Subject: [PATCH v4 14/19] KVM: arm/arm64: Move HYP IO VAs to the "idmap" range

From: [email-masked] (Christoffer Dall)
Date: Fri, 16 Feb 2018 10:25:43 +0100

On Thu, Feb 15, 2018 at 01:52:05PM +0000, Marc Zyngier wrote:
> On 18/01/18 14:39, Christoffer Dall wrote:
> > On Thu, Jan 04, 2018 at 06:43:29PM +0000, Marc Zyngier wrote:
> >> We so far mapped our HYP IO (which is essencially the GICv2 control
> >> registers) using the same method as for memory. It recently appeared
> >> that is a bit unsafe:
> >>
> >> We compute the HYP VA using the kern_hyp_va helper, but that helper
> >> is only designed to deal with kernel VAs coming from the linear map,
> >> and not from the vmalloc region... This could in turn cause some bad
> >> aliasing between the two, amplified by the upcoming VA randomisation.
> >>
> >> A solution is to come up with our very own basic VA allocator for
> >> MMIO. Since half of the HYP address space only contains a single
> >> page (the idmap), we have plenty to borrow from. Let's use the idmap
> >> as a base, and allocate downwards from it. GICv2 now lives on the
> >> other side of the great VA barrier.
> >>
> >> Signed-off-by: Marc Zyngier <[email-masked]>
> >> ---
> >>  virt/kvm/arm/mmu.c | 56 +++++++++++++++++++++++++++++++++++++++++-------------
> >>  1 file changed, 43 insertions(+), 13 deletions(-)
> >>
> >> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> >> index 6192d45d1e1a..14c5e5534f2f 100644
> >> --- a/virt/kvm/arm/mmu.c
> >> +++ b/virt/kvm/arm/mmu.c
> >> @@ -43,6 +43,9 @@ static unsigned long hyp_idmap_start;
> >>  static unsigned long hyp_idmap_end;
> >>  static phys_addr_t hyp_idmap_vector;
> >>  
> >> +static DEFINE_MUTEX(io_map_lock);
> >> +static unsigned long io_map_base;
> >> +
> >>  #define S2_PGD_SIZE	(PTRS_PER_S2_PGD * sizeof(pgd_t))
> >>  #define hyp_pgd_order get_order(PTRS_PER_PGD * sizeof(pgd_t))
> >>  
> >> @@ -502,27 +505,31 @@ static void unmap_hyp_range(pgd_t *pgdp, phys_addr_t start, u64 size)
> >>   *
> >>   * Assumes hyp_pgd is a page table used strictly in Hyp-mode and
> >>   * therefore contains either mappings in the kernel memory area (above
> >> - * PAGE_OFFSET), or device mappings in the vmalloc range (from
> >> - * VMALLOC_START to VMALLOC_END).
> >> + * PAGE_OFFSET), or device mappings in the idmap range.
> >>   *
> >> - * boot_hyp_pgd should only map two pages for the init code.
> >> + * boot_hyp_pgd should only map the idmap range, and is only used in
> >> + * the extended idmap case.
> >>   */
> >>  void free_hyp_pgds(void)
> >>  {
> >> +	pgd_t *id_pgd;
> >> +
> >>  	mutex_lock(&kvm_hyp_pgd_mutex);
> >>  
> >> +	id_pgd = boot_hyp_pgd ? boot_hyp_pgd : hyp_pgd;
> >> +
> >> +	if (id_pgd)
> >> +		unmap_hyp_range(id_pgd, io_map_base,
> >> +				hyp_idmap_start + PAGE_SIZE - io_map_base);
> >> +
> >>  	if (boot_hyp_pgd) {
> >> -		unmap_hyp_range(boot_hyp_pgd, hyp_idmap_start, PAGE_SIZE);
> >>  		free_pages((unsigned long)boot_hyp_pgd, hyp_pgd_order);
> >>  		boot_hyp_pgd = NULL;
> >>  	}
> >>  
> >>  	if (hyp_pgd) {
> >> -		unmap_hyp_range(hyp_pgd, hyp_idmap_start, PAGE_SIZE);
> >>  		unmap_hyp_range(hyp_pgd, kern_hyp_va(PAGE_OFFSET),
> >>  				(uintptr_t)high_memory - PAGE_OFFSET);
> >> -		unmap_hyp_range(hyp_pgd, kern_hyp_va(VMALLOC_START),
> >> -				VMALLOC_END - VMALLOC_START);
> >>  
> >>  		free_pages((unsigned long)hyp_pgd, hyp_pgd_order);
> >>  		hyp_pgd = NULL;
> >> @@ -721,7 +728,8 @@ int create_hyp_io_mappings(phys_addr_t phys_addr, size_t size,
> >>  			   void __iomem **kaddr,
> >>  			   void __iomem **haddr)
> >>  {
> >> -	unsigned long start, end;
> >> +	pgd_t *pgd = hyp_pgd;
> >> +	unsigned long base;
> >>  	int ret;
> >>  
> >>  	*kaddr = ioremap(phys_addr, size);
> >> @@ -733,17 +741,38 @@ int create_hyp_io_mappings(phys_addr_t phys_addr, size_t size,
> >>  		return 0;
> >>  	}
> >>  
> >> +	mutex_lock(&io_map_lock);
> >> +
> >> +	base = io_map_base - size;
> > 
> > are we guaranteed that hyp_idmap_start (and therefore io_map_base) is
> > sufficiently greater than 0 ?  I suppose that even if RAM starts at 0,
> > and the kernel was loaded at 0, the idmap page for Hyp would be at some
> > reasonable offset from the start of the kernel image?
> 
> On my kernel image:
> ffff000008080000 t _head
> ffff000008cc6000 T __hyp_idmap_text_start
> ffff000009aaa000 B swapper_pg_end
> 
> _hyp_idmap_text_start is about 12MB from the beginning of the image, and
> about 14MB from the end. Yes, it is a big kernel. But we're only mapping
> a few pages there, even with my upcoming crazy vector remapping crap. So
> the likeliness of this failing is close to zero.
> 
> Now, close to zero is not necessarily close enough. What I could do is
> to switch the allocator around on failure, so that if we can't allocate
> on one side, we can at least try to allocate on the other side. I'm
> pretty sure we'll never trigger that code, but I can implement it if you
> think that's worth it.
> 

I don't think we should necessarily implement it, my main concern is
when reading the code, the reader has to concince himself/herself why
this is always safe (and not just very likely to be safe).  I'm fine
with adding a comment that explains this instead of implementing
complicated logic though.  What do you think?

> > 
> >> +	base &= ~(size - 1);
> > 
> > I'm not sure I understand this line.  Wouldn't it make more sense to use
> > PAGE_SIZE here?
> 
> This is trying to align the base of the allocation to its natural size
> (8kB aligned on an 8kB boundary, for example), which is what other
> allocators in the kernel do. I've now added a roundup_pow_of_two(size)
> so that we're guaranteed to deal with those.
> 

Ah right, it's just that I wasn't thinking of the size in terms of
always being page aligned, so couldn't you here attempt two 4K
allocations on a 64K page system, where one would overwrite the other?

Thanks,
-Christoffer