linux-kernel - Re: [PATCH v1 1/5] powerpc/mm: sanity_check_fault() should work for all, not only BOOK3S

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Date:   Tue, 8 Sep 2020 10:56:29 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Nicholas Piggin <npiggin@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v1 1/5] powerpc/mm: sanity_check_fault() should work for
 all, not only BOOK3S



Le 08/09/2020 à 10:43, Nicholas Piggin a écrit :
> Excerpts from Christophe Leroy's message of August 7, 2020 3:15 am:
>> The verification and message introduced by commit 374f3f5979f9
>> ("powerpc/mm/hash: Handle user access of kernel address gracefully")
>> applies to all platforms, it should not be limited to BOOK3S.
>>
>> Make the BOOK3S version of sanity_check_fault() the one for all,
>> and bail out earlier if not BOOK3S.
>>
>> Fixes: 374f3f5979f9 ("powerpc/mm/hash: Handle user access of kernel address gracefully")
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>> ---
>>   arch/powerpc/mm/fault.c | 8 +++-----
>>   1 file changed, 3 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
>> index 925a7231abb3..2efa34d7e644 100644
>> --- a/arch/powerpc/mm/fault.c
>> +++ b/arch/powerpc/mm/fault.c
>> @@ -303,7 +303,6 @@ static inline void cmo_account_page_fault(void)
>>   static inline void cmo_account_page_fault(void) { }
>>   #endif /* CONFIG_PPC_SMLPAR */
>>   
>> -#ifdef CONFIG_PPC_BOOK3S
>>   static void sanity_check_fault(bool is_write, bool is_user,
>>   			       unsigned long error_code, unsigned long address)
>>   {
>> @@ -320,6 +319,9 @@ static void sanity_check_fault(bool is_write, bool is_user,
>>   		return;
>>   	}
>>   
>> +	if (!IS_ENABLED(CONFIG_PPC_BOOK3S))
>> +		return;
> 
> Seems okay. Why is address == -1 special though? I guess it's because
> it may not be an exploit kernel reference but a buggy pointer underflow?
> In that case -1 doesn't seem like it would catch very much. Would it be
> better to test for high bit set for example ((long)address < 0) ?

See 
https://github.com/linuxppc/linux/commit/0f9aee0cb9da7db7d96f63cfa2dc5e4f1bffeb87#diff-f9658f412252f3bb3093e0a95b37f3ac

-1 is what mmap() returns on error, if the app uses that as a pointer 
that's a programming error not an exploit.

Euh .. If you test (long)address < 0, then the entire kernel falls into 
that range as usually it goes from 0xc0000000 to 0xffffffff

But we could skip the top page entirely, anyway it is never mapped.

> 
> Anyway for your patch
> 
> Reviewed-by: Nicholas Piggin <npiggin@...il.com>

Thanks
Christophe