Re: [PATCH] LSM: Convert security_hook_heads into explicit array of struct list_head

[Tetsuo Handa]

Kees Cook wrote:
> On Sat, May 27, 2017 at 4:17 AM, Tetsuo Handa
> <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
> > Commit 3dfc9b02864b19f4 ("LSM: Initialize security_hook_heads upon
> > registration.") treats "struct security_hook_heads" as an implicit array
> > of "struct list_head" so that we can eliminate code for static
> > initialization. Although we haven't encountered compilers which do not
> > treat sizeof(security_hook_heads) != sizeof(struct list_head) *
> > (sizeof(security_hook_heads) / sizeof(struct list_head)), Casey does not
> > like the assumption that a structure of N elements can be assumed to be
> > the same as an array of N elements.
> >
> > Now that Kees found that randstruct complains such casting
> >
> >   security/security.c: In function 'security_init':
> >   security/security.c:59:20: note: found mismatched op0 struct pointer types: 'struct list_head' and 'struct security_hook_heads'
> >
> >     struct list_head *list = (struct list_head *) &security_hook_heads;
> >
> > and Christoph thinks that we should fix it rather than make randstruct
> > whitelist it, this patch fixes it.
> >
> > It would be possible to revert commit 3dfc9b02864b19f4, but this patch
> > converts security_hook_heads into an explicit array of struct list_head
> > by introducing an enum, due to reasons explained below.
> 
> Like Casey, I had confused this patch with the other(?) that resulted
> in dropped type checking. This just switches from named list_heads to
> indexed list_heads, which is fine now that the BUG_ON exists to
> sanity-check the index being used.

Casey, are you just confused as well?

> 
> > In MM subsystem, a sealable memory allocator patch was proposed, and
> > the LSM hooks ("struct security_hook_heads security_hook_heads" and
> > "struct security_hook_list ...[]") will benefit from this allocator via
> > protection using set_memory_ro()/set_memory_rw(), and that allocator
> > will remove CONFIG_SECURITY_WRITABLE_HOOKS config option. Thus, we will
> > likely be moving to that direction.
> 
> It's unlikely that smalloc will allow unsealing after initialization,
> so the SELinux disabling case will remain, IIUC.

LKM-based LSM modules will need it. Look at the result of a recent poll at
https://distrowatch.com/weekly.php?pollnumber=102&myaction=SeeVote&issue=20170522#poll .
We are still failing to provide users "a security module that individual user
can afford enabling". And we know that we cannot merge all security modules
into mainline. Thus, allowing LKM-based LSM modules is inevitable.

> > @@ -179,7 +182,8 @@ void __init security_add_hooks(struct security_hook_list *hooks, int count,
> >         do {                                                    \
> >                 struct security_hook_list *P;                   \
> >                                                                 \
> > -               list_for_each_entry(P, &security_hook_heads.FUNC, list) \
> > +               list_for_each_entry(P, &security_hook_heads     \
> > +                                   [LSM_##FUNC], list)         \
> 
> Can this be unsplit so the [...] remains next to security_hook_heads?

These are needed for passing 80 columns check by scripts/checkpatch.pl .
Should we ignore that warning or rename security_hook_heads to e.g. SHH ?

> Otherwise, yeah, I can be convinced to take this. :) Thanks for
> persisting with this, I think it makes sense now.

Thank you.