linux-kernel - [PATCH v2 3/3] netfilter: x_tables: Use correct memory barriers.

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Date:   Mon,  8 Mar 2021 14:24:13 +1300
From:   Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz>
To:     pablo@...filter.org, kadlec@...filter.org, fw@...len.de
Cc:     subashab@...eaurora.org, netfilter-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz>
Subject: [PATCH v2 3/3] netfilter: x_tables: Use correct memory barriers.

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@...iedtelesis.co.nz>
---
 include/linux/netfilter/x_tables.h | 2 +-
 net/netfilter/x_tables.c           | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/netfilter/x_tables.h b/include/linux/netfilter/x_tables.h
index 5deb099d156d..8ec48466410a 100644
--- a/include/linux/netfilter/x_tables.h
+++ b/include/linux/netfilter/x_tables.h
@@ -376,7 +376,7 @@ static inline unsigned int xt_write_recseq_begin(void)
 	 * since addend is most likely 1
 	 */
 	__this_cpu_add(xt_recseq.sequence, addend);
-	smp_wmb();
+	smp_mb();

 	return addend;
 }
diff --git a/net/netfilter/x_tables.c b/net/netfilter/x_tables.c
index af22dbe85e2c..a2b50596b87e 100644
--- a/net/netfilter/x_tables.c
+++ b/net/netfilter/x_tables.c
@@ -1387,7 +1387,7 @@ xt_replace_table(struct xt_table *table,
 	table->private = newinfo;

 	/* make sure all cpus see new ->private value */
-	smp_wmb();
+	smp_mb();

 	/*
 	 * Even though table entries have now been swapped, other CPU's
-- 
2.30.1