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Date: Fri, 03 Apr 2020 00:18:06 +0530
From: "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
To: linuxppc-dev@...ts.ozlabs.org, Russell Currey <ruscur@...sell.cc>
Cc: ajd@...ux.ibm.com, dja@...ens.net, kernel-hardening@...ts.openwall.com,
npiggin@...il.com
Subject: Re: [PATCH v8 2/7] powerpc/kprobes: Mark newly allocated probes as RO
Naveen N. Rao wrote:
> Russell Currey wrote:
>> With CONFIG_STRICT_KERNEL_RWX=y and CONFIG_KPROBES=y, there will be one
>> W+X page at boot by default. This can be tested with
>> CONFIG_PPC_PTDUMP=y and CONFIG_PPC_DEBUG_WX=y set, and checking the
>> kernel log during boot.
>>
>> powerpc doesn't implement its own alloc() for kprobes like other
>> architectures do, but we couldn't immediately mark RO anyway since we do
>> a memcpy to the page we allocate later. After that, nothing should be
>> allowed to modify the page, and write permissions are removed well
>> before the kprobe is armed.
>>
>> The memcpy() would fail if >1 probes were allocated, so use
>> patch_instruction() instead which is safe for RO.
>>
>> Reviewed-by: Daniel Axtens <dja@...ens.net>
>> Signed-off-by: Russell Currey <ruscur@...sell.cc>
>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>> ---
>> arch/powerpc/kernel/kprobes.c | 17 +++++++++++++----
>> 1 file changed, 13 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
>> index 81efb605113e..fa4502b4de35 100644
>> --- a/arch/powerpc/kernel/kprobes.c
>> +++ b/arch/powerpc/kernel/kprobes.c
>> @@ -24,6 +24,8 @@
>> #include <asm/sstep.h>
>> #include <asm/sections.h>
>> #include <linux/uaccess.h>
>> +#include <linux/set_memory.h>
>> +#include <linux/vmalloc.h>
>>
>> DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
>> DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
>> @@ -102,6 +104,16 @@ kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
>> return addr;
>> }
>>
>> +void *alloc_insn_page(void)
>> +{
>> + void *page = vmalloc_exec(PAGE_SIZE);
>> +
>> + if (page)
>> + set_memory_ro((unsigned long)page, 1);
>> +
>> + return page;
>> +}
>> +
>
> This crashes for me with KPROBES_SANITY_TEST during the kretprobe test.
That isn't needed to reproduce this. After bootup, disabling optprobes
also shows the crash with kretprobes:
sysctl debug.kprobes-optimization=0
The problem happens to be with patch_instruction() in
arch_prepare_kprobe(). During boot, on kprobe init, we register a probe
on kretprobe_trampoline for use with kretprobes (see
arch_init_kprobes()). This results in an instruction slot being
allocated, and arch_prepare_kprobe() to be called for copying the
instruction (nop) at kretprobe_trampoline. patch_instruction() is
failing resulting in corrupt instruction which we try to emulate/single
step causing the crash.
- Naveen
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