[PATCH] net: procfs: Fix RCU stall and soft lockup in ptype_seq_next()

[fengwei_yin@linux.alibaba.com]

From: Yin Fengwei <fengwei_yin@linux.alibaba.com>

On an arm64 server platform, fuzz testing can trigger an RCU stall
followed by a soft lockup in ptype_seq_show(). The backtrace shows
the stall occurs in traverse.part.0(), which is called from
seq_read_iter() during /proc/net/ptype access:

[  295.799369] rcu: INFO: rcu_preempt self-detected stall on CPU
[  295.799937] rcu:     1-....: (60005 ticks this GP) idle=0f94/1/0x4000000000000000 softirq=5198/5199 fqs=29992
[  295.800708] rcu:     (t=60008 jiffies g=11197 q=1553 ncpus=2)
[  295.801152] CPU: 1 UID: 0 PID: 2695 Comm: test2 Tainted: G		L      6.19.0-rc7+ #19 PREEMPT(voluntary)
[  295.801961] Tainted: [L]=SOFTLOCKUP
[  295.802245] Hardware name: linux,dummy-virt (DT)
[  295.802616] pstate: 83400005 (Nzcv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
[  295.803171] pc : traverse.part.0+0x80/0x190
[  295.803511] lr : traverse.part.0+0x4c/0x190
[  295.803850] sp : ffff800082dabb20
[  295.804117] x29: ffff800082dabb20 x28: ffff000004571280 x27: 0000000000000000
[  295.804680] x26: 0000000000000000 x25: ffff00000471fee0 x24: ffff00000471fed0
[  295.805245] x23: 00000000000080db x22: 0000000000000033 x21: 0000000000000000
[  295.805814] x20: ffff000003719160 x19: ffff00000471fea8 x18: 0000000000000000
[  295.806378] x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
[  295.806941] x14: 0000000000000000 x13: 0a6e6f6974636e75 x12: 4620202020202065
[  295.807507] x11: 0000000000000000 x10: 0000000000000001 x9 : ffffc27ab28f881c
[  295.808081] x8 : 000000000000000a x7 : ffffc27ab3a8db42 x6 : 000000000000000a
[  295.808652] x5 : 0000000000000000 x4 : 0000000000001000 x3 : ffffc27ab328a7f8
[  295.809219] x2 : ffff00000471fed0 x1 : ffff000003719160 x0 : ffff00000471fea8
[  295.809785] Call trace:
[  295.809983]  traverse.part.0+0x80/0x190 (P)
[  295.810322]  seq_read_iter+0x2f8/0x4f8
[  295.810625]  seq_read+0xe4/0x120
[  295.810890]  proc_reg_read+0x9c/0xf8
[  295.811185]  do_loop_readv_writev.part.0+0xbc/0x118
[  295.811574]  vfs_readv+0x174/0x1d8
[  295.811848]  do_preadv+0x90/0xf8
[  295.812110]  __arm64_sys_preadv+0x24/0x38
[  295.812435]  invoke_syscall+0x4c/0x110
[  295.812737]  el0_svc_common.constprop.0+0x44/0xe8
[  295.813113]  do_el0_svc+0x20/0x30
[  295.813382]  el0_svc+0x38/0x178
[  295.813636]  el0t_64_sync_handler+0x98/0xe0
[  295.813969]  el0t_64_sync+0x184/0x188

The root cause is in ptype_seq_next(): when iterating over packet
types, it's possible that a packet type entry (pt) has been removed,
its dev set to NULL, and pt->af_packet_net is not initialized.
In that case, the function may return the same 'nxt' pointer indefinitely.
This results in an infinite loop under RCU read-side critical section,
causing an RCU stall and eventually a soft lockup.

Fix the issue by properly handling the case where 'nxt' points to
an empty list, ensuring forward progress in the iterator.

Signed-off-by: Yin Fengwei <fengwei_yin@linux.alibaba.com>
---
 net/core/net-procfs.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/core/net-procfs.c b/net/core/net-procfs.c
index 70e0e9a3b650..eb8c34bb1f0a 100644
--- a/net/core/net-procfs.c
+++ b/net/core/net-procfs.c
@@ -231,7 +231,7 @@ static void *ptype_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 	pt = v;
 	nxt = pt->list.next;
 	if (pt->dev) {
-		if (nxt != &pt->dev->ptype_all)
+		if (!list_empty(nxt) && nxt != &pt->dev->ptype_all)
 			goto found;
 
 		dev = pt->dev;
@@ -247,7 +247,7 @@ static void *ptype_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 
 	if (pt->af_packet_net) {
 net_ptype_all:
-		if (nxt != &net->ptype_all && nxt != &net->ptype_specific)
+		if (!list_empty(nxt) && nxt != &net->ptype_all && nxt != &net->ptype_specific)
 			goto found;
 
 		if (nxt == &net->ptype_all) {
@@ -267,6 +267,9 @@ static void *ptype_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 			return NULL;
 		nxt = ptype_base[hash].next;
 	}
+
+	if (list_empty(nxt))
+		return NULL;
 found:
 	return list_entry(nxt, struct packet_type, list);
 }
-- 
2.43.5