Date: Sat, 02 Nov 2019 21:45:18 +1100 From: Michael Ellerman <mpe@...erman.id.au> To: Russell Currey <ruscur@...sell.cc>, linuxppc-dev@...ts.ozlabs.org Cc: Russell Currey <ruscur@...sell.cc>, christophe.leroy@....fr, joel@....id.au, ajd@...ux.ibm.com, dja@...ens.net, npiggin@...il.com, kernel-hardening@...ts.openwall.com Subject: Re: [PATCH v5 2/5] powerpc/kprobes: Mark newly allocated probes as RO Russell Currey <ruscur@...sell.cc> writes: > 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. > > Thus mark newly allocated probes as read-only once it's safe to do so. > > Signed-off-by: Russell Currey <ruscur@...sell.cc> > --- > arch/powerpc/kernel/kprobes.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c > index 2d27ec4feee4..2610496de7c7 100644 > --- a/arch/powerpc/kernel/kprobes.c > +++ b/arch/powerpc/kernel/kprobes.c > @@ -24,6 +24,7 @@ > #include <asm/sstep.h> > #include <asm/sections.h> > #include <linux/uaccess.h> > +#include <linux/set_memory.h> > > DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL; > DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk); > @@ -131,6 +132,8 @@ int arch_prepare_kprobe(struct kprobe *p) > (unsigned long)p->ainsn.insn + sizeof(kprobe_opcode_t)); > } > > + set_memory_ro((unsigned long)p->ainsn.insn, 1); > + That comes from: p->ainsn.insn = get_insn_slot(); Which ends up in __get_insn_slot() I think. And that looks very much like it's going to hand out multiple slots per page, which isn't going to work because you've just marked the whole page RO. So I would expect this to crash on the 2nd kprobe that's installed. Have you tested it somehow? I think this code should just use patch_instruction() rather than memcpy(). cheers
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