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, 84 insertions, 0 deletions

diff --git a/Documentation/admin-guide/sysctl/user.rst b/Documentation/admin-guide/sysctl/user.rst
new file mode 100644
index 000000000..c45824589
--- /dev/null
+++ b/Documentation/admin-guide/sysctl/user.rst
@@ -0,0 +1,84 @@
+=================================
+Documentation for /proc/sys/user/
+=================================
+
+kernel version 4.9.0
+
+Copyright (c) 2016		Eric Biederman <ebiederm@xmission.com>
+
+------------------------------------------------------------------------------
+
+This file contains the documentation for the sysctl files in
+/proc/sys/user.
+
+The files in this directory can be used to override the default
+limits on the number of namespaces and other objects that have
+per user per user namespace limits.
+
+The primary purpose of these limits is to stop programs that
+malfunction and attempt to create a ridiculous number of objects,
+before the malfunction becomes a system wide problem.  It is the
+intention that the defaults of these limits are set high enough that
+no program in normal operation should run into these limits.
+
+The creation of per user per user namespace objects are charged to
+the user in the user namespace who created the object and
+verified to be below the per user limit in that user namespace.
+
+The creation of objects is also charged to all of the users
+who created user namespaces the creation of the object happens
+in (user namespaces can be nested) and verified to be below the per user
+limits in the user namespaces of those users.
+
+This recursive counting of created objects ensures that creating a
+user namespace does not allow a user to escape their current limits.
+
+Currently, these files are in /proc/sys/user:
+
+max_cgroup_namespaces
+=====================
+
+  The maximum number of cgroup namespaces that any user in the current
+  user namespace may create.
+
+max_ipc_namespaces
+==================
+
+  The maximum number of ipc namespaces that any user in the current
+  user namespace may create.
+
+max_mnt_namespaces
+==================
+
+  The maximum number of mount namespaces that any user in the current
+  user namespace may create.
+
+max_net_namespaces
+==================
+
+  The maximum number of network namespaces that any user in the
+  current user namespace may create.
+
+max_pid_namespaces
+==================
+
+  The maximum number of pid namespaces that any user in the current
+  user namespace may create.
+
+max_time_namespaces
+===================
+
+  The maximum number of time namespaces that any user in the current
+  user namespace may create.
+
+max_user_namespaces
+===================
+
+  The maximum number of user namespaces that any user in the current
+  user namespace may create.
+
+max_uts_namespaces
+==================
+
+  The maximum number of user namespaces that any user in the current
+  user namespace may create.