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

Hi,

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

From: Alexei Starovoitov <redacted>
Date: Sat, 22 Dec 2018 15:59:54 -0800

quoted

On Sat, Dec 22, 2018 at 03:07:22PM -0800, David Miller wrote:

quoted

From: "Gustavo A. R. Silva" <redacted>
Date: Fri, 21 Dec 2018 14:49:01 -0600

quoted

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 <redacted>

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[].