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

[Alexei Starovoitov]

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.