxx>
> > > Link: https://lore.kernel.org/r/f5f1a4d8699613f8c02ce762807228c841c2e26f.1621363275.git.rgb@xxxxxxxxxx
> > > ---
> >
> > [...]
> >
> > > 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.

I haven't heard anything from Richard so I put together a patch which
should fix the problem (link below).  It's currently untested, but
I've got a kernel building now with the patch ...

https://lore.kernel.org/linux-audit/164444111699.153511.15656610495968926251.stgit@olly/T/#u

-- 
paul-moore.com