Re: [patch] mmap() MAP_TEXT implementation (to use for shared libraries)

From: Konstantin Belousov <kostikbel_at_gmail.com>
Date: Fri, 7 Sep 2012 19:02:08 +0300
On Fri, Sep 07, 2012 at 05:12:37PM +0200, Svatopluk Kraus wrote:
> On Tue, Sep 4, 2012 at 6:00 PM, John Baldwin <jhb_at_freebsd.org> wrote:
> > On Tuesday, September 04, 2012 9:00:39 am Konstantin Belousov wrote:
> >> On Tue, Sep 04, 2012 at 02:49:07PM +0200, Svatopluk Kraus wrote:
> >> > On Mon, Sep 3, 2012 at 2:46 PM, Konstantin Belousov <kostikbel_at_gmail.com>
> > wrote:
> >> > > On Mon, Sep 03, 2012 at 12:35:08PM +0200, Svatopluk Kraus wrote:
> >> > >> Hi,
> >> > >>
> >> > >>   I found out that while the running excecutables and a dynamic linker
> >> > >> are protected against writing (ETXTBSY), the loaded shared libraries
> >> > >> are not protected. The libraries are mapped by mmap() in dynamic
> >> > >> linker (rtld) and there is no way how to set VV_TEXT flag on the
> >> > >> libraries vnodes in mmap() code.
> >> > >>
> >> > >>   In linux compability code \compat\linux\linux_misc.c, linux_uselib()
> >> > >> sets VV_TEXT flags on a library vnode. In Solaris, MAP_TEXT flag
> >> > >> exists which informs mmap() that the mapped region will be used
> >> > >> primarily for executing instructions (for better MMU utilization).
> >> > >> With these on mind, I propose to implement MAP_TEXT option in mmap()
> >> > >> and in case that underlying object is a vnode, set VV_TEXT flag on it.
> >> > >>
> >> > >>   I already have implemented it and with rtld map_object() patch it
> >> > >> works fine for me (of course). The rtld patch looks easy, however I'm
> >> > >> not sure about mmap patch.
> >> > >>
> >> > >>   After some investigation, it looks that VV_TEXT once set on a vnode
> >> > >> remains set until last reference on the vnode is left. So, I don't
> >> > >> bother with VV_TEXT unset in munmap() to be consistent. The
> >> > >> executables and dynamic linker are activated in kernel, so VV_TEXT is
> >> > >> set before activation and cleared if something failed. Shared library
> >> > >> activation is done in dynamic linker (i.e., in userland). It's done in
> >> > >> steps and mmaping the library is one from them. So, I think that
> >> > >> VV_TEXT can be set in mmap() just after everything is finished
> >> > >> successfully.
> >> > > This is right, the object reference counter is also used as
> >> > > VV_TEXT counter. It is somewhat unaccurate, but in practice does
> >> > > not cause issues.
> >> > >
> >> > >>
> >> > >>   The patch itself is implemented in vm_mmap_vnode(). If I want to set
> >> > >> VV_TEXT flag on a vnode, I need an exclusive lock. In current code,
> >> > >> the exclusive lock flag is (mis)used as a flag for
> >> > >> vnode_pager_update_writecount() call. (I hope that I didn't miss
> >> > >> something.) So, the patch is bigger slightly.
> >> > >>
> >> > >>   I defined the MAP_TEXT flag in extented flags sections. However, I'm
> >> > >> feeling the relation to MAP_STACK flag, but not sure if and when
> >> > >> reserved flags (in other flags section) can be re-used.
> >> > >>
> >> > >>        Svata
> >> > >>
> >> > >>
> >> > >>   Index: libexec/rtld-elf/map_object.c
> >> > >> ===================================================================
> >> > >> --- libexec/rtld-elf/map_object.c     (revision 239770)
> >> > >> +++ libexec/rtld-elf/map_object.c     (working copy)
> >> > >> _at__at_ -199,7 +199,8 _at__at_
> >> > >>       data_prot = convert_prot(segs[i]->p_flags);
> >> > >>       data_flags = convert_flags(segs[i]->p_flags) | MAP_FIXED;
> >> > >>       if (mmap(data_addr, data_vlimit - data_vaddr, data_prot,
> >> > >> -       data_flags | MAP_PREFAULT_READ, fd, data_offset) == (caddr_t)
> > -1) {
> >> > >> +       data_flags | MAP_PREFAULT_READ | MAP_TEXT, fd, data_offset) ==
> >> > >> +         (caddr_t) -1) {
> >> > > I am not sure that we shall mark all segments mappings with MAP_TEXT.
> >> > > I understand the logic of the change, since we do not want data segment
> >> > > to be changed under us. Still, having MAP_TEXT for non-text segments
> > looks
> >> > > strange.
> >> > >
> >> >
> >> > I agree. However, only way how to recognize a text segment is an
> >> > executable flag set. The new patch for map_object.c is following:
> >> >
> >> > Index: libexec/rtld-elf/map_object.c
> >> > ===================================================================
> >> > --- libexec/rtld-elf/map_object.c   (revision 239770)
> >> > +++ libexec/rtld-elf/map_object.c   (working copy)
> >> > _at__at_ -442,5 +442,10 _at__at_
> >> >       */
> >> >      if (!(elfflags & PF_W))
> >> >     flags |= MAP_NOCORE;
> >> > +    /*
> >> > +     * Executable mappings are marked "MAP_TEXT".
> >> > +     */
> >> > +    if (elfflags & PF_X)
> >> > +   flags |= MAP_TEXT;
> >> >      return flags;
> >> >  }
> >> >
> >> >
> >> > >>           _rtld_error("%s: mmap of data failed: %s", path,
> >> > >>               rtld_strerror(errno));
> >> > >>           goto error1;
> >> > >> Index: sys/vm/vm_mmap.c
> >> > >> ===================================================================
> >> > >> --- sys/vm/vm_mmap.c  (revision 239770)
> >> > >> +++ sys/vm/vm_mmap.c  (working copy)
> >> > >> _at__at_ -1258,10 +1258,13 _at__at_
> >> > >>       struct mount *mp;
> >> > >>       struct ucred *cred;
> >> > >>       int error, flags, locktype, vfslocked;
> >> > >> +     int writeable_shared;
> >> > >>
> >> > >>       mp = vp->v_mount;
> >> > >>       cred = td->td_ucred;
> >> > >> -     if ((*maxprotp & VM_PROT_WRITE) && (*flagsp & MAP_SHARED))
> >> > >> +     flags = *flagsp;
> >> > >> +     writeable_shared = ((*maxprotp & VM_PROT_WRITE) && (flags &
> > MAP_SHARED));
> >> > >> +     if (writeable_shared || ((flags & MAP_TEXT) != 0))
> >> > >>               locktype = LK_EXCLUSIVE;
> >> > >>       else
> >> > >>               locktype = LK_SHARED;
> >> > >> _at__at_ -1271,7 +1274,6 _at__at_
> >> > >>               return (error);
> >> > >>       }
> >> > >>       foff = *foffp;
> >> > >> -     flags = *flagsp;
> >> > >>       obj = vp->v_object;
> >> > >>       if (vp->v_type == VREG) {
> >> > >>               /*
> >> > >> _at__at_ -1294,7 +1296,7 _at__at_
> >> > >>                               return (error);
> >> > >>                       }
> >> > >>               }
> >> > >> -             if (locktype == LK_EXCLUSIVE) {
> >> > >> +             if (writeable_shared) {
> >> > >>                       *writecounted = TRUE;
> >> > >>                       vnode_pager_update_writecount(obj, 0, objsize);
> >> > >>               }
> >> > >> _at__at_ -1337,6 +1339,14 _at__at_
> >> > >>               error = ENOMEM;
> >> > >>               goto done;
> >> > >>       }
> >> > >> +     /*
> >> > >> +      * If MAP_TEXT is announced, set VV_TEXT so no one can write
> >> > >> +      * to the executable.
> >> > >> +      */
> >> > >> +     if ((flags & MAP_TEXT) != 0) {
> >> > >> +             ASSERT_VOP_ELOCKED(vp, "vv_text");
> >> > >> +             vp->v_vflag |= VV_TEXT;
> >> > >> +     }
> >> > > I do not think we want to set VV_TEXT for device vnodes.
> >> > >
> >> >
> >> > I agree too. However, my patch doesn't set VV_TEXT for device vnodes.
> >> > Device vnodes never enter into patched part of code.
> >> Hm, yes.
> >>
> >> Anyway, after thinking about the patch more, I see two issues:
> >>
> >> 1. You are setting VV_TEXT without checking v_writecount. This basically
> >>    nullifies the main reason for the patch, since existing writer can still
> >>    write or truncate the shared library after the mapping.
> >>
> >> 2. I do not see what would prevent malicious local user from mmaping
> >>    arbitrary file readonly with MAP_TEXT, thus blocking any modifications
> >>    to the file. Note that this is not a problem for executables, because
> >>    kernel only sets VV_TEXT on executables if +x permission is set and
> >>    file is valid binary which kernel is able to execute.
> >>
> >>    E.g. you might block log writes with VV_TEXT, or other user editing
> >>    session or whatever, having just read access to corresponding files.
> >>
> >> Am I wrong ?
> >
> > Hmm, I do think 2) is a bit of a show-stopper.  I do wonder why one needs
> > MAP_TEXT at all or if you could key this off of mmap() with PROT_EXEC?
> > Do we require +x permissions for PROT_EXEC?  No, it seems we only require
> > a file opened with FREAD.  Hmm, perhaps rtld could open a separate fd for
> > PROT_EXEC mappings that used O_EXEC and mmap()'ing an O_EXEC fd could enable
> > VV_TEXT?  That would require a file to have +x permisson for an mmap() to
> > enable VV_TEXT.  It would also make MAP_TEXT unneeded.
> 
> It sounds good for me. I will try to patch it this way. However, do
> you think that will be acceptable to set +x permission to shared
> libraries in general?

Setting +x on shared libraries can be done. But setting VV_TEXT for
such mappings is definitely non-standard behaviour, that could cause
locking surprises for unaware system administrator. The issuw would be
very hard to diagnose.

Received on Fri Sep 07 2012 - 14:02:16 UTC

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