syzbot and Ilya faced the splats when %XDP_PASS happens for bpf_test_run
after skb PP recycling was enabled for {__,}xdp_build_skb_from_frame():
BUG: kernel NULL pointer dereference, address: 0000000000000d28
RIP: 0010:memset_erms+0xd/0x20 arch/x86/lib/memset_64.S:66
[...]
Call Trace:
<TASK>
__finalize_skb_around net/core/skbuff.c:321 [inline]
__build_skb_around+0x232/0x3a0 net/core/skbuff.c:379
build_skb_around+0x32/0x290 net/core/skbuff.c:444
__xdp_build_skb_from_frame+0x121/0x760 net/core/xdp.c:622
xdp_recv_frames net/bpf/test_run.c:248 [inline]
xdp_test_run_batch net/bpf/test_run.c:334 [inline]
bpf_test_run_xdp_live+0x1289/0x1930 net/bpf/test_run.c:362
bpf_prog_test_run_xdp+0xa05/0x14e0 net/bpf/test_run.c:1418
[...]
This happens due to that it calls xdp_scrub_frame(), which nullifies
xdpf->data. bpf_test_run code doesn't reinit the frame when the XDP
program doesn't adjust head or tail. Previously, %XDP_PASS meant the
page will be released from the pool and returned to the MM layer, but
now it does return to the Pool with the nullified xdpf->data, which
doesn't get reinitialized then.
So, in addition to checking whether the head and/or tail have been
adjusted, check also for a potential XDP frame corruption. xdpf->data
is 100% affected and also xdpf->flags is the field closest to the
metadata / frame start. Checking for these two should be enough for
non-extreme cases.
Fixes: 9c94bbf9a87b ("xdp: recycle Page Pool backed skbs built from XDP frames")
Reported-by: syzbot+e1d1b65f7c32f2a86a9f@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://lore.kernel.org/bpf/000000000000f1985705f6ef2243@xxxxxxxxxx
Reported-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/bpf/e07dd94022ad5731705891b9487cc9ed66328b94.camel@xxxxxxxxxxxxx
Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
---
net/bpf/test_run.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 71226f68270d..8d6b31209bd6 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -208,6 +208,16 @@ static void xdp_test_run_teardown(struct xdp_test_data *xdp)
kfree(xdp->skbs);
}
+static bool frame_was_changed(const struct xdp_page_head *head)
+{
+ /* xdp_scrub_frame() zeroes the data pointer, flags is the last field,
+ * i.e. has the highest chances to be overwritten. If those two are
+ * untouched, it's most likely safe to skip the context reset.
+ */
+ return head->frm.data != head->orig_ctx.data ||
+ head->frm.flags != head->orig_ctx.flags;
+}
+
static bool ctx_was_changed(struct xdp_page_head *head)
{
return head->orig_ctx.data != head->ctx.data ||
@@ -217,7 +227,7 @@ static bool ctx_was_changed(struct xdp_page_head *head)
static void reset_ctx(struct xdp_page_head *head)
{
- if (likely(!ctx_was_changed(head)))
+ if (likely(!frame_was_changed(head) && !ctx_was_changed(head)))
return;
head->ctx.data = head->orig_ctx.data;
--
2.39.2
