linux-kernel - Re: [PATCH] ARM: check __ex_table in do

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Message-ID: <20191107092404.GV25745@shell.armlinux.org.uk>
Date:   Thu, 7 Nov 2019 09:24:04 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Lvqiang <Lvqiang.Huang@...soc.com>
Cc:     ebiederm@...ssion.com, dave.hansen@...ux.intel.com,
        anshuman.khandual@....com, akpm@...ux-foundation.org,
        f.fainelli@...il.com, will@...nel.org, tglx@...utronix.de,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: check __ex_table in do_bad()

On Thu, Nov 07, 2019 at 03:45:13PM +0800, Lvqiang wrote:
> 
> We got many crashs in for_each_frame+0x18 arch/arm/lib/backtrace.S
>     1003: ldr r2, [sv_pc, #-4]
> 
> The backtrace is
>     dump_backtrace
>     show_stack
>     sched_show_task
>     show_state_filter
>     sysrq_handle_showstate_blocked
>     __handle_sysrq
>     write_sysrq_trigger
>     proc_reg_write
>     __vfs_write
>     vfs_write
>     sys_write
> 
> Related Kernel config
>     CONFIG_CPU_SW_DOMAIN_PAN=y
>     # CONFIG_ARM_UNWIND is not set
>     CONFIG_FRAME_POINTER=y
> 
> The task A was dumping the stack of an UN task B. However, the task B

What is "an UN task B"?

> scheduled to run on another CPU, which cause it stack content changed.
> Then, task A may hit a page domain fault and die().
>     [520.661314] Unhandled fault: page domain fault (0x01b) at 0x32848c02

So, the backtrace code is trying to access userspace.  It isn't supposed
to be accessing userspace - there are no guarantees that userspace will
be using frame pointers.  That is the bug.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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