net: core: Fix Spectre v1 vulnerability

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>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/filter.c

@@ -73,6 +73,7 @@
 #include <linux/seg6_local.h>
 #include <net/seg6.h>
 #include <net/seg6_local.h>
+#include <linux/nospec.h>
 
 /**
  *	sk_filter_trim_cap - run a packet through a socket filter
@@ -1038,6 +1039,7 @@ static int bpf_check_classic(const struct sock_filter *filter,
 	bool anc_found;
 	int pc;
 
+	flen = array_index_nospec(flen, BPF_MAXINSNS + 1);
 	/* Check the filter code now */
 	for (pc = 0; pc < flen; pc++) {
 		const struct sock_filter *ftest = &filter[pc];