ARM: 9485/1: mm: acquire mmap write lock around show_pte() for user faultsgrafted

When CONFIG_DEBUG_USER=y, and cmdline "user_debug=31" is set,
a user fault may trigger show_pte() without any lock.
If another thread in the same process concurrently calls munmap(),
the page table pages may be freed while show_pte() is still traversing
them, causing a use-after-free in show_pte().

If CONFIG_ARM_LPAE=y, this may cause a kernel panic if the pages table
of PMD are freed when show_pte() is running.

Acquire mmap_write_lock() around show_pte() for user faults to fix the
contention.

For user faults, additionally restrict that show_pte() is called only
when the addr is a user-space address (addr < TASK_SIZE). This is because
the lock of tsk->mm only protects the virtual memory of user address space,
furthermore, dumping the page tables of a kernel-space address for user
faults is unnecessary and may have security implications.

Keep everything unchanged for kernel faults, because the kernel is
already in the "oops" state, acquiring a lock may risk a deadlock.

Co-developed-by: Qi Xi <xiqi2@huawei.com>

Fixes: 6d021b724481 ("ARM: dump pgd, pmd and pte states on unhandled data abort faults")
Link: https://lore.kernel.org/20260716014022.2823-1-xieyuanbin1@huawei.com
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Signed-off-by: Qi Xi <xiqi2@huawei.com>
Signed-off-by: Xie Yuanbin <xieyuanbin1@huawei.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

1 files changed, 53 insertions, 0 deletions

diff --git a/include/net/dsfield.h b/include/net/dsfield.h
new file mode 100644
index 000000000..a59a57ffc
--- /dev/null
+++ b/include/net/dsfield.h
@@ -0,0 +1,53 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* include/net/dsfield.h - Manipulation of the Differentiated Services field */
+
+/* Written 1998-2000 by Werner Almesberger, EPFL ICA */
+
+
+#ifndef __NET_DSFIELD_H
+#define __NET_DSFIELD_H
+
+#include <linux/types.h>
+#include <linux/ip.h>
+#include <linux/ipv6.h>
+#include <asm/byteorder.h>
+
+
+static inline __u8 ipv4_get_dsfield(const struct iphdr *iph)
+{
+	return iph->tos;
+}
+
+
+static inline __u8 ipv6_get_dsfield(const struct ipv6hdr *ipv6h)
+{
+	return ntohs(*(__force const __be16 *)ipv6h) >> 4;
+}
+
+
+static inline void ipv4_change_dsfield(struct iphdr *iph,__u8 mask,
+    __u8 value)
+{
+        __u32 check = ntohs((__force __be16)iph->check);
+	__u8 dsfield;
+
+	dsfield = (iph->tos & mask) | value;
+	check += iph->tos;
+	if ((check+1) >> 16) check = (check+1) & 0xffff;
+	check -= dsfield;
+	check += check >> 16; /* adjust carry */
+	iph->check = (__force __sum16)htons(check);
+	iph->tos = dsfield;
+}
+
+
+static inline void ipv6_change_dsfield(struct ipv6hdr *ipv6h,__u8 mask,
+    __u8 value)
+{
+	__be16 *p = (__force __be16 *)ipv6h;
+
+	*p = (*p & htons((((u16)mask << 4) | 0xf00f))) | htons((u16)value << 4);
+}
+
+
+#endif