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

[Gustavo A. R. Silva]

Hi,

On 12/22/18 8:40 PM, David Miller wrote:
> 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 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.