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Date: Tue, 07 Jan 2020 21:48:33 +1100
From: Michael Ellerman <mpe@...erman.id.au>
To: Christophe Leroy <christophe.leroy@....fr>, Russell Currey <ruscur@...sell.cc>
Cc: kernel-hardening@...ts.openwall.com, npiggin@...il.com, dja@...ens.net, ajd@...ux.ibm.com, joel@....id.au, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v6 3/5] powerpc/mm/ptdump: debugfs handler for W+X checks at runtime

Christophe Leroy <christophe.leroy@....fr> writes:
> Russell Currey <ruscur@...sell.cc> a écrit :
>
>> Very rudimentary, just
>>
>> 	echo 1 > [debugfs]/check_wx_pages
>>
>> and check the kernel log.  Useful for testing strict module RWX.
>
> For testing strict module RWX you could instead implement  
> module_arch_freeing_init() and call  ptdump_check_wx() from there.

That could get expensive on large systems, not sure if we want it
enabled by default?

cheers


>> diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
>> index 4e1d39847462..7c14c9728bc0 100644
>> --- a/arch/powerpc/Kconfig.debug
>> +++ b/arch/powerpc/Kconfig.debug
>> @@ -370,7 +370,7 @@ config PPC_PTDUMP
>>  	  If you are unsure, say N.
>>
>>  config PPC_DEBUG_WX
>> -	bool "Warn on W+X mappings at boot"
>> +	bool "Warn on W+X mappings at boot & enable manual checks at runtime"
>>  	depends on PPC_PTDUMP
>>  	help
>>  	  Generate a warning if any W+X mappings are found at boot.
>> @@ -384,7 +384,9 @@ config PPC_DEBUG_WX
>>  	  of other unfixed kernel bugs easier.
>>
>>  	  There is no runtime or memory usage effect of this option
>> -	  once the kernel has booted up - it's a one time check.
>> +	  once the kernel has booted up, it only automatically checks once.
>> +
>> +	  Enables the "check_wx_pages" debugfs entry for checking at runtime.
>>
>>  	  If in doubt, say "Y".
>>
>> diff --git a/arch/powerpc/mm/ptdump/ptdump.c  
>> b/arch/powerpc/mm/ptdump/ptdump.c
>> index 2f9ddc29c535..b6cba29ae4a0 100644
>> --- a/arch/powerpc/mm/ptdump/ptdump.c
>> +++ b/arch/powerpc/mm/ptdump/ptdump.c
>> @@ -4,7 +4,7 @@
>>   *
>>   * This traverses the kernel pagetables and dumps the
>>   * information about the used sections of memory to
>> - * /sys/kernel/debug/kernel_pagetables.
>> + * /sys/kernel/debug/kernel_page_tables.
>>   *
>>   * Derived from the arm64 implementation:
>>   * Copyright (c) 2014, The Linux Foundation, Laura Abbott.
>> @@ -409,6 +409,25 @@ void ptdump_check_wx(void)
>>  	else
>>  		pr_info("Checked W+X mappings: passed, no W+X pages found\n");
>>  }
>> +
>> +static int check_wx_debugfs_set(void *data, u64 val)
>> +{
>> +	if (val != 1ULL)
>> +		return -EINVAL;
>> +
>> +	ptdump_check_wx();
>> +
>> +	return 0;
>> +}
>> +
>> +DEFINE_SIMPLE_ATTRIBUTE(check_wx_fops, NULL, check_wx_debugfs_set,  
>> "%llu\n");
>> +
>> +static int ptdump_check_wx_init(void)
>> +{
>> +	return debugfs_create_file("check_wx_pages", 0200, NULL,
>> +				   NULL, &check_wx_fops) ? 0 : -ENOMEM;
>> +}
>> +device_initcall(ptdump_check_wx_init);
>>  #endif
>>
>>  static int ptdump_init(void)
>> --
>> 2.24.1

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