net: filter: return -EINVAL if BPF_S_ANC* operation is not supported

Currently, we return -EINVAL for malicious or wrong BPF filters.
However, this is not done for BPF_S_ANC* operations, which makes it
more difficult to detect if it's actually supported or not by the
BPF machine. Therefore, we should also return -EINVAL if K is within
the SKF_AD_OFF universe and the ancillary operation did not match.

Cc: Ani Sinha <ani@aristanetworks.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
---
 net/core/filter.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

On 12/12/2012 10:31 AM, Daniel Borkmann wrote:
> Currently, we return -EINVAL for malicious or wrong BPF filters.
> However, this is not done for BPF_S_ANC* operations, which makes it
> more difficult to detect if it's actually supported or not by the
> BPF machine. Therefore, we should also return -EINVAL if K is within
> the SKF_AD_OFF universe and the ancillary operation did not match.
>
> Cc: Ani Sinha <ani@aristanetworks.com>
> Cc: Eric Dumazet <eric.dumazet@gmail.com>
> Signed-off-by: Daniel Borkmann <dborkman@redhat.com>

(Sorry, this is intended for net-next.)
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On Wed, 2012-12-12 at 10:31 +0100, Daniel Borkmann wrote:
> Currently, we return -EINVAL for malicious or wrong BPF filters.
> However, this is not done for BPF_S_ANC* operations, which makes it
> more difficult to detect if it's actually supported or not by the
> BPF machine. Therefore, we should also return -EINVAL if K is within
> the SKF_AD_OFF universe and the ancillary operation did not match.
> 
> Cc: Ani Sinha <ani@aristanetworks.com>
> Cc: Eric Dumazet <eric.dumazet@gmail.com>
> Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
> ---
>  net/core/filter.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index c23543c..de9bed4 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -531,7 +531,7 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
>  		[BPF_JMP|BPF_JSET|BPF_K] = BPF_S_JMP_JSET_K,
>  		[BPF_JMP|BPF_JSET|BPF_X] = BPF_S_JMP_JSET_X,
>  	};
> -	int pc;
> +	int pc, anc_found;
>  
>  	if (flen == 0 || flen > BPF_MAXINSNS)
>  		return -EINVAL;
> @@ -592,8 +592,10 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
>  		case BPF_S_LD_W_ABS:
>  		case BPF_S_LD_H_ABS:
>  		case BPF_S_LD_B_ABS:
> +			anc_found = 0;
>  #define ANCILLARY(CODE) case SKF_AD_OFF + SKF_AD_##CODE:	\
>  				code = BPF_S_ANC_##CODE;	\
> +				anc_found = 1;			\
>  				break
>  			switch (ftest->k) {
>  			ANCILLARY(PROTOCOL);
> @@ -610,6 +612,10 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
>  			ANCILLARY(VLAN_TAG);
>  			ANCILLARY(VLAN_TAG_PRESENT);
>  			}
> +
> +			/* ancillary operation unkown or unsupported */
> +			if (anc_found == 0 && ftest->k >= SKF_AD_OFF)
> +				return -EINVAL;
>  		}
>  		ftest->code = code;
>  	}

Several points :

1) This might break a userland filter that was previously working, by
returning 0 when load_pointer() returns NULL.

Specifying an offset bigger than skb->len is not _invalid_, it only
makes a filter returns 0, because load_pointer() returns NULL.

2) This wont help applications running on old kernels where your patch
wont be applied, as already mentioned yesterday.

3) Misses a "Reported-by" tag

4) anc_found is a boolean


To be truly portable, userland should not rely on kernel doing a full
validation of ancillaries. 



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On 12/12/2012 01:22 PM, Eric Dumazet wrote:
> On Wed, 2012-12-12 at 10:31 +0100, Daniel Borkmann wrote:
>> Currently, we return -EINVAL for malicious or wrong BPF filters.
>> However, this is not done for BPF_S_ANC* operations, which makes it
>> more difficult to detect if it's actually supported or not by the
>> BPF machine. Therefore, we should also return -EINVAL if K is within
>> the SKF_AD_OFF universe and the ancillary operation did not match.
>>
>> Cc: Ani Sinha <ani@aristanetworks.com>
>> Cc: Eric Dumazet <eric.dumazet@gmail.com>
>> Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
>> ---
>>   net/core/filter.c | 8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index c23543c..de9bed4 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -531,7 +531,7 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
>>   		[BPF_JMP|BPF_JSET|BPF_K] = BPF_S_JMP_JSET_K,
>>   		[BPF_JMP|BPF_JSET|BPF_X] = BPF_S_JMP_JSET_X,
>>   	};
>> -	int pc;
>> +	int pc, anc_found;
>>
>>   	if (flen == 0 || flen > BPF_MAXINSNS)
>>   		return -EINVAL;
>> @@ -592,8 +592,10 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
>>   		case BPF_S_LD_W_ABS:
>>   		case BPF_S_LD_H_ABS:
>>   		case BPF_S_LD_B_ABS:
>> +			anc_found = 0;
>>   #define ANCILLARY(CODE) case SKF_AD_OFF + SKF_AD_##CODE:	\
>>   				code = BPF_S_ANC_##CODE;	\
>> +				anc_found = 1;			\
>>   				break
>>   			switch (ftest->k) {
>>   			ANCILLARY(PROTOCOL);
>> @@ -610,6 +612,10 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
>>   			ANCILLARY(VLAN_TAG);
>>   			ANCILLARY(VLAN_TAG_PRESENT);
>>   			}
>> +
>> +			/* ancillary operation unkown or unsupported */
>> +			if (anc_found == 0 && ftest->k >= SKF_AD_OFF)
>> +				return -EINVAL;
>>   		}
>>   		ftest->code = code;
>>   	}
>
> Several points :
>
> 1) This might break a userland filter that was previously working, by
> returning 0 when load_pointer() returns NULL.
>
> Specifying an offset bigger than skb->len is not _invalid_, it only
> makes a filter returns 0, because load_pointer() returns NULL.

I think it will not break for code, that calls load_pointer() in such a
circumstance which passed the sk_chk_filter() test. However, it will
"break" for code that calls ...

   { BPF_LD | BPF_(W|H|B) | BPF_ABS, 0, 0, <K> },

... where <K> is in [0xfffff000, 0xffffffff] _and_ <K> is not an ancillary.

But ...

Assuming some old code will have such an instruction where <K> is between
[0xfffff000, 0xffffffff] and it doesn't know ancillary operations, then
this will give a non-expected/unwanted behavior as well (since we do not
return the BPF machine with 0 as it probably was the case before anc.ops,
but load sth. into the accumulator instead and continue with the next
instruction, for instance), right? Thus, following this argumentation, user
space code would already have been broken by introducing ancillary
operations into the BPF machine per se.

This is probably just an assumption, but code that does such a direct load,
e.g. "load word at packet offset 0xffffffff into accumulator" ("ld [0xffffffff]")
is quite broken, isn't it? Isn't the whole assumption of ancillary operations
that no-one intentionally calls things like "ld [0xffffffff]" and expect this
word to be loaded from the packet offset?

> 2) This wont help applications running on old kernels where your patch
> wont be applied, as already mentioned yesterday.

Agreed, but leaving old kernels aside, it would be nice if newer kernels
could validate that, so at least from kernel <xyz> onwards it could be
checked _for sure_ if anc.op <abc> is present and can be used.

> 3) Misses a "Reported-by" tag
>
> 4) anc_found is a boolean

3 + 4 agreed, sorry for that. I could do a v2 of the patch with 3 + 4 fixed
and resubmit it, if there's interest ...

> To be truly portable, userland should not rely on kernel doing a full
> validation of ancillaries.
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On Wed, Dec 12, 2012 at 8:25 AM, Daniel Borkmann <dborkman@redhat.com> wrote:
> On 12/12/2012 01:22 PM, Eric Dumazet wrote:
>>
>> On Wed, 2012-12-12 at 10:31 +0100, Daniel Borkmann wrote:
>>>
>>> Currently, we return -EINVAL for malicious or wrong BPF filters.
>>> However, this is not done for BPF_S_ANC* operations, which makes it
>>> more difficult to detect if it's actually supported or not by the
>>> BPF machine. Therefore, we should also return -EINVAL if K is within
>>> the SKF_AD_OFF universe and the ancillary operation did not match.
>>>
>>> Cc: Ani Sinha <ani@aristanetworks.com>
>>> Cc: Eric Dumazet <eric.dumazet@gmail.com>
>>> Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
>>> ---
>>>   net/core/filter.c | 8 +++++++-
>>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>> index c23543c..de9bed4 100644
>>> --- a/net/core/filter.c
>>> +++ b/net/core/filter.c
>>> @@ -531,7 +531,7 @@ int sk_chk_filter(struct sock_filter *filter,
>>> unsigned int flen)
>>>                 [BPF_JMP|BPF_JSET|BPF_K] = BPF_S_JMP_JSET_K,
>>>                 [BPF_JMP|BPF_JSET|BPF_X] = BPF_S_JMP_JSET_X,
>>>         };
>>> -       int pc;
>>> +       int pc, anc_found;
>>>
>>>         if (flen == 0 || flen > BPF_MAXINSNS)
>>>                 return -EINVAL;
>>> @@ -592,8 +592,10 @@ int sk_chk_filter(struct sock_filter *filter,
>>> unsigned int flen)
>>>                 case BPF_S_LD_W_ABS:
>>>                 case BPF_S_LD_H_ABS:
>>>                 case BPF_S_LD_B_ABS:
>>> +                       anc_found = 0;
>>>   #define ANCILLARY(CODE) case SKF_AD_OFF + SKF_AD_##CODE:      \
>>>                                 code = BPF_S_ANC_##CODE;        \
>>> +                               anc_found = 1;                  \
>>>                                 break
>>>                         switch (ftest->k) {
>>>                         ANCILLARY(PROTOCOL);
>>> @@ -610,6 +612,10 @@ int sk_chk_filter(struct sock_filter *filter,
>>> unsigned int flen)
>>>                         ANCILLARY(VLAN_TAG);
>>>                         ANCILLARY(VLAN_TAG_PRESENT);
>>>                         }
>>> +
>>> +                       /* ancillary operation unkown or unsupported */
>>> +                       if (anc_found == 0 && ftest->k >= SKF_AD_OFF)
>>> +                               return -EINVAL;
>>>                 }
>>>                 ftest->code = code;
>>>         }
>>
>>
>> Several points :
>>
>> 1) This might break a userland filter that was previously working, by
>> returning 0 when load_pointer() returns NULL.
>>
>> Specifying an offset bigger than skb->len is not _invalid_, it only
>> makes a filter returns 0, because load_pointer() returns NULL.
>
>
> I think it will not break for code, that calls load_pointer() in such a
> circumstance which passed the sk_chk_filter() test. However, it will
> "break" for code that calls ...
>
>   { BPF_LD | BPF_(W|H|B) | BPF_ABS, 0, 0, <K> },
>
> ... where <K> is in [0xfffff000, 0xffffffff] _and_ <K> is not an ancillary.
>
> But ...
>
> Assuming some old code will have such an instruction where <K> is between
> [0xfffff000, 0xffffffff] and it doesn't know ancillary operations, then
> this will give a non-expected/unwanted behavior as well (since we do not
> return the BPF machine with 0 as it probably was the case before anc.ops,
> but load sth. into the accumulator instead and continue with the next
> instruction, for instance), right? Thus, following this argumentation, user
> space code would already have been broken by introducing ancillary
> operations into the BPF machine per se.
>
> This is probably just an assumption, but code that does such a direct load,
> e.g. "load word at packet offset 0xffffffff into accumulator" ("ld
> [0xffffffff]")
> is quite broken, isn't it? Isn't the whole assumption of ancillary
> operations
> that no-one intentionally calls things like "ld [0xffffffff]" and expect
> this
> word to be loaded from the packet offset?
>
>
>> 2) This wont help applications running on old kernels where your patch
>> wont be applied, as already mentioned yesterday.
>
>
> Agreed, but leaving old kernels aside, it would be nice if newer kernels
> could validate that, so at least from kernel <xyz> onwards it could be
> checked _for sure_ if anc.op <abc> is present and can be used.
>

I second that. It would be nice to have a clean way to know whether a
particular ancilliary operation is supported by the kernel. After all,
the latest kernel of today will be ancient one soon enough ;)
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