[PATCH] bpf: fix possible spectre-v1 in find_and_alloc_map()

[Mark Rutland]

It's possible for userspace to control attr->map_type. Sanitize it when
using it as an array index to prevent an out-of-bounds value being used
under speculation.

Found by smatch.

Signed-off-by: Mark Rutland <mark.rutl...@arm.com>
Cc: Alexei Starovoitov <a...@kernel.org>
Cc: Dan Carpenter <dan.carpen...@oracle.com>
Cc: Daniel Borkmann <dan...@iogearbox.net>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: netdev@vger.kernel.org
---
 kernel/bpf/syscall.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

I found this when running smatch over a v4.17-rc2 arm64 allyesconfig kernel.

IIUC this may allow for a speculative branch to an arbitrary gadget when we
subsequently call ops->map_alloc_check(attr).

Mark.

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 4ca46df19c9a..8a7acd0dbeb6 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -26,6 +26,7 @@
 #include <linux/cred.h>
 #include <linux/timekeeping.h>
 #include <linux/ctype.h>
+#include <linux/nospec.h>
 
 #define IS_FD_ARRAY(map) ((map)->map_type == BPF_MAP_TYPE_PROG_ARRAY || \
                           (map)->map_type == BPF_MAP_TYPE_PERF_EVENT_ARRAY || \
@@ -102,12 +103,14 @@ const struct bpf_map_ops bpf_map_offload_ops = {
 static struct bpf_map *find_and_alloc_map(union bpf_attr *attr)
 {
        const struct bpf_map_ops *ops;
+       u32 type = attr->map_type;
        struct bpf_map *map;
        int err;
 
-       if (attr->map_type >= ARRAY_SIZE(bpf_map_types))
+       if (type >= ARRAY_SIZE(bpf_map_types))
                return ERR_PTR(-EINVAL);
-       ops = bpf_map_types[attr->map_type];
+       type = array_index_nospec(type, ARRAY_SIZE(bpf_map_types));
+       ops = bpf_map_types[type];
        if (!ops)
                return ERR_PTR(-EINVAL);
 
@@ -122,7 +125,7 @@ static struct bpf_map *find_and_alloc_map(union bpf_attr 
*attr)
        if (IS_ERR(map))
                return map;
        map->ops = ops;
-       map->map_type = attr->map_type;
+       map->map_type = type;
        return map;
 }
 
-- 
2.11.0