From: Michael Ellerman mpe@ellerman.id.au
commit 8adddf349fda0d3de2f6bb41ddf838cbf36a8ad2 upstream.
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.
We can see this using `dv` in xmon which also uses pmd_huge():
0:mon> dv c000000000000000 pgd @ 0xc000000001740000 pgdp @ 0xc000000001740000 = 0x80000000ffffb009 pudp @ 0xc0000000ffffb000 = 0x80000000ffffa009 pmdp @ 0xc0000000ffffa000 = 0xc00000000000018f <- this is a PTE 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@vger.kernel.org # v4.7+ Reported-by: Joel Stanley joel@jms.id.au Signed-off-by: Michael Ellerman mpe@ellerman.id.au Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/powerpc/configs/skiroot_defconfig | 1 + arch/powerpc/platforms/Kconfig.cputype | 2 +- 2 files changed, 2 insertions(+), 1 deletion(-)
--- a/arch/powerpc/configs/skiroot_defconfig +++ b/arch/powerpc/configs/skiroot_defconfig @@ -195,6 +195,7 @@ CONFIG_UDF_FS=m CONFIG_MSDOS_FS=m CONFIG_VFAT_FS=m CONFIG_PROC_KCORE=y +CONFIG_HUGETLBFS=y CONFIG_TMPFS=y CONFIG_TMPFS_POSIX_ACL=y # CONFIG_MISC_FILESYSTEMS is not set --- a/arch/powerpc/platforms/Kconfig.cputype +++ b/arch/powerpc/platforms/Kconfig.cputype @@ -330,7 +330,7 @@ config ARCH_ENABLE_SPLIT_PMD_PTLOCK
config PPC_RADIX_MMU bool "Radix MMU Support" - depends on PPC_BOOK3S_64 + depends on PPC_BOOK3S_64 && HUGETLB_PAGE select ARCH_HAS_GIGANTIC_PAGE if (MEMORY_ISOLATION && COMPACTION) || CMA default y help