Messages in this thread | | | Date | Fri, 23 Sep 2016 20:34:43 +0200 | From | Jann Horn <> | Subject | Re: [PATCH 09/12] x86/process: Pin the target stack in get_wchan() |
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On Fri, Sep 23, 2016 at 11:28:26AM -0700, Kees Cook wrote: > On Fri, Sep 23, 2016 at 12:43 AM, Jann Horn <jann@thejh.net> wrote: > > On Thu, Sep 22, 2016 at 03:44:37PM -0700, Andy Lutomirski wrote: > >> On Fri, Sep 16, 2016 at 7:00 PM, Jann Horn <jann@thejh.net> wrote: > >> > On Tue, Sep 13, 2016 at 02:29:29PM -0700, Andy Lutomirski wrote: > >> >> This will prevent a crash if get_wchan() runs after the task stack > >> >> is freed. > >> > > >> > I think I found some more stuff. Have a look at KSTK_EIP() and KSTK_ESP(), I think > >> > they read from the saved userspace registers area at the top of the kernel stack? > >> > > >> > Used on remote processes in: > >> > vma_is_stack_for_task() (via /proc/$pid/maps) > >> > >> This isn't used in /proc/$pid/maps -- it's only used in > >> /proc/$pid/task/$tid/maps. I wonder if anyone actually cares about it > >> -- it certainly won't work reliably. > >> > >> I could pin the stack in vma_is_stack_for_task, but it seems > >> potentially better to me to change it to vma_is_stack_for_current() > >> and remove the offending caller in /proc, replacing it with "return > >> 0". Thoughts? > > > > I just scrolled through the debian codesearch results for "\[stack\]" - > > there seem to only be 105 across all of debian's packages, many of them > > duplicates - and I didn't see any that looked like they used the tid map. > > So I think this might work. > > > > ( https://codesearch.debian.net/search?q=%22%5C%5Bstack%5C%5D%22 ) > > > > > >> > do_task_stat() (/proc/$pid/stat) > >> > >> Like this: > >> > >> mm = get_task_mm(task); > >> if (mm) { > >> vsize = task_vsize(mm); > >> if (permitted) { > >> eip = KSTK_EIP(task); > >> esp = KSTK_ESP(task); > >> } > >> } > >> > >> Can we just delete this outright? It seems somewhere between mostly > >> and entirely useless, and it also seems dangerous. Until very > >> recently, on x86_64, this would have been a potential info leak, as > >> SYSCALL followed closely by a hardware interrupt would cause *kernel* > >> values to land in task_pt_regs(). I don't even want to think about > >> what this code does if the task is in vm86 mode. I wouldn't be at all > >> surprised if non-x86 architectures have all kinds of interesting > >> thinks happen if you do this to a task that isn't running normal > >> non-atomic kernel code at the time. > >> > >> I would advocate for unconditionally returning zeros in these two stat fields. > > > > I'd like that a lot. > > > > I guess the two things that might theoretically use it are ptrace users > > and (very theoretically) sampling profiling stuff or so? > > > > In gdb, the only code I can find that reads this is in gdb/linux-nat.c, but > > it's behind an "#ifdef 0": > > > > #if 0 /* Don't know how architecture-dependent the rest is... > > Anyway the signal bitmap info is available from "status". */ > > if (fscanf (procfile, "%lu ", <mp) > 0) /* FIXME arch? */ > > printf_filtered (_("Kernel stack pointer: 0x%lx\n"), ltmp); > > if (fscanf (procfile, "%lu ", <mp) > 0) /* FIXME arch? */ > > printf_filtered (_("Kernel instr pointer: 0x%lx\n"), ltmp); > > [...] > > > > strace and ltrace don't seem to be using it. > > Does CRIU use this? I wouldn't expect so, since they're using ptrace, > IIUC, to freeze/restore.
As far as I can tell:
parse_pid_stat() parses them into a struct proc_pid_stat as "esp" and "eip", but those struct members are never used (like, probably, most other members of that struct).
child_opened_proc.c just opens /proc/%d/stat and then closes it again immediately.
So in summary: I don't think so. [unhandled content-type:application/pgp-signature] | |