On Tue, Apr 8, 2014 at 5:41 AM, Jon Medhurst (Tixy) <t...@linaro.org> wrote: > On Fri, 2014-04-04 at 17:07 -0700, Kees Cook wrote: >> On Fri, Apr 4, 2014 at 12:58 PM, Rabin Vincent <ra...@rab.in> wrote: > [...] >> > You need a TLB flush. I had a flush_tlb_all() in my example patch, >> > http://lists.infradead.org/pipermail/linux-arm-kernel/2014-April/244335.html, >> > but the following is probably nicer (on top of this patch): >> > >> > diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c >> > index 9bea524..a92c45a 100644 >> > --- a/arch/arm/mm/init.c >> > +++ b/arch/arm/mm/init.c >> > @@ -741,6 +741,8 @@ static inline bool arch_has_strict_perms(void) >> > addr += SECTION_SIZE) \ >> > section_update(addr, perms[i].mask, \ >> > perms[i].field); \ >> > + \ >> > + flush_tlb_kernel_range(perms[i].start, perms[i].end); \ >> > } \ >> > } >> > >> >> When I do this, I hang the system, and get a WARN due to the tlb call >> attempting to flush on all CPUs, I think: >> >> [ 34.246034] WARNING: at >> /mnt/host/source/src/third_party/kernel-next/kernel/smp.c:466 >> smp_call_function_many+0xac/0x26c() >> ... >> [ 34.246617] Backtrace: >> [ 34.246697] [<c010d3b8>] (unwind_backtrace+0x0/0x118) from >> [<c060b9d8>] (dump_stack+0x28/0x30) >> [ 34.246765] [<c060b9d8>] (dump_stack+0x28/0x30) from [<c0123044>] >> (warn_slowpath_null+0x44/0x5c) >> [ 34.246824] [<c0123044>] (warn_slowpath_null+0x44/0x5c) from >> [<c017426c>] (smp_call_function_many+0xac/0x26c) >> [ 34.246881] [<c017426c>] (smp_call_function_many+0xac/0x26c) from >> [<c0174468>] (smp_call_function+0x3c/0x48) >> [ 34.246937] [<c0174468>] (smp_call_function+0x3c/0x48) from >> [<c010c0fc>] (broadcast_tlb_a15_erratum+0x40/0x4c) >> [ 34.246994] [<c010c0fc>] (broadcast_tlb_a15_erratum+0x40/0x4c) from >> [<c010c590>] (flush_tlb_kernel_range+0x74/0xa0) >> [ 34.247046] [<c010c590>] (flush_tlb_kernel_range+0x74/0xa0) from >> [<c011403c>] (set_kernel_text_rw+0xd8/0xec) >> [ 34.247099] [<c011403c>] (set_kernel_text_rw+0xd8/0xec) from >> [<c010c878>] (__ftrace_modify_code+0x14/0x28) >> [ 34.247156] [<c010c878>] (__ftrace_modify_code+0x14/0x28) from >> [<c0184318>] (stop_machine_cpu_stop+0xc0/0x114) >> [ 34.247212] [<c0184318>] (stop_machine_cpu_stop+0xc0/0x114) from >> [<c01841cc>] (cpu_stopper_thread+0xd8/0x164) >> [ 34.247266] [<c01841cc>] (cpu_stopper_thread+0xd8/0x164) from >> [<c0145c14>] (kthread+0xc8/0xd8) >> [ 34.247323] [<c0145c14>] (kthread+0xc8/0xd8) from [<c0106118>] >> (ret_from_fork+0x14/0x20) >> >> Using local_flush_tlb_kernel_range() fixed it though. > > What about if another CPU had a TLB entry with the old permissions in? > Or do you consider that the likelihood and consequences of that aren't > significant?
The purpose of the function is to temporarily make text writable, do the write, and then restore read-only. Since only the writer needs to care about TLB state, this works fine. It's actually nice that only the current CPU can make text writes. -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
