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Thu, 21 May 2026 05:16:42 -0700 (PDT) Received: from fedora ([171.243.49.69]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-36a46c53220sm753820a91.13.2026.05.21.05.16.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 21 May 2026 05:16:41 -0700 (PDT) From: lazyming To: netdev@vger.kernel.org Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, w@1wt.eu, security@kernel.org, linux-kernel@vger.kernel.org, lazyming , stable@vger.kernel.org Subject: [PATCH net] net: skbuff: fix missing zerocopy reference in pskb_carve helpers Date: Thu, 21 May 2026 19:16:28 +0700 Message-ID: <20260521121628.309924-1-minhnguyen.080505@gmail.com> X-Mailer: git-send-email 2.54.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" pskb_carve_inside_header() and pskb_carve_inside_nonlinear() both copy the old skb_shared_info header into a new buffer via memcpy(), which includes the destructor_arg pointer (uarg) for MSG_ZEROCOPY skbs. Neither function calls net_zcopy_get() for the new shinfo, creating an unaccounted holder: every skb_shared_info with destructor_arg set will call skb_zcopy_clear() once when freed, but the corresponding net_zcopy_get() was never called for the new copy. Repeated calls drive uarg->refcnt to zero prematurely, freeing ubuf_info_msgzc while TX skbs still hold live destructor_arg pointers. KASAN reports use-after-free on a freed ubuf_info_msgzc: BUG: KASAN: slab-use-after-free in skb_release_data+0x77b/0x810 Read of size 8 at addr ffff88801574d3e8 by task poc/220 Call Trace: skb_release_data+0x77b/0x810 kfree_skb_list_reason+0x13e/0x610 skb_release_data+0x4cd/0x810 sk_skb_reason_drop+0xf3/0x340 skb_queue_purge_reason+0x282/0x440 rds_tcp_inc_free+0x1e/0x30 rds_recvmsg+0x354/0x1780 __sys_recvmsg+0xdf/0x180 Allocated by task 219: msg_zerocopy_realloc+0x157/0x7b0 tcp_sendmsg_locked+0x2892/0x3ba0 Freed by task 219: ip_recv_error+0x74a/0xb10 tcp_recvmsg+0x475/0x530 The skb consuming the late access still referenced the same uarg via shinfo->destructor_arg copied by pskb_carve_inside_nonlinear() without a refcount bump. This has been verified to be reliably exploitable: a working proof-of-concept achieves full root privilege escalation from an unprivileged local user on a default kernel configuration. The fix follows the pattern of pskb_expand_head() which has the same memcpy/cloned structure. For pskb_carve_inside_header(), net_zcopy_get() is placed after skb_orphan_frags() succeeds, so the orphan error path needs no cleanup. For pskb_carve_inside_nonlinear(), net_zcopy_get() is placed after all failure points and just before skb_release_data(), so no error path needs cleanup at all -- matching pskb_expand_head() more closely and avoiding the need for a balancing net_zcopy_put(). Fixes: 6fa01ccd8830 ("skbuff: Add pskb_extract() helper function") Cc: stable@vger.kernel.org Assisted-by: Claude:claude-sonnet-4-6 Signed-off-by: lazyming Reviewed-by: Willem de Bruijn --- net/core/skbuff.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/net/core/skbuff.c b/net/core/skbuff.c index 44ac121cf..6a1a2c203 100644 --- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -6810,6 +6810,8 @@ static int pskb_carve_inside_header(struct sk_buff *s= kb, const u32 off, skb_kfree_head(data); return -ENOMEM; } + if (skb_zcopy(skb)) + net_zcopy_get(skb_zcopy(skb)); for (i =3D 0; i < skb_shinfo(skb)->nr_frags; i++) skb_frag_ref(skb, i); if (skb_has_frag_list(skb)) @@ -6953,6 +6955,8 @@ static int pskb_carve_inside_nonlinear(struct sk_buff= *skb, const u32 off, skb_kfree_head(data); return -ENOMEM; } + if (skb_zcopy(skb)) + net_zcopy_get(skb_zcopy(skb)); skb_release_data(skb, SKB_CONSUMED); =20 skb->head =3D data; base-commit: 94e3dd6874bf04d5939bc8431b9f7852f3a4a121 --=20 2.54.0