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

[Paul Moore]

On Wed, Feb 9, 2022 at 4:41 PM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
> On 2022-02-09 10:57, Paul Moore 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.
>
> 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 would have thought so, but I've now run this multiple times on both
affected and patched kernels but I don't see the page fault on my test
system.

Anyway, that test has now been merged with the audit-testsuite as well
as some cleanup on top to test for the new OPENAT2 record when
applicable.

-- 
paul-moore.com