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authorMichael Ellerman <mpe@ellerman.id.au>2019-04-16 23:59:02 +1000
committerMichael Ellerman <mpe@ellerman.id.au>2019-04-17 18:50:26 +1000
commit8adddf349fda0d3de2f6bb41ddf838cbf36a8ad2 (patch)
tree38ec4223a93318178fe7ce35b5e382a3441b7808 /arch
parentcf7cf6977f531acd5dfe55250d0ee8cbbb6f1ae8 (diff)
powerpc/mm/radix: Make Radix require HUGETLB_PAGE
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>
Diffstat (limited to 'arch')
-rw-r--r--arch/powerpc/configs/skiroot_defconfig1
-rw-r--r--arch/powerpc/platforms/Kconfig.cputype2
2 files changed, 2 insertions, 1 deletions
diff --git a/arch/powerpc/configs/skiroot_defconfig b/arch/powerpc/configs/skiroot_defconfig
index 5ba131c30f6b..1bcd468ab422 100644
--- a/arch/powerpc/configs/skiroot_defconfig
+++ b/arch/powerpc/configs/skiroot_defconfig
@@ -266,6 +266,7 @@ CONFIG_UDF_FS=m
CONFIG_MSDOS_FS=m
CONFIG_VFAT_FS=m
CONFIG_PROC_KCORE=y
+CONFIG_HUGETLBFS=y
# CONFIG_MISC_FILESYSTEMS is not set
# CONFIG_NETWORK_FILESYSTEMS is not set
CONFIG_NLS=y
diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
index 842b2c7e156a..50cd09b4e05d 100644
--- a/arch/powerpc/platforms/Kconfig.cputype
+++ b/arch/powerpc/platforms/Kconfig.cputype
@@ -324,7 +324,7 @@ 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