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Thu, 14 May 2026 23:07:43 -0700 (PDT) Received: from v4bel ([58.123.110.97]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2bd5d23044csm44639995ad.78.2026.05.14.23.07.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 14 May 2026 23:07:42 -0700 (PDT) Date: Fri, 15 May 2026 15:07:37 +0900 From: Hyunwoo Kim To: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, kerneljasonxing@gmail.com, kuniyu@google.com, mhal@rbox.co, jiayuan.chen@linux.dev, steffen.klassert@secunet.com, ben@decadent.org.uk, herbert@gondor.apana.org.au, dsahern@kernel.org, sultan@kerneltoast.com, sd@queasysnail.net, malin89@huawei.com, tanjingguo@huawei.com Cc: netdev@vger.kernel.org, stable@vger.kernel.org, imv4bel@gmail.com Subject: Re: [PATCH net v4] net: skbuff: propagate shared-frag marker through frag-transfer helpers Message-ID: References: Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, May 15, 2026 at 02:55:35PM +0900, Hyunwoo Kim wrote: > Two frag-transfer helpers (__pskb_copy_fclone() and skb_shift()) fail > to propagate the SKBFL_SHARED_FRAG bit in skb_shinfo()->flags when > moving frags from source to destination. __pskb_copy_fclone() defers > the rest of the shinfo metadata to skb_copy_header() after copying > frag descriptors, but that helper only carries over gso_{size,segs, > type} and never touches skb_shinfo()->flags; skb_shift() moves frag > descriptors directly and leaves flags untouched. As a result, the > destination skb keeps a reference to the same externally-owned or > page-cache-backed pages while reporting skb_has_shared_frag() as > false. > > The mismatch is harmful in any in-place writer that uses > skb_has_shared_frag() to decide whether shared pages must be detoured > through skb_cow_data(). ESP input is one such writer (esp4.c, > esp6.c), and a single nft 'dup to ' rule -- or any other > nf_dup_ipv4() / xt_TEE caller -- is enough to land a pskb_copy()'d > skb in esp_input() with the marker stripped, letting an unprivileged > user write into the page cache of a root-owned read-only file via > authencesn-ESN stray writes. > > Set SKBFL_SHARED_FRAG on the destination whenever frag descriptors > were actually moved from the source. skb_copy() and skb_copy_expand() > share skb_copy_header() too but linearize all paged data into freshly > allocated head storage and emerge with nr_frags == 0, so > skb_has_shared_frag() returns false on its own; they need no change. > > The same omission exists in skb_gro_receive() and skb_gro_receive_list(). > The former moves the incoming skb's frag descriptors into the > accumulator's last sub-skb via two paths (a direct frag-move loop and > the head_frag + memcpy path); the latter chains the incoming skb whole > onto p's frag_list. Downstream skb_segment() reads only > skb_shinfo(p)->flags, and skb_segment_list() reuses each sub-skb's > shinfo as the nskb -- both p and lp must carry the marker. > > The same omission also exists in tcp_clone_payload(), which builds an > MTU probe skb by moving frag descriptors from skbs on sk_write_queue > into a freshly allocated nskb. The helper falls into the same family > and warrants the same fix for consistency; no TCP TX-side in-place > writer is currently known to reach a user page through this gap, but > a future consumer depending on the marker would regress silently. > > Fixes: cef401de7be8 ("net: fix possible wrong checksum generation") > Fixes: f4c50a4034e6 ("xfrm: esp: avoid in-place decrypt on shared skb frags") > Suggested-by: Sabrina Dubroca > Suggested-by: Sultan Alsawaf > Suggested-by: Ben Hutchings Since they are asking for credit, I will add them: Suggested-by: Lin Ma Suggested-by: Jingguo Tan > Cc: stable@vger.kernel.org > Signed-off-by: Hyunwoo Kim > --- > Changes in v4: > - Include the tcp_clone_payload() propagation suggested by Sabrina. > - Drop the skb_try_coalesce() change; addressed by commit f84eca581739. > - v3: https://lore.kernel.org/all/agW4vC0r8QOUKtRT@v4bel/ > > Changes in v3: > - Include the skb_gro_receive() audit patch suggested by Sultan > - v2: https://lore.kernel.org/all/agToIEDI4TaTNLRb@v4bel/ > > Changes in v2: > - Also propagate SHARED_FRAG in skb_try_coalesce() and skb_shift() > - v1: https://lore.kernel.org/all/agRfuVOeMI5pbHhY@v4bel/ > --- > net/core/gro.c | 4 ++++ > net/core/skbuff.c | 3 +++ > net/ipv4/tcp_output.c | 1 + > 3 files changed, 8 insertions(+) > > diff --git a/net/core/gro.c b/net/core/gro.c > index 31d21de5b15a..9f8960789b2c 100644 > --- a/net/core/gro.c > +++ b/net/core/gro.c > @@ -213,10 +213,12 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb) > p->data_len += len; > p->truesize += delta_truesize; > p->len += len; > + skb_shinfo(p)->flags |= skbinfo->flags & SKBFL_SHARED_FRAG; > if (lp != p) { > lp->data_len += len; > lp->truesize += delta_truesize; > lp->len += len; > + skb_shinfo(lp)->flags |= skbinfo->flags & SKBFL_SHARED_FRAG; > } > NAPI_GRO_CB(skb)->same_flow = 1; > return 0; > @@ -244,6 +246,8 @@ int skb_gro_receive_list(struct sk_buff *p, struct sk_buff *skb) > p->truesize += skb->truesize; > p->len += skb->len; > > + skb_shinfo(p)->flags |= skb_shinfo(skb)->flags & SKBFL_SHARED_FRAG; > + > NAPI_GRO_CB(skb)->same_flow = 1; > > return 0; > diff --git a/net/core/skbuff.c b/net/core/skbuff.c > index 9c4e8d331d6d..7cd388504297 100644 > --- a/net/core/skbuff.c > +++ b/net/core/skbuff.c > @@ -2248,6 +2248,7 @@ struct sk_buff *__pskb_copy_fclone(struct sk_buff *skb, int headroom, > skb_frag_ref(skb, i); > } > skb_shinfo(n)->nr_frags = i; > + skb_shinfo(n)->flags |= skb_shinfo(skb)->flags & SKBFL_SHARED_FRAG; > } > > if (skb_has_frag_list(skb)) { > @@ -4349,6 +4350,8 @@ int skb_shift(struct sk_buff *tgt, struct sk_buff *skb, int shiftlen) > tgt->ip_summed = CHECKSUM_PARTIAL; > skb->ip_summed = CHECKSUM_PARTIAL; > > + skb_shinfo(tgt)->flags |= skb_shinfo(skb)->flags & SKBFL_SHARED_FRAG; > + > skb_len_add(skb, -shiftlen); > skb_len_add(tgt, shiftlen); > > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c > index f9d8755705f7..6e4bb411dc04 100644 > --- a/net/ipv4/tcp_output.c > +++ b/net/ipv4/tcp_output.c > @@ -2626,6 +2626,7 @@ static int tcp_clone_payload(struct sock *sk, struct sk_buff *to, > todo = min_t(int, skb_frag_size(fragfrom), > probe_size - len); > len += todo; > + skb_shinfo(to)->flags |= skb_shinfo(skb)->flags & SKBFL_SHARED_FRAG; > if (lastfrag && > skb_frag_page(fragfrom) == skb_frag_page(lastfrag) && > skb_frag_off(fragfrom) == skb_frag_off(lastfrag) + > -- > 2.43.0 >