linux-kernel - Re: [PATCH v2 10/10] powerpc/32s: Implement Kernel Userspace Access Protection

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Message-ID: <e297ba05-e82d-d269-20d2-5bd3914b1d14@c-s.fr>
Date:   Tue, 23 Apr 2019 11:26:39 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, ruscur@...sell.cc
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 10/10] powerpc/32s: Implement Kernel Userspace Access
 Protection



Le 18/04/2019 à 08:55, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@....fr> writes:
>> diff --git a/arch/powerpc/include/asm/book3s/32/kup.h b/arch/powerpc/include/asm/book3s/32/kup.h
>> index 5f97c742ca71..b3560b2de435 100644
>> --- a/arch/powerpc/include/asm/book3s/32/kup.h
>> +++ b/arch/powerpc/include/asm/book3s/32/kup.h
>> @@ -37,6 +37,113 @@
> ...
>> +
>> +static inline void allow_user_access(void __user *to, const void __user *from, u32 size)
>> +{
>> +	u32 addr = (__force u32)to;
>> +	u32 end = min(addr + size, TASK_SIZE);
>> +
>> +	if (!addr || addr >= TASK_SIZE || !size)
>> +		return;
>> +
>> +	current->thread.kuap = (addr & 0xf0000000) | ((((end - 1) >> 28) + 1) & 0xf);
>> +	kuap_update_sr(mfsrin(addr) & ~SR_KS, addr, end);	/* Clear Ks */
>> +}
> 
> When rebasing on my v6 I changed the above to:
> 
> static inline void allow_user_access(void __user *to, const void __user *from, u32 size)
> {
> 	u32 addr, end;
> 
> 	if (__builtin_constant_p(to) && to == NULL)
> 		return;

Good point, it avoids keeping the test compiled in when the 'to' is not 
NULL.

> 
> 	addr = (__force u32)to;
> 
> 	if (!addr || addr >= TASK_SIZE || !size)
> 		return;

Then the !addr test isn't needed anymore I think.

Christophe

> 
> 	end = min(addr + size, TASK_SIZE);
> 	current->thread.kuap = (addr & 0xf0000000) | ((((end - 1) >> 28) + 1) & 0xf);
> 	kuap_update_sr(mfsrin(addr) & ~SR_KS, addr, end);	/* Clear Ks */
> }
> 
> Which I think achieves the same result. It does boot :)
> 
>> +
>> +static inline void prevent_user_access(void __user *to, const void __user *from, u32 size)
>> +{
>> +	u32 addr = (__force u32)to;
>> +	u32 end = min(addr + size, TASK_SIZE);
>> +
>> +	if (!addr || addr >= TASK_SIZE || !size)
>> +		return;
>> +
>> +	current->thread.kuap = 0;
>> +	kuap_update_sr(mfsrin(addr) | SR_KS, addr, end);	/* set Ks */
>> +}
>> +
>> +static inline void allow_read_from_user(const void __user *from, unsigned long size)
>> +{
>> +}
> 
> And I dropped that.
> 
> cheers
>