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

[David Miller]

From: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
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@xxxxxxxxxxxxxx>
>> 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@xxxxxxxxxxxxxx>
>> 
>> 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 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.