Re: [PATCH v10 05/10] powerpc/bpf: Write protect JIT code

From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Michael Ellerman <mpe@ellerman.id.au>,
	Jordan Niethe <jniethe5@gmail.com>,
	linuxppc-dev@lists.ozlabs.org
Cc: naveen.n.rao@linux.ibm.com, cmr@codefail.de, ajd@linux.ibm.com,
	npiggin@gmail.com, dja@axtens.net
Subject: Re: [PATCH v10 05/10] powerpc/bpf: Write protect JIT code
Date: Wed, 31 Mar 2021 12:39:44 +0200	[thread overview]
Message-ID: <9034d909-170f-7e9c-02a2-e329d613ed7c@csgroup.eu> (raw)
In-Reply-To: <87wntnwqw9.fsf@mpe.ellerman.id.au>

Le 31/03/2021 à 12:37, Michael Ellerman a écrit :
> Jordan Niethe <jniethe5@gmail.com> writes:
> 
>> Once CONFIG_STRICT_MODULE_RWX is enabled there will be no need to
>> override bpf_jit_free() because it is now possible to set images
>> read-only. So use the default implementation.
>>
>> Also add the necessary call to bpf_jit_binary_lock_ro() which will
>> remove write protection and add exec protection to the JIT image after
>> it has finished being written.
>>
>> Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
>> ---
>> v10: New to series
>> ---
>>   arch/powerpc/net/bpf_jit_comp.c   | 5 ++++-
>>   arch/powerpc/net/bpf_jit_comp64.c | 4 ++++
>>   2 files changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
>> index e809cb5a1631..8015e4a7d2d4 100644
>> --- a/arch/powerpc/net/bpf_jit_comp.c
>> +++ b/arch/powerpc/net/bpf_jit_comp.c
>> @@ -659,12 +659,15 @@ void bpf_jit_compile(struct bpf_prog *fp)
>>   		bpf_jit_dump(flen, proglen, pass, code_base);
>>   
>>   	bpf_flush_icache(code_base, code_base + (proglen/4));
>> -
>>   #ifdef CONFIG_PPC64
>>   	/* Function descriptor nastiness: Address + TOC */
>>   	((u64 *)image)[0] = (u64)code_base;
>>   	((u64 *)image)[1] = local_paca->kernel_toc;
>>   #endif
>> +	if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX)) {
>> +		set_memory_ro((unsigned long)image, alloclen >> PAGE_SHIFT);
>> +		set_memory_x((unsigned long)image, alloclen >> PAGE_SHIFT);
>> +	}
> 
> You don't need to check the ifdef in a caller, there are stubs that
> compile to nothing when CONFIG_ARCH_HAS_SET_MEMORY=n.

I was about to do the same comment, but ....

CONFIG_STRICT_MODULE_RWX is not CONFIG_ARCH_HAS_SET_MEMORY

> 
>> diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
>> index aaf1a887f653..1484ad588685 100644
>> --- a/arch/powerpc/net/bpf_jit_comp64.c
>> +++ b/arch/powerpc/net/bpf_jit_comp64.c
>> @@ -1240,6 +1240,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
>>   	fp->jited_len = alloclen;
>>   
>>   	bpf_flush_icache(bpf_hdr, (u8 *)bpf_hdr + (bpf_hdr->pages * PAGE_SIZE));
>> +	if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
>> +		bpf_jit_binary_lock_ro(bpf_hdr);
> 
> Do we need the ifdef here either? Looks like it should be safe to call
> due to the stubs.

Same

> 
>> @@ -1262,6 +1264,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
>>   }
>>   
>>   /* Overriding bpf_jit_free() as we don't set images read-only. */
>> +#ifndef CONFIG_STRICT_MODULE_RWX
> 
> Did you test without this and notice something broken?
> 
> Looking at the generic version I can't tell why we need to override
> this. Maybe we don't (anymore?) ?
> 
> cheers
> 
>>   void bpf_jit_free(struct bpf_prog *fp)
>>   {
>>   	unsigned long addr = (unsigned long)fp->bpf_func & PAGE_MASK;
>> @@ -1272,3 +1275,4 @@ void bpf_jit_free(struct bpf_prog *fp)
>>   
>>   	bpf_prog_unlock_free(fp);
>>   }
>> +#endif
>> -- 
>> 2.25.1