net: skbuff: propagate shared-frag marker through frag-transfer helpers

commit 48f6a5356a33dd78e7144ae1faef95ffc990aae0 upstream.

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 <local>' 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.

The same omission exists in skb_segment(): the per-iteration flag
merge takes only head_skb's flag, and the inner switch that rebinds
frag_skb to list_skb on head_skb-frags exhaustion does not fold the
new frag_skb's flag into nskb.  Fold frag_skb's flag at both sites
so segments drawing frags from frag_list members carry the marker.

Fixes: cef401de7b ("net: fix possible wrong checksum generation")
Fixes: f4c50a4034e6 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
Suggested-by: Sabrina Dubroca <sd@queasysnail.net>
Suggested-by: Sultan Alsawaf <sultan@kerneltoast.com>
Suggested-by: Ben Hutchings <ben@decadent.org.uk>
Suggested-by: Lin Ma <malin89@huawei.com>
Suggested-by: Jingguo Tan <tanjingguo@huawei.com>
Suggested-by: Aaron Esau <aaron1esau@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Tested-by: Rajat Gupta <rajat.gupta@oss.qualcomm.com>
Link: https://patch.msgid.link/ageeJfJHwgzmKXbh@v4bel
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
[bwh: Backported to 5.10:
 - Set the SKBTX_SHARED_FRAG flag in skb_shared_info::tx_flags,
   instead of SKBFL_SHARED_FRAG in skb_shared_info::flags
 - skb_gro_receive() and skb_gro_receive_list() are in skbuff.c here
 - Drop change to tcp_clone_payload(), which does not exist here
 - Adjust context in skb_shift()
]
Signed-off-by: Ben Hutchings <benh@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[uli: backport to 4.19]
Signed-off-by: Ulrich Hecht <uli@kernel.org>
Reviewed-by: Pavel Machek <pavel@nabladev.com>
Reviewed-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.x90@mail.toshiba>

net/core/skbuff.c

@@ -1425,6 +1425,7 @@ 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)->tx_flags |= skb_shinfo(skb)->tx_flags & SKBTX_SHARED_FRAG;
 	}
 
 	if (skb_has_frag_list(skb)) {
@@ -3240,6 +3241,8 @@ onlymerged:
 	tgt->ip_summed = CHECKSUM_PARTIAL;
 	skb->ip_summed = CHECKSUM_PARTIAL;
 
+	skb_shinfo(tgt)->tx_flags |= skb_shinfo(skb)->tx_flags & SKBTX_SHARED_FRAG;
+
 	/* Yak, is it really working this way? Some helper please? */
 	skb->len -= shiftlen;
 	skb->data_len -= shiftlen;
@@ -3754,7 +3757,8 @@ normal:
 		skb_copy_from_linear_data_offset(head_skb, offset,
 						 skb_put(nskb, hsize), hsize);
 
-		skb_shinfo(nskb)->tx_flags |= skb_shinfo(head_skb)->tx_flags &
+		skb_shinfo(nskb)->tx_flags |= (skb_shinfo(head_skb)->tx_flags |
+					       skb_shinfo(frag_skb)->tx_flags) &
 					      SKBTX_SHARED_FRAG;
 
 		if (skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))
@@ -3771,6 +3775,9 @@ normal:
 				nfrags = skb_shinfo(list_skb)->nr_frags;
 				frag = skb_shinfo(list_skb)->frags;
 				frag_skb = list_skb;
+
+				skb_shinfo(nskb)->tx_flags |= skb_shinfo(frag_skb)->tx_flags & SKBTX_SHARED_FRAG;
+
 				if (!skb_headlen(list_skb)) {
 					BUG_ON(!nfrags);
 				} else {
@@ -3987,10 +3994,12 @@ done:
 	p->data_len += len;
 	p->truesize += delta_truesize;
 	p->len += len;
+	skb_shinfo(p)->tx_flags |= skbinfo->tx_flags & SKBTX_SHARED_FRAG;
 	if (lp != p) {
 		lp->data_len += len;
 		lp->truesize += delta_truesize;
 		lp->len += len;
+		skb_shinfo(lp)->tx_flags |= skbinfo->tx_flags & SKBTX_SHARED_FRAG;
 	}
 	NAPI_GRO_CB(skb)->same_flow = 1;
 	return 0;