From: Mark Tomlinson <mark.tomlinson@alliedtelesis.co.nz>
Date: Mon, 8 Mar 2021 14:24:13 +1300
Subject: netfilter: x_tables: Use correct memory barriers.
Git-commit: 175e476b8cdf2a4de7432583b49c871345e4f8a1
Patch-mainline: v5.12-rc5
References: bsc#1184208 CVE-2021-29650

When a new table value was assigned, it was followed by a write memory
barrier. This ensured that all writes before this point would complete
before any writes after this point. However, to determine whether the
rules are unused, the sequence counter is read. To ensure that all
writes have been done before these reads, a full memory barrier is
needed, not just a write memory barrier. The same argument applies when
incrementing the counter, before the rules are read.

Changing to using smp_mb() instead of smp_wmb() fixes the kernel panic
reported in cc00bcaa5899 (which is still present), while still
maintaining the same speed of replacing tables.

The smb_mb() barriers potentially slow the packet path, however testing
has shown no measurable change in performance on a 4-core MIPS64
platform.

Fixes: 7f5c6d4f665b ("netfilter: get rid of atomic ops in fast path")
Signed-off-by: Mark Tomlinson <mark.tomlinson@alliedtelesis.co.nz>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

 [ bp: Add the second smp_wmb() line in xt_replace_table() from
   d3d40f237480 ("Revert "netfilter: x_tables: Switch synchronization to RCU"")
   so that the table->private assignment is "sandwiched" by the two barriers
   like it is done upstream. ]

Acked-by: Borislav Petkov <bp@suse.de>
---
 include/linux/netfilter/x_tables.h |    2 +-
 net/netfilter/x_tables.c           |    3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)

--- a/include/linux/netfilter/x_tables.h
+++ b/include/linux/netfilter/x_tables.h
@@ -369,7 +369,7 @@ static inline unsigned int xt_write_recs
 	 * since addend is most likely 1
 	 */
 	__this_cpu_add(xt_recseq.sequence, addend);
-	smp_wmb();
+	smp_mb();
 
 	return addend;
 }
--- a/net/netfilter/x_tables.c
+++ b/net/netfilter/x_tables.c
@@ -1191,6 +1191,9 @@ xt_replace_table(struct xt_table *table,
 	smp_wmb();
 	table->private = newinfo;
 
+	/* make sure all cpus see new ->private value */
+	smp_mb();
+
 	/*
 	 * Even though table entries have now been swapped, other CPU's
 	 * may still be using the old entries. This is okay, because