On 4/18/24 12:18 AM, Toke Høiland-Jørgensen wrote:
When redirecting a packet using XDP, the bpf_redirect_map() helper will set
up the redirect destination information in struct bpf_redirect_info (using
the __bpf_xdp_redirect_map() helper function), and the xdp_do_redirect()
function will read this information after the XDP program returns and pass
the frame on to the right redirect destination.
When using the BPF_F_BROADCAST flag to do multicast redirect to a whole
map, __bpf_xdp_redirect_map() sets the 'map' pointer in struct
bpf_redirect_info to point to the destination map to be broadcast. And
xdp_do_redirect() reacts to the value of this map pointer to decide whether
it's dealing with a broadcast or a single-value redirect. However, if the
destination map is being destroyed before xdp_do_redirect() is called, the
map pointer will be cleared out (by bpf_clear_redirect_map()) without
waiting for any XDP programs to stop running. This causes xdp_do_redirect()
to think that the redirect was to a single target, but the target pointer
is also NULL (since broadcast redirects don't have a single target), so
this causes a crash when a NULL pointer is passed to dev_map_enqueue().
To fix this, change xdp_do_redirect() to react directly to the presence of
the BPF_F_BROADCAST flag in the 'flags' value in struct bpf_redirect_info
to disambiguate between a single-target and a broadcast redirect. And only
read the 'map' pointer if the broadcast flag is set, aborting if that has
been cleared out in the meantime. This prevents the crash, while keeping
the atomic (cmpxchg-based) clearing of the map pointer itself, and without
adding any more checks in the non-broadcast fast path.
Fixes: e624d4ed4aa8 ("xdp: Extend xdp_redirect_map with broadcast support")
Reported-and-tested-by: syzbot+af9492708df9797198d6@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
---
net/core/filter.c | 42 ++++++++++++++++++++++++++++++++----------
1 file changed, 32 insertions(+), 10 deletions(-)
diff --git a/net/core/filter.c b/net/core/filter.c
index 786d792ac816..8120c3dddf5e 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4363,10 +4363,12 @@ static __always_inline int __xdp_do_redirect_frame(struct bpf_redirect_info *ri,
enum bpf_map_type map_type = ri->map_type;
void *fwd = ri->tgt_value;
u32 map_id = ri->map_id;
+ u32 flags = ri->flags;
struct bpf_map *map;
int err;
ri->map_id = 0; /* Valid map id idr range: [1,INT_MAX[ */
+ ri->flags = 0;
ri->map_type = BPF_MAP_TYPE_UNSPEC;
if (unlikely(!xdpf)) {
@@ -4378,11 +4380,20 @@ static __always_inline int __xdp_do_redirect_frame(struct bpf_redirect_info *ri,
case BPF_MAP_TYPE_DEVMAP:
fallthrough;
case BPF_MAP_TYPE_DEVMAP_HASH:
- map = READ_ONCE(ri->map);
- if (unlikely(map)) {
+ if (unlikely(flags & BPF_F_BROADCAST)) {
+ map = READ_ONCE(ri->map);
+
+ /* The map pointer is cleared when the map is being torn
+ * down by bpf_clear_redirect_map()
Thanks for the details explanation in the commit message. All make sense.
It could be a dumb question.
From reading the "waits for...NAPI being the relevant context here..." comment
in dev_map_free(), I wonder if moving synchronize_rcu() before
bpf_clear_redirect_map() would also work? Actually, does it need to call
bpf_clear_redirect_map(). The on-going xdp_do_redirect() should be the last one
using the map in ri->map anyway and no xdp prog can set it again to ri->map.
+ */
+ if (unlikely(!map)) {
+ err = -ENOENT;
+ break;
+ }
+
WRITE_ONCE(ri->map, NULL);
err = dev_map_enqueue_multi(xdpf, dev, map,
- ri->flags & BPF_F_EXCLUDE_INGRESS);
+ flags & BPF_F_EXCLUDE_INGRESS);
} else {
err = dev_map_enqueue(fwd, xdpf, dev);
}
