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, 50 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-block-loop b/Documentation/ABI/testing/sysfs-block-loop
new file mode 100644
index 000000000..627f4eb87
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-block-loop
@@ -0,0 +1,50 @@
+What:		/sys/block/loopX/loop/autoclear
+Date:		Aug, 2010
+KernelVersion:	v2.6.37
+Contact:	linux-block@vger.kernel.org
+Description:
+		(RO) Shows if the device is in autoclear mode or not ( "1" or
+		"0"). Autoclear (if set) indicates that the loopback device will
+		self-distruct after last close.
+
+What:		/sys/block/loopX/loop/backing_file
+Date:		Aug, 2010
+KernelVersion:	v2.6.37
+Contact:	linux-block@vger.kernel.org
+Description:
+		(RO) The path of the backing file that the loop device maps its
+		data blocks to.
+
+What:		/sys/block/loopX/loop/offset
+Date:		Aug, 2010
+KernelVersion:	v2.6.37
+Contact:	linux-block@vger.kernel.org
+Description:
+		(RO) Start offset (in bytes).
+
+What:		/sys/block/loopX/loop/sizelimit
+Date:		Aug, 2010
+KernelVersion:	v2.6.37
+Contact:	linux-block@vger.kernel.org
+Description:
+		(RO) The size (in bytes) that the block device maps, starting
+		from the offset.
+
+What:		/sys/block/loopX/loop/partscan
+Date:		Aug, 2011
+KernelVersion:	v3.10
+Contact:	linux-block@vger.kernel.org
+Description:
+		(RO) Shows if automatic partition scanning is enabled for the
+		device or not ("1" or "0"). This can be requested individually
+		per loop device during its setup by setting LO_FLAGS_PARTSCAN in
+		in the ioctl request. By default, no partition tables are
+		scanned.
+
+What:		/sys/block/loopX/loop/dio
+Date:		Aug, 2015
+KernelVersion:	v4.10
+Contact:	linux-block@vger.kernel.org
+Description:
+		(RO) Shows if direct IO is being used to access backing file or
+		not ("1 or "0").