Re: [PATCH 0/2]: ARM: Enable THREAD_INFO_IN_TASK

Russell King (Oracle)
2021-09-02T16:54:54Z

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@amazon.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.

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