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SubjectRe: [BUGFIX PATCH -tip] kprobes/x86: Fix to copy RIP relative instruction correctly
Hi Ingo,

Could you pick this fix to urgent branch?

Thank you,

On Fri, 24 Aug 2018 16:57:19 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> On Thu, 23 Aug 2018 21:41:09 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > On Fri, 24 Aug 2018 02:16:12 +0900
> > Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >
> > > Dump of assembler code from 0xffffffffa000207a to 0xffffffffa00020ea:
> > > 54 push %rsp
> > > ...
> > > 48 83 c4 08 add $0x8,%rsp
> > > 9d popfq
> > > 48 89 f0 mov %rsi,%rax
> > > 8b 35 82 7d db e2 mov -0x1d24827e(%rip),%esi
> > > # 0xffffffff82db9e67 <nr_cpu_ids+3>
> > >
> > > As it shows, the 2nd mov accesses *(nr_cpu_ids+3) instead of
> > > *nr_cpu_ids. This leads a kernel freeze because cpumask_next()
> > > always returns 0 and for_each_cpu() never ended.
> >
> > Ouch! Nice catch.
> >
> > >
> > > Fixing this by adding len correctly to real RIP address while
> > > copying.
> > >
> > > Fixes: 63fef14fc98a ("kprobes/x86: Make insn buffer always ROX and use text_poke()")
> > > Reported-by: Michael Rodin <michael@rodin.online>
> > > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> > > Cc: stable@vger.kernel.org
> > > ---
> > > arch/x86/kernel/kprobes/opt.c | 3 ++-
> > > 1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
> > > index eaf02f2e7300..e92672b8b490 100644
> > > --- a/arch/x86/kernel/kprobes/opt.c
> > > +++ b/arch/x86/kernel/kprobes/opt.c
> > > @@ -189,7 +189,8 @@ static int copy_optimized_instructions(u8 *dest, u8 *src, u8 *real)
> > > int len = 0, ret;
> > >
> > > while (len < RELATIVEJUMP_SIZE) {
> > > - ret = __copy_instruction(dest + len, src + len, real, &insn);
> > > + ret = __copy_instruction(dest + len, src + len, real + len,
> > > + &insn);
> > > if (!ret || !can_boost(&insn, src + len))
> > > return -EINVAL;
> > > len += ret;
> >
> > Looking at the change that broke this we have:
> >
> > > -static int copy_optimized_instructions(u8 *dest, u8 *src)
> > > +static int copy_optimized_instructions(u8 *dest, u8 *src, u8 *real)
> > > {
> > > struct insn insn;
> > > int len = 0, ret;
> > >
> > > while (len < RELATIVEJUMP_SIZE) {
> > > - ret = __copy_instruction(dest + len, src + len, &insn);
> > > + ret = __copy_instruction(dest + len, src + len, real, &insn);
> >
> > Where "real" was added as a parameter to __copy_instruction. Note that
> > we pass in "dest + len" but not "real + len" as you patch fixes.
> > __copy_instruction was changed by the bad commit with:
> >
> > > -int __copy_instruction(u8 *dest, u8 *src, struct insn *insn)
> > > +int __copy_instruction(u8 *dest, u8 *src, u8 *real, struct insn *insn)
> > > {
> > > kprobe_opcode_t buf[MAX_INSN_SIZE];
> > > unsigned long recovered_insn =
> > > @@ -387,11 +388,11 @@ int __copy_instruction(u8 *dest, u8 *src, struct insn *insn)
> > > * have given.
> > > */
> > > newdisp = (u8 *) src + (s64) insn->displacement.value
> > > - - (u8 *) dest;
> > > + - (u8 *) real;
> >
> > "real" replaces "dest", which was the first parameter to __copy_instruction.
> >
> > > return 0;
> >
> > And:
> >
> > > int arch_prepare_optimized_kprobe(struct optimized_kprobe *op,
> > > struct kprobe *__unused)
> > > {
> > > - u8 *buf;
> > > - int ret;
> > > + u8 *buf = NULL, *slot;
> > > + int ret, len;
> > > long rel;
> > >
> > > if (!can_optimize((unsigned long)op->kp.addr))
> > > return -EILSEQ;
> > >
> > > - op->optinsn.insn = get_optinsn_slot();
> > > - if (!op->optinsn.insn)
> > > + buf = kzalloc(MAX_OPTINSN_SIZE, GFP_KERNEL);
> > > + if (!buf)
> > > return -ENOMEM;
> > >
> > > + op->optinsn.insn = slot = get_optinsn_slot();
> > > + if (!slot) {
> > > + ret = -ENOMEM;
> > > + goto out;
> > > + }
> > > +
> > > /*
> > > * Verify if the address gap is in 2GB range, because this uses
> > > * a relative jump.
> > > */
> > > - rel = (long)op->optinsn.insn - (long)op->kp.addr + RELATIVEJUMP_SIZE;
> > > + rel = (long)slot - (long)op->kp.addr + RELATIVEJUMP_SIZE;
> > > if (abs(rel) > 0x7fffffff) {
> > > - __arch_remove_optimized_kprobe(op, 0);
> > > - return -ERANGE;
> > > + ret = -ERANGE;
> > > + goto err;
> > > }
> > >
> > > - buf = (u8 *)op->optinsn.insn;
> >
> > "slot" is equivalent to the old "buf".
> >
> > > - set_memory_rw((unsigned long)buf & PAGE_MASK, 1);
> > > + /* Copy arch-dep-instance from template */
> > > + memcpy(buf, &optprobe_template_entry, TMPL_END_IDX);
> > >
> > > /* Copy instructions into the out-of-line buffer */
> > > - ret = copy_optimized_instructions(buf + TMPL_END_IDX, op->kp.addr);
> > > - if (ret < 0) {
> > > - __arch_remove_optimized_kprobe(op, 0);
> > > - return ret;
> > > - }
> > > + ret = copy_optimized_instructions(buf + TMPL_END_IDX, op->kp.addr,
> > > + slot + TMPL_END_IDX);
> >
> > We pass in "real" as "slot + TMPL_END_IDX" and "dest" as "buf +
> > TMPL_END_IDX", thus to make it be equivalent to the code before this
> > commit, "real" should have "+ len" added to it in order to be
> > equivalent to what was there before.
>
> Right! The broken commit splits trampoline buffer into
> "temporary" destination buffer and "real" trampoline buffer,
> and use the "real" address for RIP-relative adjustment.
> However, I forgot to introduce update the "real" address
> in the copying loop.
>
> >
> > That said...
> >
> > Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
>
> Thanks!
>
>
> --
> Masami Hiramatsu <mhiramat@kernel.org>


--
Masami Hiramatsu <mhiramat@kernel.org>

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