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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: 566ca99af0 ("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>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
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In order to build the documentation, use ``make htmldocs`` or
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https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.