Re: [PATCH] arm64: kaslr: Use standard early random function

[Guenter Roeck]

On 8/7/20 3:17 AM, Mark Rutland wrote:
> Hi Guenter,
> 
> On Thu, Aug 06, 2020 at 05:49:04PM -0700, Guenter Roeck wrote:
>> Commit 585524081ecd ("random: random.h should include archrandom.h, not
>> the other way around") tries to fix a problem with recursive inclusion
>> of linux/random.h and arch/archrandom.h for arm64. Unfortunately, this
>> results in the following compile error if ARCH_RANDOM is disabled.
>>
>> arch/arm64/kernel/kaslr.c: In function 'kaslr_early_init':
>> arch/arm64/kernel/kaslr.c:128:6: error: implicit declaration of function
>> '__early_cpu_has_rndr'; did you mean '__early_pfn_to_nid'?
>> [-Werror=implicit-function-declaration]
>>   if (__early_cpu_has_rndr()) {
>>       ^~~~~~~~~~~~~~~~~~~~
>>       __early_pfn_to_nid
>> arch/arm64/kernel/kaslr.c:131:7: error: implicit declaration of function
>> '__arm64_rndr' [-Werror=implicit-function-declaration]
>>    if (__arm64_rndr(&raw))
>>        ^~~~~~~~~~~~
>>
>> Problem is that arch/archrandom.h is only included from linux/random.h if
>> ARCH_RANDOM is enabled. If not, __arm64_rndr() and __early_cpu_has_rndr()
>> are undeclared, causing the problem.
>>
>> Use arch_get_random_seed_long_early() instead of arm64 specific functions
>> to solve the problem. As a side effect of this change, the code no longer
>> bypasses ARCH_RANDOM, which I consider desirable (after all, ARCH_RANDOM
>> was disabled for a reason).
> 
> There's no bypass of ARCH_RANDOM; the bits KASLR depends on are empty
> stubs when ARCH_RANDOM is not selected. I added the common early
> functions after this code was written.
> 
>> Reported-by: Qian Cai <cai@xxxxxx>
>> Fixes: 585524081ecd ("random: random.h should include archrandom.h, not the other way around")
> 
> This is where things broke; there was no need to change kaslr.c's
> include of <asm/archrandom.h>, since kaslr.c only depends on the RNDR
> bits defined there./
> 

Problem is that, prior to the above patch, asm/archrandom.h used to include
linux/random.h, which resulted in a recursion. That only worked because
some other include file had already included linux/random.h in kaslr.c.
In v5.7.y, with gcc 7.x and older, that include did not happen, causing
a compile failure.

In file included from ./arch/arm64/include/asm/archrandom.h:9:0,
                 from arch/arm64/kernel/kaslr.c:14:
./include/linux/random.h: In function ‘arch_get_random_seed_long_early’:
./include/linux/random.h:149:9: error: implicit declaration of function ‘arch_get_random_seed_long’

>> Fixes: 2e8e1ea88cbc ("arm64: Use v8.5-RNG entropy for KASLR seed")
> 
> I don't think this tag is necessary; this commit built and worked fine,
> and there wasn't any ARCH_RANDOM bypass to speak of.
> 
>> Cc: Qian Cai <cai@xxxxxx>
>> Cc: Mark Brown <broonie@xxxxxxxxxx>
>> Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> 
> This patch itself looks fine, but as above I think the commit message is
> misleading w.r.t. bypassing ARCH_RANDOM, and the second fixes tag isn't
> necessary.
> 

You are correct. I'll change that and resubmit.

Thanks,
Guenter

> With those bits gone:
> 
> Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
> 
> Mark.
> 
>> ---
>>  arch/arm64/kernel/kaslr.c | 12 +++++-------
>>  1 file changed, 5 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
>> index 9ded4237e1c1..b181e0544b79 100644
>> --- a/arch/arm64/kernel/kaslr.c
>> +++ b/arch/arm64/kernel/kaslr.c
>> @@ -84,6 +84,7 @@ u64 __init kaslr_early_init(u64 dt_phys)
>>  	void *fdt;
>>  	u64 seed, offset, mask, module_range;
>>  	const u8 *cmdline, *str;
>> +	unsigned long raw;
>>  	int size;
>>  
>>  	/*
>> @@ -122,15 +123,12 @@ u64 __init kaslr_early_init(u64 dt_phys)
>>  	}
>>  
>>  	/*
>> -	 * Mix in any entropy obtainable architecturally, open coded
>> -	 * since this runs extremely early.
>> +	 * Mix in any entropy obtainable architecturally if enabled
>> +	 * and supported.
>>  	 */
>> -	if (__early_cpu_has_rndr()) {
>> -		unsigned long raw;
>>  
>> -		if (__arm64_rndr(&raw))
>> -			seed ^= raw;
>> -	}
>> +	if (arch_get_random_seed_long_early(&raw))
>> +		seed ^= raw;
>>  
>>  	if (!seed) {
>>  		kaslr_status = KASLR_DISABLED_NO_SEED;
>> -- 
>> 2.17.1
>>