linux-kernel - Re: [PATCH 0/2]: ARM: Enable THREAD_INFO_IN

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Date:   Thu, 2 Sep 2021 17:53:53 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Keith Packard <keithp@...thp.com>
Cc:     linux-kernel@...r.kernel.org, Abbott Liu <liuwenliang@...wei.com>,
        Alexander Sverdlin <alexander.sverdlin@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Hartley Sweeten <hsweeten@...ionengravers.com>,
        Jens Axboe <axboe@...nel.dk>, Jian Cai <jiancai@...gle.com>,
        Joe Perches <joe@...ches.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-arm-kernel@...ts.infradead.org,
        Maninder Singh <maninder1.s@...sung.com>,
        Manivannan Sadhasivam <mani@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nick Desaulniers <ndesaulniers@...ogle.com>,
        Nicolas Pitre <nico@...xnic.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Vaneet Narang <v.narang@...sung.com>,
        "Wolfram Sang (Renesas)" <wsa+renesas@...g-engineering.com>,
        YiFei Zhu <yifeifz2@...inois.edu>,
        Keith Packard <keithpac@...zon.com>
Subject: Re: [PATCH 0/2]: ARM: Enable THREAD_INFO_IN_TASK

On Thu, Sep 02, 2021 at 08:54:26AM -0700, Keith Packard wrote:
> Placing thread_info in the kernel stack leaves it vulnerable to stack
> overflow attacks. This short series addresses that by using the
> existing THREAD_INFO_IN_TASK infrastructure.
> 
> As this is my first patch in this part of the kernel, I'm looking for
> feedback about the general approach as well as specific comments on
> places where I've missed something.
> 
> I've only run this on armhf running under qemu, so while I've tried to
> make patches for other code paths, I haven't been able to test those.
> 
> (yes, I know checkpatch.pl complains about whitespace in asm-offsets.c, I
> decided to leave the existing whitespace alone)
> 
> Signed-off-by: Keith Packard <keithpac@...zon.com>

I think you're introducing a circular dependency with this for
certain kernel configurations:

E.g. Have you tried running this with CONFIG_CPU_V6 enabled?

+#define raw_smp_processor_id() this_cpu_read(cpu_number)
+#define __smp_processor_id() __this_cpu_read(cpu_number)
+
+DECLARE_PER_CPU_READ_MOSTLY(unsigned int, cpu_number);

this_cpu_read() is defined as:

#define this_cpu_read(pcp) __pcpu_size_call_return(this_cpu_read_, pcp)
(which will call this_cpu_read_4)

#define this_cpu_read_4(pcp)            this_cpu_generic_read(pcp)
=> __this_cpu_generic_read_nopreempt()
=>         ___ret = READ_ONCE(*raw_cpu_ptr(&(pcp)));

#define raw_cpu_ptr(ptr)                                                \
({                                                                      \
        __verify_pcpu_ptr(ptr);                                         \
        arch_raw_cpu_ptr(ptr);                                          \
})

#ifndef arch_raw_cpu_ptr
#define arch_raw_cpu_ptr(ptr) SHIFT_PERCPU_PTR(ptr, __my_cpu_offset)
#endif

#ifndef __my_cpu_offset
#define __my_cpu_offset per_cpu_offset(raw_smp_processor_id())
#endif

... which then leads back to a use of raw_smp_processor_id(), thereby
creating a circular loop of preprocessor definitions that the compiler
can't resolve.

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