Date:	Mon, 24 Aug 2015 10:46:38 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Doug Anderson <dianders@...omium.org>
Cc:	Aapo Vienamo <avienamo@...dia.com>,
	Nicolas Pitre <nico@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] arm: kgdb: patch_text() in kgdb_arch_set_breakpoint() may sleep

On Sun, Aug 23, 2015 at 7:45 PM, Doug Anderson <dianders@...omium.org> wrote:
> On Wed, Aug 5, 2015 at 8:50 AM, Aapo Vienamo <avienamo@...dia.com> wrote:
>> Hi,
>>
>> The breakpoint setting code in arch/arm/kernel/kgdb.c calls
>> patch_text(), which ends up trying to sleep while in interrupt context.
>> The bug was introduced by commit: 23a4e40 arm: kgdb: Handle read-only
>> text / modules. The resulting behavior is "BUG: scheduling while
>> atomic..." when setting a breakpoint in kgdb. This was tested on an
>> Nvidia Jetson TK1 board with 4.2.0-rc5-next-20150805 kernel.
>>
>> Regards,
>> Aapo Vienamo
>
> Aapo,
>
> Including the stack trace with this would have been helpful, though
> it's not too hard to reproduce.  Here it is:
>
> [  416.510559] BUG: scheduling while atomic: swapper/0/0/0x00010007
> [  416.516554] Modules linked in:
> [  416.519614] CPU: 0 PID: 0 Comm: swapper/0 Not tainted
> 4.2.0-rc7-00133-geb63b34 #1073
> [  416.527341] Hardware name: Rockchip (Device Tree)
> [  416.532042] [<c0017a4c>] (unwind_backtrace) from [<c00133d4>]
> (show_stack+0x20/0x24)
> [  416.539772] [<c00133d4>] (show_stack) from [<c05400e8>]
> (dump_stack+0x84/0xb8)
> [  416.546983] [<c05400e8>] (dump_stack) from [<c004913c>]
> (__schedule_bug+0x54/0x6c)
> [  416.554540] [<c004913c>] (__schedule_bug) from [<c054065c>]
> (__schedule+0x80/0x668)
> [  416.562183] [<c054065c>] (__schedule) from [<c0540cfc>] (schedule+0xb8/0xd4)
> [  416.569219] [<c0540cfc>] (schedule) from [<c0543a3c>]
> (schedule_timeout+0x2c/0x234)
> [  416.576861] [<c0543a3c>] (schedule_timeout) from [<c05417c0>]
> (wait_for_common+0xf4/0x188)
> [  416.585109] [<c05417c0>] (wait_for_common) from [<c0541874>]
> (wait_for_completion+0x20/0x24)
> [  416.593531] [<c0541874>] (wait_for_completion) from [<c00a0104>]
> (__stop_cpus+0x58/0x70)
> [  416.601608] [<c00a0104>] (__stop_cpus) from [<c00a0580>]
> (stop_cpus+0x3c/0x54)
> [  416.608817] [<c00a0580>] (stop_cpus) from [<c00a06c4>]
> (__stop_machine+0xcc/0xe8)
> [  416.616286] [<c00a06c4>] (__stop_machine) from [<c00a0714>]
> (stop_machine+0x34/0x44)
> [  416.624016] [<c00a0714>] (stop_machine) from [<c00173e8>]
> (patch_text+0x28/0x34)
> [  416.631399] [<c00173e8>] (patch_text) from [<c001733c>]
> (kgdb_arch_set_breakpoint+0x40/0x4c)
> [  416.639823] [<c001733c>] (kgdb_arch_set_breakpoint) from
> [<c00a0d68>] (kgdb_validate_break_address+0x2c/0x60)
> [  416.649719] [<c00a0d68>] (kgdb_validate_break_address) from
> [<c00a0e90>] (dbg_set_sw_break+0x1c/0xdc)
> [  416.658922] [<c00a0e90>] (dbg_set_sw_break) from [<c00a2e88>]
> (gdb_serial_stub+0x9c4/0xba4)
> [  416.667259] [<c00a2e88>] (gdb_serial_stub) from [<c00a11cc>]
> (kgdb_cpu_enter+0x1f8/0x60c)
> [  416.675423] [<c00a11cc>] (kgdb_cpu_enter) from [<c00a18cc>]
> (kgdb_handle_exception+0x19c/0x1d0)
> [  416.684106] [<c00a18cc>] (kgdb_handle_exception) from [<c0016f7c>]
> (kgdb_compiled_brk_fn+0x30/0x3c)
> [  416.693135] [<c0016f7c>] (kgdb_compiled_brk_fn) from [<c00091a4>]
> (do_undefinstr+0x1a4/0x20c)
> [  416.701643] [<c00091a4>] (do_undefinstr) from [<c001400c>]
> (__und_svc_finish+0x0/0x34)
> [  416.709543] Exception stack(0xc07c1ce8 to 0xc07c1d30)
> [  416.714584] 1ce0:                   00000000 c07c6504 c086e290
> c086e294 c086e294 c086e290
> [  416.722745] 1d00: c07c6504 00000067 00000001 c07c2100 00000027
> c07c1d4c c07c1d50 c07c1d30
> [  416.730905] 1d20: c00a0990 c00a08d0 60000193 ffffffff
> [  416.735947] [<c001400c>] (__und_svc_finish) from [<c00a08d0>]
> (kgdb_breakpoint+0x58/0x94)
> [  416.744110] [<c00a08d0>] (kgdb_breakpoint) from [<c00a0990>]
> (sysrq_handle_dbg+0x58/0x6c)
> [  416.752273] [<c00a0990>] (sysrq_handle_dbg) from [<c02c230c>]
> (__handle_sysrq+0xac/0x15c)
> [  416.760437] [<c02c230c>] (__handle_sysrq) from [<c02c23ec>]
> (handle_sysrq+0x30/0x34)
>
>
> Kees: I think you've dealt with a lot more of these types of issues
> than I have.  Any quick thoughts?  If not I can put it on my long-term
> list of things to do, but until then we could always just post a
> Revert...

I don't think a revert is in order here. CONFIG_DEBUG_RODATA could be
turned off for builds where you need kgdb while this bug gets found. I
don't actually see where we've gone wrong, though. Looks like
scheduling happened while waiting for CPUs to stop? Where did we enter
atomic?

Perhaps we need to test if we're already atomic in patch_text, and
only call stop_machine if we need to?

Untested (and likely mangled by gmail):

diff --git a/arch/arm/kernel/patch.c b/arch/arm/kernel/patch.c
index 69bda1a5707e..855696bfe072 100644
--- a/arch/arm/kernel/patch.c
+++ b/arch/arm/kernel/patch.c
@@ -124,5 +124,8 @@ void __kprobes patch_text(void *addr, unsigned int insn)
                .insn = insn,
        };

-       stop_machine(patch_text_stop_machine, &patch, NULL);
+       if (unlikely(in_atomic_preempt_off()))
+               patch_text_stop_machine(&patch);
+       else
+               stop_machine(patch_text_stop_machine, &patch, NULL);
 }


-Kees

-- 
Kees Cook
Chrome OS Security
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