Subject: Re: [PATCH RFC 02/17] arm64: place kernel in its own L0 page table entry

From: "Russell King (Oracle)" <[email-masked]>
Date: Mon, 12 Jun 2023 16:04:58 +0100

On Mon, Jun 12, 2023 at 12:14:54PM +0100, Mark Rutland wrote:
> Hi Russell,
> 
> On Tue, May 30, 2023 at 03:04:40PM +0100, Russell King (Oracle) wrote:
> > Kernel text replication needs to maintain separate per-node page
> > tables for the kernel text. In order to do this without affecting
> > other kernel memory mappings, placing the kernel such that it does
> > not share a L0 page table entry with any other mapping is desirable.
> > 
> > Prior to this commit, the layout without KASLR was:
> > 
> > +----------+
> > |  vmalloc |
> > +----------+
> > |  Kernel  |
> > +----------+ MODULES_END, VMALLOC_START, KIMAGE_VADDR =
> > |  Modules |                 MODULES_VADDR + MODULES_VSIZE
> > +----------+ MODULES_VADDR = _PAGE_END(VA_BITS_MIN)
> > | VA space |
> > +----------+ 0
> > 
> > This becomes:
> > 
> > +----------+
> > |  vmalloc |
> > +----------+ VMALLOC_START = MODULES_END + PGDIR_SIZE
> > |  Kernel  |
> > +----------+ MODULES_END, KIMAGE_VADDR = _PAGE_END(VA_BITS_MIN) + PGDIR_SIZE
> > |  Modules |
> > +----------+ MODULES_VADDR = MODULES_END - MODULES_VSIZE
> > | VA space |
> > +----------+ 0
> 
> With KSASLR we may randomize the kernel and module space over a substantial
> portion of the vmalloc range. Are you expecting that text replication is going
> to restruct that range, or that we'd make it mutually exclusive with KASLR?

In the patch that adds the REPLICATE_KTEXT config option, I've made it
exclusive with RANDOMIZE_BASE, but this change in layout isn't dependent
on REPLICATE_KTEXT.

I've tested it with RANDOMIZE_BASE=y, and nothing seems to get upset,
so I believe that this patch doesn't cause any negative issues.

> I also note that the L0 table could have as few as two entries (with 16K pages
> and 4 levels). So either we'd need to also mess with an L1 table, or make text
> replication mutually exclusive with such configurations.

Ah, thanks for pointing that out - I was hoping to avoid needing
to touch anything but L0 tables.

However, it brings up a question: are there any NUMA systems that would
have just two entries in the L0 table? I suspect NUMA systems have lots
of RAM, and so would want a page table layout that results in multiple
L0 entries.

> > This assumes MODULES_VSIZE (128M) <= PGDIR_SIZE.
> 
> As a heads-up, we've just changed MODULES_VSIZE to be 2G in
> 
>   https://lore.kernel.org/linux-arm-kernel/[email-masked]/
> 
> .. which is queued in the arm64 for-next/module-alloc branch:
> 
>   https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/module-alloc

Ok - so I need to get a bit more clever about calculating MODULES_END
and KIMAGE_VADDR

> > One side effect of this change is that KIMAGE_VADDR's definition now
> > includes PGDIR_SIZE (to leave room for the modules) but this is not
> > defined when asm/memory.h is included. This means KIMAGE_VADDR can
> > not be used in inline functions within this file, so we convert
> > kaslr_offset() and kaslr_enabled() to be macros instead.
> 
> That series above also decoupled kaslr_enabled() from kaslr_offset(), 
> so we'd only need to change kaslr_offset().

Ok, I'll take a look to see how my changes are impacted.

> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index 4829abe017e9..baf74d0c43c9 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -478,7 +478,8 @@ void __init create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
> >  static void update_mapping_prot(phys_addr_t phys, unsigned long virt,
> >  				phys_addr_t size, pgprot_t prot)
> >  {
> > -	if ((virt >= PAGE_END) && (virt < VMALLOC_START)) {
> > +	if ((virt >= PAGE_END) && (virt < VMALLOC_START) &&
> > +	    !is_kernel(virt)) {
> >  		pr_warn("BUG: not updating mapping for %pa at 0x%016lx - outside kernel range\n",
> >  			&phys, virt);
> >  		return;
> 
> I think the existing conditions here aren't quite right, and have become bogus
> over time, and I don't think that the is_kernel() check is necessary here.
> 
> Originally, back in commit:
> 
>   c1cc1552616d0f35 ("arm64: MMU initialisation")
> 
> We had:
> 
> 	if (virt < VMALLOC_START) {
> 		pr_warning("BUG: not creating mapping for 0x%016llx at 0x%016lx - outside kernel range\n",
> 			   phys, virt);
> 		return;
> 	}
> 
> ... which checked that the VA range we were manipulating was in the TTBR1 VA
> range, as at the time, VMALLOC_START happened to be the lowest TTBR1 address.
> 
> That didn't substantially change until commit:
> 
>   14c127c957c1c607 ("arm64: mm: Flip kernel VA space")
> 
> ... when the test was changed to:
> 
> 	if ((virt >= VA_START) && (virt < VMALLOC_START)) {
> 		pr_warn("BUG: not creating mapping for %pa at 0x%016lx - outside kernel range\n",
> 			&phys, virt);
> 		return;
> 	}
> 
> Note: in that commit, VA_START was actually the end of the linear map (which
> was itself a the start of the TTBR1 address space), so this is just checking if
> we're poking a small portion of the TTBR1 address space, rather than if we're
> poking *outside* of the TTBR1 address space.
> 
> That doesn't make much sense, and I'm pretty sure that was a thinko rather than
> an intentional change of semantic.
> 
> I "fixed" that without thinking in commit:
> 
>   77ad4ce69321abbe ("arm64: memory: rename VA_START to PAGE_END")
> 
> ... making that:
> 
> 	if ((virt >= PAGE_END) && (virt < VMALLOC_START)) {
> 		pr_warn("BUG: not creating mapping for %pa at 0x%016lx - outside kernel range\n",
> 			&phys, virt);
> 		return;
> 	}
> 
> ... but clearly it has lost the original semantic and doesn't make much sense.
> 
> I think the test should actually be something like:
> 
> 	/* Must be a TTBR1 address */
> 	if (virt < PAGE_OFFSET ) {
> 		...
> 	}
> 
> ... and then we won't randomly trip for kernel mappings if those fall between
> the linear map and vmalloc range.

Okay, so that sounds like if this is fixed, then I won't need to patch
it! Yay!

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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