Date: Tue, 4 Sep 2012 12:00:27 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-current@freebsd.org Cc: Konstantin Belousov <kostikbel@gmail.com>, Svatopluk Kraus <onwahe@gmail.com> Subject: Re: [patch] mmap() MAP_TEXT implementation (to use for shared libraries) Message-ID: <201209041200.27100.jhb@freebsd.org> In-Reply-To: <20120904130039.GX33100@deviant.kiev.zoral.com.ua> References: <CAFHCsPX6HrCXHA%2BS31Dz9QP8eiwbo21Vzju4K4paohciu2vPTw@mail.gmail.com> <CAFHCsPWBkU23kk-vnMoahMUBkyfbJXoH=jj=DTqwV520mGC5Fw@mail.gmail.com> <20120904130039.GX33100@deviant.kiev.zoral.com.ua>
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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@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)
> > >> @@ -199,7 +199,8 @@
> > >> 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)
> > @@ -442,5 +442,10 @@
> > */
> > 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)
> > >> @@ -1258,10 +1258,13 @@
> > >> 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;
> > >> @@ -1271,7 +1274,6 @@
> > >> return (error);
> > >> }
> > >> foff = *foffp;
> > >> - flags = *flagsp;
> > >> obj = vp->v_object;
> > >> if (vp->v_type == VREG) {
> > >> /*
> > >> @@ -1294,7 +1296,7 @@
> > >> return (error);
> > >> }
> > >> }
> > >> - if (locktype == LK_EXCLUSIVE) {
> > >> + if (writeable_shared) {
> > >> *writecounted = TRUE;
> > >> vnode_pager_update_writecount(obj, 0, objsize);
> > >> }
> > >> @@ -1337,6 +1339,14 @@
> > >> 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.
--
John Baldwin
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