net: skbuff: fix missing zerocopy reference in pskb_carve helpers

Description

In the Linux kernel, the following vulnerability has been resolved: net: skbuff: fix missing zerocopy reference in pskb_carve helpers 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().

Affected Products

Also Affects

Downstream vendors/products affected by this vulnerability

References

• https://git.kernel.org/stable/c/8dbed691e43a50903658130bde0fcb5abc425b37
• https://git.kernel.org/stable/c/9b40bdc2a3298225dffab8158208a0d8c6300578
• https://git.kernel.org/stable/c/fd470f0a97b8e9a125f520265d2f3b088ffb5b8a
• https://git.kernel.org/stable/c/ceafb893b12f23331dcc5ff9587e643c3a40ee9f
• https://git.kernel.org/stable/c/2e0e74c59b2761a414d9f48d7bee1e45220b2427
• https://git.kernel.org/stable/c/96a4713ae041cc85e712bac682cd2e644004d6c6
• https://git.kernel.org/stable/c/474d6c771d798bca84f0a140b611e36743511e18
• https://git.kernel.org/stable/c/98d0912e9f841e5529a5b89a972805f34cb1c69d

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