netdev - [PATCH v2 bpf 3/3] bpf: Fix syscall's stackmap lookup potential deadlock

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Message-ID: <20190131021245.1905869-4-ast@kernel.org>
Date:   Wed, 30 Jan 2019 18:12:45 -0800
From:   Alexei Starovoitov <ast@...nel.org>
To:     <davem@...emloft.net>
CC:     <daniel@...earbox.net>, <peterz@...radead.org>,
        <edumazet@...gle.com>, <jannh@...gle.com>,
        <netdev@...r.kernel.org>, <kernel-team@...com>
Subject: [PATCH v2 bpf 3/3] bpf: Fix syscall's stackmap lookup potential deadlock

From: Martin KaFai Lau <kafai@...com>

The map_lookup_elem used to not acquiring spinlock
in order to optimize the reader.

It was true until commit 557c0c6e7df8 ("bpf: convert stackmap to pre-allocation")
The syscall's map_lookup_elem(stackmap) calls bpf_stackmap_copy().
bpf_stackmap_copy() may find the elem no longer needed after the copy is done.
If that is the case, pcpu_freelist_push() saves this elem for reuse later.
This push requires a spinlock.

If a tracing bpf_prog got run in the middle of the syscall's
map_lookup_elem(stackmap) and this tracing bpf_prog is calling
bpf_get_stackid(stackmap) which also requires the same pcpu_freelist's
spinlock, it may end up with a dead lock situation as reported by
Eric Dumazet in https://patchwork.ozlabs.org/patch/1030266/

The situation is the same as the syscall's map_update_elem() which
needs to acquire the pcpu_freelist's spinlock and could race
with tracing bpf_prog.  Hence, this patch fixes it by protecting
bpf_stackmap_copy() with this_cpu_inc(bpf_prog_active)
to prevent tracing bpf_prog from running.

A later syscall's map_lookup_elem commit f1a2e44a3aec ("bpf: add queue and stack maps")
also acquires a spinlock and races with tracing bpf_prog similarly.
Hence, this patch is forward looking and protects the majority
of the map lookups.  bpf_map_offload_lookup_elem() is the exception
since it is for network bpf_prog only (i.e. never called by tracing
bpf_prog).

Fixes: 557c0c6e7df8 ("bpf: convert stackmap to pre-allocation")
Reported-by: Eric Dumazet <eric.dumazet@...il.com>
Acked-by: Alexei Starovoitov <ast@...nel.org>
Signed-off-by: Martin KaFai Lau <kafai@...com>
Signed-off-by: Alexei Starovoitov <ast@...nel.org>
---
 kernel/bpf/syscall.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index b155cd17c1bd..8577bb7f8be6 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -713,8 +713,13 @@ static int map_lookup_elem(union bpf_attr *attr)

 	if (bpf_map_is_dev_bound(map)) {
 		err = bpf_map_offload_lookup_elem(map, key, value);
-	} else if (map->map_type == BPF_MAP_TYPE_PERCPU_HASH ||
-		   map->map_type == BPF_MAP_TYPE_LRU_PERCPU_HASH) {
+		goto done;
+	}
+
+	preempt_disable();
+	this_cpu_inc(bpf_prog_active);
+	if (map->map_type == BPF_MAP_TYPE_PERCPU_HASH ||
+	    map->map_type == BPF_MAP_TYPE_LRU_PERCPU_HASH) {
 		err = bpf_percpu_hash_copy(map, key, value);
 	} else if (map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY) {
 		err = bpf_percpu_array_copy(map, key, value);
@@ -744,7 +749,10 @@ static int map_lookup_elem(union bpf_attr *attr)
 		}
 		rcu_read_unlock();
 	}
+	this_cpu_dec(bpf_prog_active);
+	preempt_enable();

+done:
 	if (err)
 		goto free_value;

-- 
2.20.0