[RFC PATCH] net: Fix one page_pool page leak from skb_frag_unref

From: Dragos Tatulea
Date: Wed Apr 24 2024 - 12:57:30 EST

The patch referenced in the fixes tag introduced the skb_frag_unref API.
This API still has a dark corner: when skb->pp_recycled is false and a
fragment being referenced is backed by a page_pool page, skb_frag_unref
can leak the page out of the page_pool, like in the following example:

BUG: Bad page state in process swapper/4 pfn:103423
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x103423000 pfn:0x103423
flags: 0x200000000000000(node=0|zone=2)
page_type: 0xffffffff()
raw: 0200000000000000 dead000000000040 ffff888106f38000 0000000000000000
raw: 0000000103423000 0000000000000041 00000000ffffffff 0000000000000000
page dumped because: page_pool leak
Modules linked in: act_mirred act_csum act_pedit act_gact cls_flower
act_ct nf_flow_table sch_ingress xt_conntrack xt_MASQUERADE
nf_conntrack_netlink nfnetlink xt_addrtype iptable_nat nf_nat
br_netfilter overlay rpcrdma rdma_ucm ib_iser libiscsi
scsi_transport_iscsi ib_umad rdma_cm ib_ipoib iw_cm ib_cm mlx5_ib
ib_uverbs ib_core zram zsmalloc mlx5_core fuse CPU: 4 PID: 0 Comm:
swapper/4 Not tainted 6.9.0-rc4+ #2
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
Call Trace:
<IRQ>
dump_stack_lvl+0x53/0x70
bad_page+0x6f/0xf0
free_unref_page_prepare+0x271/0x420
free_unref_page+0x38/0x120
___pskb_trim+0x261/0x390
skb_segment+0xf60/0x1040
tcp_gso_segment+0xe8/0x4e0
inet_gso_segment+0x155/0x3d0
skb_mac_gso_segment+0x99/0x100
__skb_gso_segment+0xb4/0x160
? netif_skb_features+0x95/0x2f0
validate_xmit_skb+0x16c/0x330
validate_xmit_skb_list+0x4c/0x70
sch_direct_xmit+0x23e/0x350
__dev_queue_xmit+0x334/0xbc0
tcf_mirred_to_dev+0x2a5/0x3e0 [act_mirred]
tcf_mirred_act+0xd7/0x4b0 [act_mirred]
? tcf_pedit_act+0x6f/0x540 [act_pedit]
tcf_action_exec+0x82/0x170
fl_classify+0x1ee/0x200 [cls_flower]
? tcf_mirred_to_dev+0x2a5/0x3e0 [act_mirred]
? mlx5e_completion_event+0x39/0x40 [mlx5_core]
? mlx5_eq_comp_int+0x1d7/0x1f0 [mlx5_core]
tcf_classify+0x26a/0x470
tc_run+0xa2/0x120
? handle_irq_event+0x53/0x80
? kvm_clock_get_cycles+0x11/0x20
__netif_receive_skb_core.constprop.0+0x932/0xee0
__netif_receive_skb_list_core+0xfe/0x1f0
netif_receive_skb_list_internal+0x1b5/0x2b0
napi_gro_complete.constprop.0+0xee/0x120
dev_gro_receive+0x3f4/0x710
napi_gro_receive+0x7d/0x220
mlx5e_handle_rx_cqe_mpwrq+0x10d/0x1d0 [mlx5_core]
mlx5e_poll_rx_cq+0x8b/0x6f0 [mlx5_core]
mlx5e_napi_poll+0xdc/0x6c0 [mlx5_core]
__napi_poll+0x25/0x1b0
net_rx_action+0x2c1/0x330
__do_softirq+0xcb/0x28c
irq_exit_rcu+0x69/0x90
common_interrupt+0x85/0xa0
</IRQ>
<TASK>
asm_common_interrupt+0x26/0x40
RIP: 0010:default_idle+0x17/0x20
Code: 00 4d 29 c8 4c 01 c7 4c 29 c2 e9 76 ff ff ff cc cc cc cc f3 0f 1e
fa 8b 05 76 3f 0a 01 85 c0 7e 07 0f 00 2d 1d 74 41 00 fb f4 <fa> c3 0f
1f 80 00 00 00 00 f3 0f 1e fa 65 48 8b 35 04 b3 42 7e f0
RSP: 0018:ffff88810087bed8 EFLAGS: 00000246
RAX: 0000000000000000 RBX: ffff8881008415c0 RCX: 000000e116d359fb
RDX: 0000000000000000 RSI: ffffffff8223e1d1 RDI: 000000000003f214
RBP: 0000000000000004 R08: 000000000003f214 R09: 000000e116d359fb
R10: 000000e116d359fb R11: 000000000005dfee R12: 0000000000000004
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
default_idle_call+0x3d/0xf0
do_idle+0x1ce/0x1e0
cpu_startup_entry+0x29/0x30
start_secondary+0x109/0x130
common_startup_64+0x129/0x138
</TASK>

How it happens:
-> skb_segment
-> clone skb into nskb
-> call __pskb_trim(nskb)
-> call pskb_expand_head(nskb) because nskb is cloned
-> call skb_release_data(nskb) because nskb is cloned
-> nskb->pp_recycle = 0
-> skb_unref(nskb->frag[i], nskb)
-> nskb->pp_recycle is false, frag is page_pool page
-> Fragment is released via put_page(frag page), hence leaking
from the page_pool.

Something tells me that this is not be the only issue of this kind...

The patch itself contains a suggested fix for this specific bug: it
forces the unref in ___pskb_trim to recycle to the page_pool. If the
page is not a page_pool page, it will be dereferenced as a regular page.

The alternative would be to save the skb->pp_recycled flag before
pskb_expand_head and use it later during the unref.

Signed-off-by: Dragos Tatulea <dtatulea@xxxxxxxxxx>
Co-developed-by: Jianbo Liu <jianbol@xxxxxxxxxx>
Fixes: a580ea994fd3 ("net: mirror skb frag ref/unref helpers")
Cc: Mina Almasry <almasrymina@xxxxxxxxxx>
---
net/core/skbuff.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 37c858dc11a6..ab75b4f876ce 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2634,7 +2634,7 @@ int ___pskb_trim(struct sk_buff *skb, unsigned int len)
skb_shinfo(skb)->nr_frags = i;

for (; i < nfrags; i++)
- skb_frag_unref(skb, i);
+ __skb_frag_unref(&skb_shinfo(skb)->frags[i], true);

if (skb_has_frag_list(skb))
skb_drop_fraglist(skb);
--
2.44.0