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netfilter: x_tables: avoid leaking percpu counter pointers

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The native and compat get-entries paths copy the fixed rule entry header
from the kernelized rule blob to userspace before overwriting the entry's
counter fields with a sanitized counter snapshot.

On SMP kernels, entry->counters.pcnt contains the percpu allocation
address used by x_tables rule counters. A caller can provide a userspace
buffer that faults during the initial fixed-header copy after pcnt has
been copied but before the later sanitized counter copy runs. The syscall
then returns -EFAULT while leaving the raw percpu pointer in userspace.

Copy only the fixed entry prefix before counters from the kernelized rule
blob, then copy the sanitized counter snapshot into the counter field.
Apply this ordering to the IPv4, IPv6, and ARP native and compat
get-entries implementations so a fault cannot expose the internal percpu
counter pointer.

Fixes: 71ae0dff02 ("netfilter: xtables: use percpu rule counters")
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
This commit is contained in:
Kyle Zeng 2026-06-06 01:10:31 -07:00 • committed by Pablo Neira Ayuso
commit f7f2fbb0e8
3 changed files with 18 additions and 27 deletions
Download patch file Download diff file Expand all files Collapse all files

15
net/ipv4/netfilter/arp_tables.c
View file

@ -702,14 +702,12 @@ static int copy_entries_to_user(unsigned int total_size,
const struct xt_entry_target *t;
e = loc_cpu_entry + off;
if (copy_to_user(userptr + off, e, sizeof(*e))) {
ret = -EFAULT;
goto free_counters;
}
if (copy_to_user(userptr + off
if (copy_to_user(userptr + off, e,
offsetof(struct arpt_entry, counters)) ||
copy_to_user(userptr + off
+ offsetof(struct arpt_entry, counters),
&counters[num],
sizeof(counters[num])) != 0) {
sizeof(counters[num]))) {
ret = -EFAULT;
goto free_counters;
}
@ -1327,9 +1325,8 @@ static int compat_copy_entry_to_user(struct arpt_entry *e, void __user **dstptr,
origsize = *size;
ce = *dstptr;
if (copy_to_user(ce, e, sizeof(struct arpt_entry)) != 0 ||
copy_to_user(&ce->counters, &counters[i],
sizeof(counters[i])) != 0)
if (copy_to_user(ce, e, offsetof(struct compat_arpt_entry, counters)) ||
copy_to_user(&ce->counters, &counters[i], sizeof(counters[i])))
return -EFAULT;
*dstptr += sizeof(struct compat_arpt_entry);

15
net/ipv4/netfilter/ip_tables.c
View file

@ -832,14 +832,12 @@ copy_entries_to_user(unsigned int total_size,
const struct xt_entry_target *t;
e = loc_cpu_entry + off;
if (copy_to_user(userptr + off, e, sizeof(*e))) {
ret = -EFAULT;
goto free_counters;
}
if (copy_to_user(userptr + off
if (copy_to_user(userptr + off, e,
offsetof(struct ipt_entry, counters)) ||
copy_to_user(userptr + off
+ offsetof(struct ipt_entry, counters),
&counters[num],
sizeof(counters[num])) != 0) {
sizeof(counters[num]))) {
ret = -EFAULT;
goto free_counters;
}
@ -1228,9 +1226,8 @@ compat_copy_entry_to_user(struct ipt_entry *e, void __user **dstptr,
origsize = *size;
ce = *dstptr;
if (copy_to_user(ce, e, sizeof(struct ipt_entry)) != 0 ||
copy_to_user(&ce->counters, &counters[i],
sizeof(counters[i])) != 0)
if (copy_to_user(ce, e, offsetof(struct compat_ipt_entry, counters)) ||
copy_to_user(&ce->counters, &counters[i], sizeof(counters[i])))
return -EFAULT;
*dstptr += sizeof(struct compat_ipt_entry);

15
net/ipv6/netfilter/ip6_tables.c
View file

@ -848,14 +848,12 @@ copy_entries_to_user(unsigned int total_size,
const struct xt_entry_target *t;
e = loc_cpu_entry + off;
if (copy_to_user(userptr + off, e, sizeof(*e))) {
ret = -EFAULT;
goto free_counters;
}
if (copy_to_user(userptr + off
if (copy_to_user(userptr + off, e,
offsetof(struct ip6t_entry, counters)) ||
copy_to_user(userptr + off
+ offsetof(struct ip6t_entry, counters),
&counters[num],
sizeof(counters[num])) != 0) {
sizeof(counters[num]))) {
ret = -EFAULT;
goto free_counters;
}
@ -1244,9 +1242,8 @@ compat_copy_entry_to_user(struct ip6t_entry *e, void __user **dstptr,
origsize = *size;
ce = *dstptr;
if (copy_to_user(ce, e, sizeof(struct ip6t_entry)) != 0 ||
copy_to_user(&ce->counters, &counters[i],
sizeof(counters[i])) != 0)
if (copy_to_user(ce, e, offsetof(struct compat_ip6t_entry, counters)) ||
copy_to_user(&ce->counters, &counters[i], sizeof(counters[i])))
return -EFAULT;
*dstptr += sizeof(struct compat_ip6t_entry);