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, 127 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/dev-kmsg b/Documentation/ABI/testing/dev-kmsg
new file mode 100644
index 000000000..a377b6c09
--- /dev/null
+++ b/Documentation/ABI/testing/dev-kmsg
@@ -0,0 +1,127 @@
+What:		/dev/kmsg
+Date:		Mai 2012
+KernelVersion:	3.5
+Contact:	Kay Sievers <kay@vrfy.org>
+Description:	The /dev/kmsg character device node provides userspace access
+		to the kernel's printk buffer.
+
+		Injecting messages:
+
+		Every write() to the opened device node places a log entry in
+		the kernel's printk buffer.
+
+		The logged line can be prefixed with a <N> syslog prefix, which
+		carries the syslog priority and facility. The single decimal
+		prefix number is composed of the 3 lowest bits being the syslog
+		priority and the next 8 bits the syslog facility number.
+
+		If no prefix is given, the priority number is the default kernel
+		log priority and the facility number is set to LOG_USER (1). It
+		is not possible to inject messages from userspace with the
+		facility number LOG_KERN (0), to make sure that the origin of
+		the messages can always be reliably determined.
+
+		Accessing the buffer:
+
+		Every read() from the opened device node receives one record
+		of the kernel's printk buffer.
+
+		The first read() directly following an open() always returns
+		first message in the buffer; there is no kernel-internal
+		persistent state; many readers can concurrently open the device
+		and read from it, without affecting other readers.
+
+		Every read() will receive the next available record. If no more
+		records are available read() will block, or if O_NONBLOCK is
+		used -EAGAIN returned.
+
+		Messages in the record ring buffer get overwritten as whole,
+		there are never partial messages received by read().
+
+		In case messages get overwritten in the circular buffer while
+		the device is kept open, the next read() will return -EPIPE,
+		and the seek position be updated to the next available record.
+		Subsequent reads() will return available records again.
+
+		Unlike the classic syslog() interface, the 64 bit record
+		sequence numbers allow to calculate the amount of lost
+		messages, in case the buffer gets overwritten. And they allow
+		to reconnect to the buffer and reconstruct the read position
+		if needed, without limiting the interface to a single reader.
+
+		The device supports seek with the following parameters:
+
+		SEEK_SET, 0
+		  seek to the first entry in the buffer
+		SEEK_END, 0
+		  seek after the last entry in the buffer
+		SEEK_DATA, 0
+		  seek after the last record available at the time
+		  the last SYSLOG_ACTION_CLEAR was issued.
+
+		Other seek operations or offsets are not supported because of
+		the special behavior this device has. The device allows to read
+		or write only whole variable length messages (records) that are
+		stored in a ring buffer.
+
+		Because of the non-standard behavior also the error values are
+		non-standard. -ESPIPE is returned for non-zero offset. -EINVAL
+		is returned for other operations, e.g. SEEK_CUR. This behavior
+		and values are historical and could not be modified without the
+		risk of breaking userspace.
+
+		The output format consists of a prefix carrying the syslog
+		prefix including priority and facility, the 64 bit message
+		sequence number and the monotonic timestamp in microseconds,
+		and a flag field. All fields are separated by a ','.
+
+		Future extensions might add more comma separated values before
+		the terminating ';'. Unknown fields and values should be
+		gracefully ignored.
+
+		The human readable text string starts directly after the ';'
+		and is terminated by a '\n'. Untrusted values derived from
+		hardware or other facilities are printed, therefore
+		all non-printable characters and '\' itself in the log message
+		are escaped by "\x00" C-style hex encoding.
+
+		A line starting with ' ', is a continuation line, adding
+		key/value pairs to the log message, which provide the machine
+		readable context of the message, for reliable processing in
+		userspace.
+
+		Example::
+
+		  7,160,424069,-;pci_root PNP0A03:00: host bridge window [io  0x0000-0x0cf7] (ignored)
+		   SUBSYSTEM=acpi
+		   DEVICE=+acpi:PNP0A03:00
+		  6,339,5140900,-;NET: Registered protocol family 10
+		  30,340,5690716,-;udevd[80]: starting version 181
+
+		The DEVICE= key uniquely identifies devices the following way:
+
+		  ============  =================
+		  b12:8         block dev_t
+		  c127:3        char dev_t
+		  n8            netdev ifindex
+		  +sound:card0  subsystem:devname
+		  ============  =================
+
+		The flags field carries '-' by default. A 'c' indicates a
+		fragment of a line. Note, that these hints about continuation
+		lines are not necessarily correct, and the stream could be
+		interleaved with unrelated messages, but merging the lines in
+		the output usually produces better human readable results. A
+		similar logic is used internally when messages are printed to
+		the console, /proc/kmsg or the syslog() syscall.
+
+		By default, kernel tries to avoid fragments by concatenating
+		when it can and fragments are rare; however, when extended
+		console support is enabled, the in-kernel concatenation is
+		disabled and /dev/kmsg output will contain more fragments. If
+		the log consumer performs concatenation, the end result
+		should be the same. In the future, the in-kernel concatenation
+		may be removed entirely and /dev/kmsg users are recommended to
+		implement fragment handling.
+
+Users:		dmesg(1), userspace kernel log consumers