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Date: Fri, 21 Dec 2018 19:57:20 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>, "luto@...nel.org"
	<luto@...nel.org>
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	"jannh@...gle.com" <jannh@...gle.com>, "Dock, Deneen T"
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	<netdev@...r.kernel.org>, "Hansen, Dave" <dave.hansen@...el.com>
Subject: Re: [PATCH v2 1/4] vmalloc: New flags for safe vfree on special
 perms

On Fri, 2018-12-21 at 18:25 +0100, Ard Biesheuvel wrote:
> On Fri, 21 Dec 2018 at 18:12, Andy Lutomirski <luto@...nel.org>
> wrote:
> > > On Dec 21, 2018, at 9:39 AM, Ard Biesheuvel <
> > > ard.biesheuvel@...aro.org> wrote:
> > > 
> > > > On Wed, 12 Dec 2018 at 03:20, Andy Lutomirski <luto@...nel.org>
> > > > wrote:
> > > > 
> > > > On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
> > > > <rick.p.edgecombe@...el.com> wrote:
> > > > > This adds two new flags VM_IMMEDIATE_UNMAP and
> > > > > VM_HAS_SPECIAL_PERMS, for
> > > > > enabling vfree operations to immediately clear executable TLB
> > > > > entries to freed
> > > > > pages, and handle freeing memory with special permissions.
> > > > > 
> > > > > In order to support vfree being called on memory that might
> > > > > be RO, the vfree
> > > > > deferred list node is moved to a kmalloc allocated struct,
> > > > > from where it is
> > > > > today, reusing the allocation being freed.
> > > > > 
> > > > > arch_vunmap is a new __weak function that implements the
> > > > > actual unmapping and
> > > > > resetting of the direct map permissions. It can be overridden
> > > > > by more efficient
> > > > > architecture specific implementations.
> > > > > 
> > > > > For the default implementation, it uses architecture agnostic
> > > > > methods which are
> > > > > equivalent to what most usages do before calling vfree. So
> > > > > now it is just
> > > > > centralized here.
> > > > > 
> > > > > This implementation derives from two sketches from Dave
> > > > > Hansen and Andy
> > > > > Lutomirski.
> > > > > 
> > > > > Suggested-by: Dave Hansen <dave.hansen@...el.com>
> > > > > Suggested-by: Andy Lutomirski <luto@...nel.org>
> > > > > Suggested-by: Will Deacon <will.deacon@....com>
> > > > > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> > > > > ---
> > > > > include/linux/vmalloc.h |  2 ++
> > > > > mm/vmalloc.c            | 73
> > > > > +++++++++++++++++++++++++++++++++++++----
> > > > > 2 files changed, 69 insertions(+), 6 deletions(-)
> > > > > 
> > > > > diff --git a/include/linux/vmalloc.h
> > > > > b/include/linux/vmalloc.h
> > > > > index 398e9c95cd61..872bcde17aca 100644
> > > > > --- a/include/linux/vmalloc.h
> > > > > +++ b/include/linux/vmalloc.h
> > > > > @@ -21,6 +21,8 @@ struct notifier_block;                /* in
> > > > > notifier.h */
> > > > > #define VM_UNINITIALIZED       0x00000020      /* vm_struct
> > > > > is not fully initialized */
> > > > > #define VM_NO_GUARD            0x00000040      /* don't add
> > > > > guard page */
> > > > > #define VM_KASAN               0x00000080      /* has
> > > > > allocated kasan shadow memory */
> > > > > +#define VM_IMMEDIATE_UNMAP     0x00000200      /* flush
> > > > > before releasing pages */
> > > > > +#define VM_HAS_SPECIAL_PERMS   0x00000400      /* may be
> > > > > freed with special perms */
> > > > > /* bits [20..32] reserved for arch specific ioremap internals
> > > > > */
> > > > > 
> > > > > /*
> > > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > > index 97d4b25d0373..02b284d2245a 100644
> > > > > --- a/mm/vmalloc.c
> > > > > +++ b/mm/vmalloc.c
> > > > > @@ -18,6 +18,7 @@
> > > > > #include <linux/interrupt.h>
> > > > > #include <linux/proc_fs.h>
> > > > > #include <linux/seq_file.h>
> > > > > +#include <linux/set_memory.h>
> > > > > #include <linux/debugobjects.h>
> > > > > #include <linux/kallsyms.h>
> > > > > #include <linux/list.h>
> > > > > @@ -38,6 +39,11 @@
> > > > > 
> > > > > #include "internal.h"
> > > > > 
> > > > > +struct vfree_work {
> > > > > +       struct llist_node node;
> > > > > +       void *addr;
> > > > > +};
> > > > > +
> > > > > struct vfree_deferred {
> > > > >        struct llist_head list;
> > > > >        struct work_struct wq;
> > > > > @@ -50,9 +56,13 @@ static void free_work(struct work_struct
> > > > > *w)
> > > > > {
> > > > >        struct vfree_deferred *p = container_of(w, struct
> > > > > vfree_deferred, wq);
> > > > >        struct llist_node *t, *llnode;
> > > > > +       struct vfree_work *cur;
> > > > > 
> > > > > -       llist_for_each_safe(llnode, t, llist_del_all(&p-
> > > > > >list))
> > > > > -               __vunmap((void *)llnode, 1);
> > > > > +       llist_for_each_safe(llnode, t, llist_del_all(&p-
> > > > > >list)) {
> > > > > +               cur = container_of(llnode, struct vfree_work,
> > > > > node);
> > > > > +               __vunmap(cur->addr, 1);
> > > > > +               kfree(cur);
> > > > > +       }
> > > > > }
> > > > > 
> > > > > /*** Page table manipulation functions ***/
> > > > > @@ -1494,6 +1504,48 @@ struct vm_struct *remove_vm_area(const
> > > > > void *addr)
> > > > >        return NULL;
> > > > > }
> > > > > 
> > > > > +/*
> > > > > + * This function handles unmapping and resetting the direct
> > > > > map as efficiently
> > > > > + * as it can with cross arch functions. The three categories
> > > > > of architectures
> > > > > + * are:
> > > > > + *   1. Architectures with no set_memory implementations and
> > > > > no direct map
> > > > > + *      permissions.
> > > > > + *   2. Architectures with set_memory implementations but no
> > > > > direct map
> > > > > + *      permissions
> > > > > + *   3. Architectures with set_memory implementations and
> > > > > direct map permissions
> > > > > + */
> > > > > +void __weak arch_vunmap(struct vm_struct *area, int
> > > > > deallocate_pages)
> > > > 
> > > > My general preference is to avoid __weak functions -- they
> > > > don't
> > > > optimize well.  Instead, I prefer either:
> > > > 
> > > > #ifndef arch_vunmap
> > > > void arch_vunmap(...);
> > > > #endif
> > > > 
> > > > or
> > > > 
> > > > #ifdef CONFIG_HAVE_ARCH_VUNMAP
> > > > ...
> > > > #endif
> > > > 
> > > > 
> > > > > +{
> > > > > +       unsigned long addr = (unsigned long)area->addr;
> > > > > +       int immediate = area->flags & VM_IMMEDIATE_UNMAP;
> > > > > +       int special = area->flags & VM_HAS_SPECIAL_PERMS;
> > > > > +
> > > > > +       /*
> > > > > +        * In case of 2 and 3, use this general way of
> > > > > resetting the permissions
> > > > > +        * on the directmap. Do NX before RW, in case of X,
> > > > > so there is no W^X
> > > > > +        * violation window.
> > > > > +        *
> > > > > +        * For case 1 these will be noops.
> > > > > +        */
> > > > > +       if (immediate)
> > > > > +               set_memory_nx(addr, area->nr_pages);
> > > > > +       if (deallocate_pages && special)
> > > > > +               set_memory_rw(addr, area->nr_pages);
> > > > 
> > > > Can you elaborate on the intent here?  VM_IMMEDIATE_UNMAP means
> > > > "I
> > > > want that alias gone before any deallocation happens".
> > > > VM_HAS_SPECIAL_PERMS means "I mucked with the direct map -- fix
> > > > it for
> > > > me, please".  deallocate means "this was vfree -- please free
> > > > the
> > > > pages".  I'm not convinced that all the various combinations
> > > > make
> > > > sense.  Do we really need both flags?
> > > > 
> > > > (VM_IMMEDIATE_UNMAP is a bit of a lie, since, if
> > > > in_interrupt(), it's
> > > > not immediate.)
> > > > 
> > > > If we do keep both flags, maybe some restructuring would make
> > > > sense,
> > > > like this, perhaps.  Sorry about horrible whitespace damage.
> > > > 
> > > > if (special) {
> > > >  /* VM_HAS_SPECIAL_PERMS makes little sense without
> > > > deallocate_pages. */
> > > >  WARN_ON_ONCE(!deallocate_pages);
> > > > 
> > > >  if (immediate) {
> > > >    /* It's possible that the vmap alias is X and we're about to
> > > > make
> > > > the direct map RW.  To avoid a window where executable memory
> > > > is
> > > > writable, first mark the vmap alias NX.  This is silly, since
> > > > we're
> > > > about to *unmap* it, but this is the best we can do if all we
> > > > have to
> > > > work with is the set_memory_abc() APIs.  Architectures should
> > > > override
> > > > this whole function to get better behavior. */
> > > 
> > > So can't we fix this first? Assuming that architectures that
> > > bother to
> > > implement them will not have executable mappings in the linear
> > > region,
> > > all we'd need is set_linear_range_ro/rw() routines that default
> > > to
> > > doing nothing, and encapsulate the existing code for x86 and
> > > arm64.
> > > That way, we can handle do things in the proper order, i.e.,
> > > release
> > > the vmalloc mapping (without caring about the permissions),
> > > restore
> > > the linear alias attributes, and finally release the pages.
> > 
> > Seems reasonable, except that I think it should be
> > set_linear_range_not_present() and set_linear_range_rw(), for three
> > reasons:
> > 
> > 1. It’s not at all clear to me that we need to keep the linear
> > mapping
> > around for modules.
> > 
> 
> I'm pretty sure hibernate on arm64 will have to be fixed, since it
> expects to be able to read all valid pages via the linear map. But we
> can fix that.
Hmm, now I wonder what else might be trying to access the entire direct
map for some reason. Since the window of not present is so small,
issues could lurk for some time. I guess that should show up with XPFO
too though.

> > 2. At least on x86, the obvious algorithm to do the free operation
> > with a single flush requires it.  Someone should probably confirm
> > that
> > arm’s TLB works the same way, i.e. that no flush is needed when
> > changing from not-present (or whatever ARM calls it) to RW.
> > 
> 
> Good point. ARM is similar in this regard, although we'll probably
> clear the access flag rather than unmap the page entirely (which is
> treated the same way in terms of required TLB management)
How about set_alias_nv(not valid)/set_alias_default for the name? It
can cover the general behavior of not cacheable in the TLB.

Also, FYI for anyone that is following this - Nadav and I have
discussed merging this with the text poke patchset because of the
overlap. With the US holidays, I may not get this done and tested until
first week of January. I'll go back and make the efficient direct map
permissions part arch generic now too.

> > 3. Anyone playing with XPFO wants this facility anyway.  In fact,
> > with
> > this change, Rick’s series will more or less implement XPFO for
> > vmalloc memory :)
> > 
> > Does that seem reasonable to you?
> 
> Absolutely.

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