[PATCH net v5 1/1] net/sched: fix pedit partial COW leading to page cache corruption

[Jamal Hadi Salim <jhs@mojatatu.com>]

From: Rajat Gupta <rajat.gupta@oss.qualcomm.com>

tcf_pedit_act() computes the COW range for skb_ensure_writable()
once before the key loop using tcfp_off_max_hint, but the hint does
not account for the runtime header offset added by typed keys. This
can leave part of the write region un-COW'd.

Fix by moving skb_ensure_writable() inside the per-key loop where
the actual write offset is known, and add overflow checking on the
offset arithmetic. For negative offsets (e.g. Ethernet header edits
at ingress), use skb_cow() to COW the headroom instead. Guard
offset_valid() against INT_MIN, where negation is undefined.

Fixes: 8b796475fd78 ("net/sched: act_pedit: really ensure the skb is writable")
Reported-by: Yiming Qian <yimingqian591@gmail.com>
Reported-by: Keenan Dong <keenanat2000@gmail.com>
Reported-by: Han Guidong <2045gemini@gmail.com>
Reported-by: Zhang Cen <rollkingzzc@gmail.com>
Reviewed-by: Han Guidong <2045gemini@gmail.com>
Tested-by: Han Guidong <2045gemini@gmail.com>
Reviewed-by: Davide Caratti <dcaratti@redhat.com>
Tested-by: Davide Caratti <dcaratti@redhat.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Tested-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Tested-by: Victor Nogueira <victor@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Rajat Gupta <rajat.gupta@oss.qualcomm.com>
---
v4->v5:
 1) Dead code removal obsoleted after removing tcfp_off_max_hint as identified
    by sashiko-claude[1]. Remove dead code updating "cur".
 2) Addition of hoffset at tkey->at promotes hoffset to unsigned int,
    wraps on overflow. Claim made by both sashiko-claude[1] and sashiko-gemini[2]
    I think this is far fetched but possible. Cast tkey->at to int.
 3) Signedness mismatch in min() macro may potentially cause build issues.
    Claim made by both sashiko-claude[1] and sashiko-gemini[2].
    Go back to min_t() for now. David L. can make a better proposal later..

[1]https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260530080643.1345521-1-jhs%40mojatatu.com
[2]https://sashiko.dev/#/patchset/20260530080643.1345521-1-jhs%40mojatatu.com
---
 include/net/tc_act/tc_pedit.h |  1 -
 net/sched/act_pedit.c         | 77 +++++++++++++++++++----------------
 2 files changed, 41 insertions(+), 37 deletions(-)

diff --git a/include/net/tc_act/tc_pedit.h b/include/net/tc_act/tc_pedit.h
index f58ee15cd858..cb7b82f2cbc7 100644
--- a/include/net/tc_act/tc_pedit.h
+++ b/include/net/tc_act/tc_pedit.h
@@ -15,7 +15,6 @@ struct tcf_pedit_parms {
 	struct tc_pedit_key	*tcfp_keys;
 	struct tcf_pedit_key_ex	*tcfp_keys_ex;
 	int action;
-	u32 tcfp_off_max_hint;
 	unsigned char tcfp_nkeys;
 	unsigned char tcfp_flags;
 	struct rcu_head rcu;
diff --git a/net/sched/act_pedit.c b/net/sched/act_pedit.c
index bc20f08a2789..bd3b1da3cd63 100644
--- a/net/sched/act_pedit.c
+++ b/net/sched/act_pedit.c
@@ -16,6 +16,8 @@
 #include <linux/ip.h>
 #include <linux/ipv6.h>
 #include <linux/slab.h>
+#include <linux/overflow.h>
+#include <linux/unaligned.h>
 #include <net/ipv6.h>
 #include <net/netlink.h>
 #include <net/pkt_sched.h>
@@ -242,7 +244,6 @@ static int tcf_pedit_init(struct net *net, struct nlattr *nla,
 		goto out_free_ex;
 	}
 
-	nparms->tcfp_off_max_hint = 0;
 	nparms->tcfp_flags = parm->flags;
 	nparms->tcfp_nkeys = parm->nkeys;
 
@@ -268,14 +269,6 @@ static int tcf_pedit_init(struct net *net, struct nlattr *nla,
 						   BITS_PER_TYPE(int) - 1,
 						   nparms->tcfp_keys[i].shift);
 
-		/* The AT option can read a single byte, we can bound the actual
-		 * value with uchar max.
-		 */
-		cur += (0xff & offmask) >> nparms->tcfp_keys[i].shift;
-
-		/* Each key touches 4 bytes starting from the computed offset */
-		nparms->tcfp_off_max_hint =
-			max(nparms->tcfp_off_max_hint, cur + 4);
 	}
 
 	p = to_pedit(*a);
@@ -318,15 +311,12 @@ static void tcf_pedit_cleanup(struct tc_action *a)
 		call_rcu(&parms->rcu, tcf_pedit_cleanup_rcu);
 }
 
-static bool offset_valid(struct sk_buff *skb, int offset)
+static bool offset_valid(struct sk_buff *skb, int offset, int len)
 {
-	if (offset > 0 && offset > skb->len)
-		return false;
-
-	if  (offset < 0 && -offset > skb_headroom(skb))
+	if (offset < -(int)skb_headroom(skb))
 		return false;
 
-	return true;
+	return offset <= (int)skb->len - len;
 }
 
 static int pedit_l4_skb_offset(struct sk_buff *skb, int *hoffset, const int header_type)
@@ -393,18 +383,10 @@ TC_INDIRECT_SCOPE int tcf_pedit_act(struct sk_buff *skb,
 	struct tcf_pedit_key_ex *tkey_ex;
 	struct tcf_pedit_parms *parms;
 	struct tc_pedit_key *tkey;
-	u32 max_offset;
 	int i;
 
 	parms = rcu_dereference_bh(p->parms);
 
-	max_offset = (skb_transport_header_was_set(skb) ?
-		      skb_transport_offset(skb) :
-		      skb_network_offset(skb)) +
-		     parms->tcfp_off_max_hint;
-	if (skb_ensure_writable(skb, min(skb->len, max_offset)))
-		goto done;
-
 	tcf_lastuse_update(&p->tcf_tm);
 	tcf_action_update_bstats(&p->common, skb);
 
@@ -412,10 +394,11 @@ TC_INDIRECT_SCOPE int tcf_pedit_act(struct sk_buff *skb,
 	tkey_ex = parms->tcfp_keys_ex;
 
 	for (i = parms->tcfp_nkeys; i > 0; i--, tkey++) {
+		int write_offset, write_len;
 		int offset = tkey->off;
 		int hoffset = 0;
-		u32 *ptr, hdata;
-		u32 val;
+		u32 cur_val, val;
+		u32 *ptr;
 		int rc;
 
 		if (tkey_ex) {
@@ -433,13 +416,15 @@ TC_INDIRECT_SCOPE int tcf_pedit_act(struct sk_buff *skb,
 
 		if (tkey->offmask) {
 			u8 *d, _d;
+			int at_offset;
 
-			if (!offset_valid(skb, hoffset + tkey->at)) {
+			if (check_add_overflow(hoffset, (int)tkey->at, &at_offset) ||
+			    !offset_valid(skb, at_offset, sizeof(_d))) {
 				pr_info_ratelimited("tc action pedit 'at' offset %d out of bounds\n",
 						    hoffset + tkey->at);
 				goto bad;
 			}
-			d = skb_header_pointer(skb, hoffset + tkey->at,
+			d = skb_header_pointer(skb, at_offset,
 					       sizeof(_d), &_d);
 			if (!d)
 				goto bad;
@@ -451,31 +436,51 @@ TC_INDIRECT_SCOPE int tcf_pedit_act(struct sk_buff *skb,
 			}
 		}
 
-		if (!offset_valid(skb, hoffset + offset)) {
-			pr_info_ratelimited("tc action pedit offset %d out of bounds\n", hoffset + offset);
+		if (check_add_overflow(hoffset, offset, &write_offset)) {
+			pr_info_ratelimited("tc action pedit offset overflow\n");
 			goto bad;
 		}
 
-		ptr = skb_header_pointer(skb, hoffset + offset,
-					 sizeof(hdata), &hdata);
-		if (!ptr)
+		if (!offset_valid(skb, write_offset, sizeof(*ptr))) {
+			pr_info_ratelimited("tc action pedit offset %d out of bounds\n",
+					    write_offset);
 			goto bad;
+		}
+
+		if (write_offset < 0) {
+			if (skb_cow(skb, -write_offset))
+				goto bad;
+			if (write_offset + (int)sizeof(*ptr) > 0) {
+				if (skb_ensure_writable(skb,
+							min_t(int, skb->len,
+							      write_offset + (int)sizeof(*ptr))))
+					goto bad;
+			}
+		} else {
+			if (check_add_overflow(write_offset, (int)sizeof(*ptr),
+					       &write_len))
+				goto bad;
+			if (skb_ensure_writable(skb, min_t(int, skb->len,
+							   write_len)))
+				goto bad;
+		}
+
+		ptr = (u32 *)(skb->data + write_offset);
+		cur_val = get_unaligned(ptr);
 		/* just do it, baby */
 		switch (cmd) {
 		case TCA_PEDIT_KEY_EX_CMD_SET:
 			val = tkey->val;
 			break;
 		case TCA_PEDIT_KEY_EX_CMD_ADD:
-			val = (*ptr + tkey->val) & ~tkey->mask;
+			val = (cur_val + tkey->val) & ~tkey->mask;
 			break;
 		default:
 			pr_info_ratelimited("tc action pedit bad command (%d)\n", cmd);
 			goto bad;
 		}
 
-		*ptr = ((*ptr & tkey->mask) ^ val);
-		if (ptr == &hdata)
-			skb_store_bits(skb, hoffset + offset, ptr, 4);
+		put_unaligned((cur_val & tkey->mask) ^ val, ptr);
 	}
 
 	goto done;
-- 
2.34.1