powerpc/mm/radix: Make Radix require HUGETLB_PAGE

[Michael Ellerman]

On Wed, 2019-04-17 at 07:41:40 UTC, Michael Ellerman wrote:
> Joel reported weird crashes using skiroot_defconfig, in his case we
> jumped into an NX page:
> 
>   kernel tried to execute exec-protected page (c000000002bff4f0) - exploit attempt? (uid: 0)
>   BUG: Unable to handle kernel instruction fetch
>   Faulting instruction address: 0xc000000002bff4f0
> 
> Looking at the disassembly, we had simply branched to that address:
> 
>   c000000000c001bc  49fff335    bl     c000000002bff4f0
> 
> But that didn't match the original kernel image:
> 
>   c000000000c001bc  4bfff335    bl     c000000000bff4f0 <kobject_get+0x8>
> 
> When STRICT_KERNEL_RWX is enabled, and we're using the radix MMU, we
> call radix__change_memory_range() late in boot to change page
> protections. We do that both to mark rodata read only and also to mark
> init text no-execute. That involves walking the kernel page tables,
> and clearing _PAGE_WRITE or _PAGE_EXEC respectively.
> 
> With radix we may use hugepages for the linear mapping, so the code in
> radix__change_memory_range() uses eg. pmd_huge() to test if it has
> found a huge mapping, and if so it stops the page table walk and
> changes the PMD permissions.
> 
> However if the kernel is built without HUGETLBFS support, pmd_huge()
> is just a #define that always returns 0. That causes the code in
> radix__change_memory_range() to incorrectly interpret the PMD value as
> a pointer to a PTE page rather than as a PTE at the PMD level.
> 
> Unfortunately the combination of _PAGE_PTE and _PAGE_PRESENT in the
> high bits of the PMD entry give us 0xc in the top nibble which means
> the PMD entry happens to look like a valid pointer into the linear
> mapping.
> 
> We can see this using `dv` in xmon:
> 
>   0:mon> dv c000000000000000
>   pgd  @ 0xc000000001740000
>   pgdp @ 0xc000000001740000 = 0x80000000ffffb009
>   pudp @ 0xc0000000ffffb000 = 0x80000000ffffa009
>   pmdp @ 0xc0000000ffffa000 = 0xc00000000000018f	<- not a pointer
>   ptep @ 0xc000000000000100 = 0xa64bb17da64ab07d	<- kernel text
> 
> The end result is we treat the value at 0xc000000000000100 as a PTE
> and clear _PAGE_WRITE or _PAGE_EXEC, potentially corrupting the code
> at that address.
> 
> In Joel's specific case we cleared the sign bit in the offset of the
> branch, causing a backward branch to turn into a forward branch which
> caused us to branch into a non-executable page. However the exact
> nature of the crash depends on kernel version, compiler version, and
> other factors.
> 
> We need to fix radix__change_memory_range() to not use accessors that
> depend on HUGETLBFS, but we also have radix memory hotplug code that
> uses pmd_huge() etc that will also need fixing. So for now just
> disallow the broken combination of Radix with HUGETLBFS disabled.
> 
> The only defconfig we have that is affected is skiroot_defconfig, so
> turn on HUGETLBFS there so that it still gets Radix.
> 
> Fixes: 566ca99af026 ("powerpc/mm/radix: Add dummy radix_enabled()")
> Cc: stable at vger.kernel.org # v4.7+
> Reported-by: Joel Stanley <joel at jms.id.au>
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>

Applied to powerpc fixes.

https://git.kernel.org/powerpc/c/8adddf349fda0d3de2f6bb41ddf838cb

cheers