[PATCH net v5 1/1] net/sched: fix pedit partial COW leading to page cache corruption
From: Jamal Hadi Salim
Date: Sun May 31 2026 - 08:34:28 EST
From: Rajat Gupta <rajat.gupta@xxxxxxxxxxxxxxxx>
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@xxxxxxxxx>
Reported-by: Keenan Dong <keenanat2000@xxxxxxxxx>
Reported-by: Han Guidong <2045gemini@xxxxxxxxx>
Reported-by: Zhang Cen <rollkingzzc@xxxxxxxxx>
Reviewed-by: Han Guidong <2045gemini@xxxxxxxxx>
Tested-by: Han Guidong <2045gemini@xxxxxxxxx>
Reviewed-by: Davide Caratti <dcaratti@xxxxxxxxxx>
Tested-by: Davide Caratti <dcaratti@xxxxxxxxxx>
Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
Tested-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
Reviewed-by: Victor Nogueira <victor@xxxxxxxxxxxx>
Tested-by: Victor Nogueira <victor@xxxxxxxxxxxx>
Acked-by: Jamal Hadi Salim <jhs@xxxxxxxxxxxx>
Signed-off-by: Rajat Gupta <rajat.gupta@xxxxxxxxxxxxxxxx>
---
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