Messages in this thread | ![/](/images/icornerl.gif) | | From | Andy Lutomirski <> | Date | Fri, 14 Aug 2020 09:42:11 -0700 | Subject | Re: [LTP] [x86/entry] 2bbc68f837: ltp.ptrace08.fail |
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On Fri, Aug 14, 2020 at 7:58 AM Cyril Hrubis <chrubis@suse.cz> wrote: > > Hi! > > > do_debug is a bit of a red herring here. ptrace should not be able to > > > put a breakpoint on a kernel address, period. I would just pick a > > > fixed address that's in the kernel text range or even just in the > > > pre-KASLR text range and make sure it gets rejected. Maybe try a few > > > different addresses for good measure. > > > > I've looked at the code and it seems like this would be a bit more > > complicated since the breakpoint is set by an accident in a race and the > > call still fails. Which is why the test triggers the breakpoint and > > causes infinite loop in the kernel... > > > > I guess that we could instead read back the address with > > PTRACE_PEEKUSER, so something as: > > > > > > break_addr = ptrace(PTRACE_PEEKUSER, child_pid, > > (void *)offsetof(struct user, u_debugreg[0]), > > NULL); > > > > if (break_addr == kernel_addr) > > tst_res(TFAIL, "ptrace() set break on a kernel address"); > > So this works actually nicely, even better than the original code. > > Any hints on how to select a fixed address in the kernel range as you > pointed out in one of the previous emails? I guess that this would end > up as a per-architecture mess of ifdefs if we wanted to hardcode it. >
It's fundamentally architecture dependent. Sane architectures like s390x don't even have this concept.
--Andy
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