From: Martin KaFai Lau <martin.lau@linux.dev>
Date: Wed, 4 Feb 2026 11:34:55 -0800

On 2/4/26 7:41 AM, Michal Luczaj wrote:

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If the concern is the bpf iterator prog may use a released unix_peer(sk)
pointer, it should be fine. The unix_peer(sk) pointer is not a trusted
pointer to the bpf prog, so nothing bad will happen other than
potentially reading incorrect values.

I misremembered that following unix->peer would be marked as 
(PTR_TO_BTF_ID | PTR_UNTRUSTED). I forgot there are some legacy supports 
on the PTR_TO_BTF_ID (i.e. without PTR_UNTRUSTED marking).

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But if the prog passes a released peer pointer to a bpf helper:

BUG: KASAN: slab-use-after-free in bpf_skc_to_unix_sock+0x95/0xb0
Read of size 1 at addr ffff888110654c92 by task test_progs/1936

hmm... bpf_skc_to_unix_sock is exposed to tracing. bpf_iter is a tracing
bpf prog.

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Can you cook a patch for this ? probably like below

This can help the bpf_iter but not the other tracing prog such as fentry.

Oh well ... then bpf_skc_to_unix_sock() can be used even
with SEQ_START_TOKEN at fentry of bpf_iter_unix_seq_show() ??

It is fine. The type is void.

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How about adding notrace to all af_unix bpf iterator functions ?

but right, other functions taking [unix_]sock pointer could be audited. 
I don't know af_unix well enough to assess the blast radius or whether 
some useful functions may become untraceable.

Considering SOCK_DGRAM, the blast radus is much bigger than
I thought, so I'd avoid this way if possible by modifying
the verifier.

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The procfs iterator holds a spinlock of the hashtable from
->start/next() to ->stop() to prevent the race with unix_release_sock().

I think other (non-iterator) functions cannot do such racy
access with tracing prog.

But then there's SOCK_DGRAM where you can drop unix_peer(sk) without
releasing sk; see AF_UNSPEC in unix_dgram_connect(). I think Martin is
right, we can crash at many fentries.

BUG: KASAN: slab-use-after-free in bpf_skc_to_unix_sock+0xa4/0xb0
Read of size 2 at addr ffff888147d38890 by task test_progs/2495
Call Trace:
  dump_stack_lvl+0x5d/0x80
  print_report+0x170/0x4f3
  kasan_report+0xe1/0x180
  bpf_skc_to_unix_sock+0xa4/0xb0
  bpf_prog_564a1c39c35d86a2_unix_shutdown_entry+0x8a/0x8e
  bpf_trampoline_6442564662+0x47/0xab
  unix_shutdown+0x9/0x880
  __sys_shutdown+0xe1/0x160
  __x64_sys_shutdown+0x52/0x90
  do_syscall_64+0x6b/0x3a0
  entry_SYSCALL_64_after_hwframe+0x76/0x7e

This probably is the first case where reading a sk pointer requires a 
lock. I think it will need to be marked as PTR_UNTRUSTED in the verifier 
for the unix->peer access, so that it cannot be passed to a helper. 
There is a BTF_TYPE_SAFE_TRUSTED list. afaik, there is no untrusted one now.

Just skimmed the code, and I guess something like below would
do that ?  and if needed, we could add another helper to fetch
peer with a proper release function ?

---8<---

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3135643d5695..ef8b4dd21923 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c

@@ -7177,6 +7177,14 @@ static bool type_is_rcu_or_null(struct bpf_verifier_env *env,
 	return btf_nested_type_is_trusted(&env->log, reg, field_name, btf_id, "__safe_rcu_or_null");
 }
 
+static bool type_is_untrusted(struct bpf_verifier_env *env,
+			      struct bpf_reg_state *reg,
+			      const char *field_name, u32 btf_id)
+{
+	/* TODO: return true if field_name and btf_id is unix_sock.peer. */
+	return false;
+}
+
 static bool type_is_trusted(struct bpf_verifier_env *env,
 			    struct bpf_reg_state *reg,
 			    const char *field_name, u32 btf_id)

@@ -7307,7 +7315,9 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 		 * A regular RCU-protected pointer with __rcu tag can also be deemed
 		 * trusted if we are in an RCU CS. Such pointer can be NULL.
 		 */
-		if (type_is_trusted(env, reg, field_name, btf_id)) {
+		if (type_is_untrusted(env, reg, field_name, btf_id)) {
+			flag |= PTR_UNTRUSTED;
+		} else if (type_is_trusted(env, reg, field_name, btf_id)) {
 			flag |= PTR_TRUSTED;
 		} else if (type_is_trusted_or_null(env, reg, field_name, btf_id)) {
 			flag |= PTR_TRUSTED | PTR_MAYBE_NULL;
---8<---