Re: [PATCH 0/4] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) [BZ #26831]

[Topi Miettinen]

On 4.11.2020 16.35, Catalin Marinas wrote:
> On Wed, Nov 04, 2020 at 11:55:57AM +0200, Topi Miettinen wrote:
> > On 4.11.2020 11.29, Florian Weimer wrote:
> > > * Will Deacon:
> > >
> > > > Is there real value in this seccomp filter if it only looks at mprotect(),
> > > > or was it just implemented because it's easy to do and sounds like a good
> > > > idea?
> > >
> > > It seems bogus to me.  Everyone will just create alias mappings instead,
> > > just like they did for the similar SELinux feature.  See “Example code
> > > to avoid execmem violations” in:
> > >
> > >     <https://www.akkadia.org/drepper/selinux-mem.html>
> [...]
> > > As you can see, this reference implementation creates a PROT_WRITE
> > > mapping aliased to a PROT_EXEC mapping, so it actually reduces security
> > > compared to something that generates the code in an anonymous mapping
> > > and calls mprotect to make it executable.
> [...]
> > If a service legitimately needs executable and writable mappings (due to
> > JIT, trampolines etc), it's easy to disable the filter whenever really
> > needed with "MemoryDenyWriteExecute=no" (which is the default) in case of
> > systemd or a TE rule like "allow type_t self:process { execmem };" for
> > SELinux. But this shouldn't be the default case, since there are many
> > services which don't need W&X.
>
> I think Drepper's point is that separate X and W mappings, with enough
> randomisation, would be more secure than allowing W&X at the same
> address (but, of course, less secure than not having W at all, though
> that's not always possible).
>
> > I'd also question what is the value of BTI if it can be easily circumvented
> > by removing PROT_BTI with mprotect()?
>
> Well, BTI is a protection against JOP attacks. The assumption here is
> that an attacker cannot invoke mprotect() to disable PROT_BTI. If it
> can, it's probably not worth bothering with a subsequent JOP attack, it
> can already call functions directly.

I suppose that the target for the attacker is to eventually perform 
system calls rather than looping forever in JOP/ROP gadgets.

> I see MDWX not as a way of detecting attacks but rather plugging
> inadvertent security holes in certain programs. On arm64, such hardening
> currently gets in the way of another hardening feature, BTI.

I don't think it has to get in the way at all. Why wouldn't something 
simple like this work:

diff --git a/elf/dl-load.c b/elf/dl-load.c
index 646c5dca40..12a74d15e8 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -1170,8 +1170,13 @@ _dl_map_object_from_fd (const char *name, const 
char *origname, int fd,
            c->prot |= PROT_READ;
          if (ph->p_flags & PF_W)
            c->prot |= PROT_WRITE;
-         if (ph->p_flags & PF_X)
+         if (ph->p_flags & PF_X) {
            c->prot |= PROT_EXEC;
+#ifdef PROT_BTI
+           if (GLRO(dl_bti) & 1)
+             c->prot |= PROT_BTI;
+#endif
+         }
 #endif
          break;

diff --git a/elf/dl-support.c b/elf/dl-support.c
index 7704c101c5..22c7cc7b81 100644
--- a/elf/dl-support.c
+++ b/elf/dl-support.c
@@ -222,7 +222,7 @@ __rtld_lock_define_initialized_recursive (, 
_dl_load_write_lock)


 #ifdef HAVE_AUX_VECTOR
-int _dl_clktck;
+int _dl_clktck, _dl_bti;

 void
 _dl_aux_init (ElfW(auxv_t) *av)
@@ -294,6 +294,11 @@ _dl_aux_init (ElfW(auxv_t) *av)
       case AT_RANDOM:
        _dl_random = (void *) av->a_un.a_val;
        break;
+#ifdef PROT_BTI
+      case AT_BTI:
+       _dl_bti = (void *) av->a_un.a_val;
+       break;
+#endif
       DL_PLATFORM_AUXV
       }
   if (seen == 0xf)

Kernel sets the aux vector to indicate that BTI should be enabled for 
all segments and main exe is already protected.

-Topi