Re: [PATCH v2 1/4] vmalloc: New flags for safe vfree on special perms

Andy Lutomirski
2018-12-15T18:52:55Z

On Wed, Dec 12, 2018 at 2:01 PM Edgecombe, Rick P
<rick.p.edgecombe@intel.com> wrote:
>
> On Wed, 2018-12-12 at 11:57 -0800, Andy Lutomirski wrote:
> > On Wed, Dec 12, 2018 at 11:50 AM Edgecombe, Rick P
> > <rick.p.edgecombe@intel.com> wrote:
> > >
> > > On Tue, 2018-12-11 at 18:20 -0800, Andy Lutomirski wrote:
> > > > On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
> > > > <rick.p.edgecombe@intel.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@intel.com>
> > > > > Suggested-by: Andy Lutomirski <luto@kernel.org>
> > > > > Suggested-by: Will Deacon <will.deacon@arm.com>
> > > > > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.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
> > >
> > > Ok.
> > > >
> > > > > +{
> > > > > + 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_HAS_SPECIAL_PERMS is supposed to mean, like you said, "reset the direct
> > > map".
> > > Where VM_IMMEDIATE_UNMAP means, the vmalloc allocation has extra capabilties
> > > where we don't want to leave an enhanced capability TLB entry to the freed
> > > page.
> > >
> > > I was trying to pick names that could apply more generally for potential
> > > future
> > > special memory capabilities. Today VM_HAS_SPECIAL_PERMS does just mean reset
> > > write to the directmap and VM_IMMEDIATE_UNMAP means vmalloc mapping is
> > > executable.
> > >
> > > A present day reason for keeping both flags is, it is more efficient in the
> > > arch-agnostic implementation when freeing memory that is just RO and not
> > > executable. It saves a TLB flush.
> > >
> > > > (VM_IMMEDIATE_UNMAP is a bit of a lie, since, if in_interrupt(), it's
> > > > not immediate.)
> > >
> > > True, maybe VM_MUST_FLUSH or something else?
> > >
> > > > 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. */
> > > > set_memory_nx(...);
> > > > }
> > > >
> > > > set_memory_rw(addr, area->nr_pages);
> > > > }
> > >
> > > Ok.
> > >
> > > >
> > > > > +
> > > > > + /* Always actually remove the area */
> > > > > + remove_vm_area(area->addr);
> > > > > +
> > > > > + /*
> > > > > + * Need to flush the TLB before freeing pages in the case of
> > > > > this
> > > > > flag.
> > > > > + * As long as that's happening, unmap aliases.
> > > > > + *
> > > > > + * For 2 and 3, this will not be needed because of the
> > > > > set_memory_nx
> > > > > + * above, because the stale TLBs will be NX.
> > > >
> > > > I'm not sure I agree with this comment. If the caller asked for an
> > > > immediate unmap, we should give an immediate unmap. But I'm still not
> > > > sure I see why VM_IMMEDIATE_UNMAP needs to exist as a separate flag.
> > >
> > > Yea. I was just trying to save a TLB flush, since for today's callers that
> > > have
> > > set_memory there isn't a security downside I know of to just leaving it NX.
> > > Maybe its not worth the tradeoff of confusion? Or I can clarify that in the
> > > comment.
> >
> > Don't both of the users in your series set both flags, though? My
> > real objection to having them be separate is that, in the absence of
> > users, it's less clear exactly what they should do and the code
> > doesn't get exercised.
> The only "just RO" user today is one of the BPF allocations. I don't have a
> strong objection to combining them, just explaining the thinking. I guess if we
> could always add another flag later if it becomes more needed.
>
> > If you document that VM_IMMEDIATE_UNMAP means "I want the TLB entries
> > gone", then I can re-review the code in light of that. But then I'm
> > unconvinced by your generic implementation, since set_memory_nx()
> > seems like an odd way to go about it.
> Masami Hiramatsu pointed out if we don't do set_memory_nx before set_memory_rw,
> then there will be a small window of W^X violation. So that was the concern for
> the executable case, regardless of the semantics. I think the concern applies
> for any "special capability" permissions. Alternatively, if we remove_vm_area
> before we reset the direct map perms RW, maybe that would accomplish the same
> thing, if that's possible in a cross arch way. Maybe this is too much designing
> for hypothetical future... just was trying to avoid having to change the
> interface, and could just update the generic implementation if new permissions
> or usages come up.
>
> The set_memory_ stuff is really only needed for arm64 which seems to be the only
> other one with directmap permissions. So if it could eventually have its own
> arch_vunmap then all of the set_memory_ parts could be dropped and the default
> would just be the simple unmap then flush logic that it was originally.

I think that's probably the best solution. If there are only two
arches that have anything fancy here, let's just fix both of them up
for real.

>
> Or we have up to three flushes for the generic version and meet the name
> expectations and needed functionality today. I guess I'll just try that.
> > >
> > > > > + */
> > > > > + if (immediate && !IS_ENABLED(ARCH_HAS_SET_MEMORY))
> > > > > + vm_unmap_aliases();
> > > > > +}
> > > > > +
> > > > > static void __vunmap(const void *addr, int deallocate_pages)
> > > > > {
> > > > > struct vm_struct *area;
> > > > > @@ -1515,7 +1567,8 @@ static void __vunmap(const void *addr, int
> > > > > deallocate_pages)
> > > > > debug_check_no_locks_freed(area->addr, get_vm_area_size(area));
> > > > > debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
> > > > >
> > > > > - remove_vm_area(addr);
> > > > > + arch_vunmap(area, deallocate_pages);
> > > > > +
> > > > > if (deallocate_pages) {
> > > > > int i;
> > > > >
> > > > > @@ -1542,8 +1595,15 @@ static inline void __vfree_deferred(const void
> > > > > *addr)
> > > > > * nother cpu's list. schedule_work() should be fine with this
> > > > > too.
> > > > > */
> > > > > struct vfree_deferred *p = raw_cpu_ptr(&vfree_deferred);
> > > > > + struct vfree_work *w = kmalloc(sizeof(struct vfree_work),
> > > > > GFP_ATOMIC);
> > > > > +
> > > > > + /* If no memory for the deferred list node, give up */
> > > > > + if (!w)
> > > > > + return;
> > > >
> > > > That's nasty. I see what you're trying to do here, but I think you're
> > > > solving a problem that doesn't need solving quite so urgently. How
> > > > about dropping this part and replacing it with a comment like "NB:
> > > > this writes a word to a potentially executable address. It would be
> > > > nice if we could avoid doing this." And maybe a future patch could
> > > > more robustly avoid it without risking memory leaks.
> > >
> > > Yea, sorry I should have called this out, because I wasn't sure on how
> > > likely
> > > that was to happen. I did find some other places in the kernel with the same
> > > ignoring logic.
> > >
> > > I'll have to think though, I am not sure what the alternative is. Since the
> > > memory can be RO in the module_memfree case, the old method of re-using the
> > > allocation will no longer work. The list node could be stuffed on the
> > > vm_struct,
> > > but then the all of the spin_lock(&vmap_area_lock)'s need to be changed to
> > > work
> > > with interrupts so that the struct could be looked up. Not sure of the
> > > implications of that. Or maybe have some slow backup that resets the
> > > permissions
> > > and re-uses the allocation if kmalloc fails?
> > >
> > > I guess it could also go back to the old v1 implementation that doesn't
> > > handle
> > > RO and the directmap, and leave the W^X violation window during teardown.
> > > Then
> > > solve that problem when modules are loaded via something like Nadav's stuff.
> > >
> >
> > Hmm. Switching to spin_lock_irqsave() doesn't seem so bad to me.
> Ok.

Actually, I think I have a better solution. Just declare the
problematic case to be illegal: say that you may not free memory with
the new flags set while IRQs are off. Enforce this with a VM_WARN_ON
in the code that reads the vfree_deferred list.