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Date: Fri, 24 Aug 2018 16:57:19 +0900
From: Masami Hiramatsu <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Ingo Molnar <mingo@...hat.com>,
Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Michael Rodin <michael@...in.online>,
linux-kernel@...r.kernel.org
Subject: Re: [BUGFIX PATCH -tip] kprobes/x86: Fix to copy RIP relative
instruction correctly
On Thu, 23 Aug 2018 21:41:09 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:
> On Fri, 24 Aug 2018 02:16:12 +0900
> Masami Hiramatsu <mhiramat@...nel.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@...in.online>
> > Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> > Cc: stable@...r.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@...dmis.org>
Thanks!
--
Masami Hiramatsu <mhiramat@...nel.org>
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