Re: [PATCH] net: core: Fix Spectre v1 vulnerability

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

Hi,

On 12/22/18 8:40 PM, David Miller wrote:
> From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Date: Sat, 22 Dec 2018 15:59:54 -0800
> 
>> On Sat, Dec 22, 2018 at 03:07:22PM -0800, David Miller wrote:
>>> From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
>>> Date: Fri, 21 Dec 2018 14:49:01 -0600
>>>
>>>> flen is indirectly controlled by user-space, hence leading to
>>>> a potential exploitation of the Spectre variant 1 vulnerability.
>>>>
>>>> This issue was detected with the help of Smatch:
>>>>
>>>> net/core/filter.c:1101 bpf_check_classic() warn: potential spectre issue 'filter' [w]
>>>>
>>>> Fix this by sanitizing flen before using it to index filter at line 1101:
>>>>
>>>> 	switch (filter[flen - 1].code) {
>>>>
>>>> and through pc at line 1040:
>>>> 	
>>>> 	const struct sock_filter *ftest = &filter[pc];
>>>>
>>>> Notice that given that speculation windows are large, the policy is
>>>> to kill the speculation on the first load and not worry if it can be
>>>> completed with a dependent load/store [1].
>>>>
>>>> [1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2
>>>>
>>>> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
>>>
>>> BPF folks, I'll take this directly.
>>>
>>> Applied and queued up for -stable, thanks.
>>
>> hmm. what was the rush?
>> I think it is unnecessary change.
>> Though fp is passed initially from user space
>> it's copied into kernel struct first.
>> There is no way user space can force kernel to mispredict
>> branch in for (pc = 0; pc < flen; pc++) loop.
The following piece of code is the one that can be mispredicted, not the 
for loop:

1013         if (flen == 0 || flen > BPF_MAXINSNS)
1014                 return false;

Instead of calling array_index_nospec() inside bpf_check_basics_ok(), I 
decided to place the call close to the code that could be compromised. 
This is when accessing filter[].

--
Gustavo

>> The change doesn't harm, but I don't think it's a good idea
>> to sprinkle kernel with array_index_nospec() just because some tool
>> produced a warning.
> 
> Ok, that makes sense, I can revert.
> 
> Just let me know.
>