linux-kernel - Re: [PATCH 2/2] ARM: mm: make text and rodata read-only

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Message-ID: <1396960898.3567.55.camel@linaro1.home>
Date:	Tue, 08 Apr 2014 13:41:38 +0100
From:	"Jon Medhurst (Tixy)" <tixy@...aro.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	Rabin Vincent <rabin@....in>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Laura Abbott <lauraa@...eaurora.org>,
	Alexander Holler <holler@...oftware.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/2] ARM: mm: make text and rodata read-only

On Fri, 2014-04-04 at 17:07 -0700, Kees Cook wrote:
> On Fri, Apr 4, 2014 at 12:58 PM, Rabin Vincent <rabin@....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?

-- 
Tixy

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