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, 185 insertions, 0 deletions

diff --git a/fs/kernel_read_file.c b/fs/kernel_read_file.c
new file mode 100644
index 000000000..de32c95d8
--- /dev/null
+++ b/fs/kernel_read_file.c
@@ -0,0 +1,185 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/fs.h>
+#include <linux/fs_struct.h>
+#include <linux/kernel_read_file.h>
+#include <linux/security.h>
+#include <linux/vmalloc.h>
+
+/**
+ * kernel_read_file() - read file contents into a kernel buffer
+ *
+ * @file:	file to read from
+ * @offset:	where to start reading from (see below).
+ * @buf:	pointer to a "void *" buffer for reading into (if
+ *		*@buf is NULL, a buffer will be allocated, and
+ *		@buf_size will be ignored)
+ * @buf_size:	size of buf, if already allocated. If @buf not
+ *		allocated, this is the largest size to allocate.
+ * @file_size:	if non-NULL, the full size of @file will be
+ *		written here.
+ * @id:		the kernel_read_file_id identifying the type of
+ *		file contents being read (for LSMs to examine)
+ *
+ * @offset must be 0 unless both @buf and @file_size are non-NULL
+ * (i.e. the caller must be expecting to read partial file contents
+ * via an already-allocated @buf, in at most @buf_size chunks, and
+ * will be able to determine when the entire file was read by
+ * checking @file_size). This isn't a recommended way to read a
+ * file, though, since it is possible that the contents might
+ * change between calls to kernel_read_file().
+ *
+ * Returns number of bytes read (no single read will be bigger
+ * than SSIZE_MAX), or negative on error.
+ *
+ */
+ssize_t kernel_read_file(struct file *file, loff_t offset, void **buf,
+			 size_t buf_size, size_t *file_size,
+			 enum kernel_read_file_id id)
+{
+	loff_t i_size, pos;
+	ssize_t copied;
+	void *allocated = NULL;
+	bool whole_file;
+	int ret;
+
+	if (offset != 0 && (!*buf || !file_size))
+		return -EINVAL;
+
+	if (!S_ISREG(file_inode(file)->i_mode))
+		return -EINVAL;
+
+	ret = deny_write_access(file);
+	if (ret)
+		return ret;
+
+	i_size = i_size_read(file_inode(file));
+	if (i_size <= 0) {
+		ret = -EINVAL;
+		goto out;
+	}
+	/* The file is too big for sane activities. */
+	if (i_size > SSIZE_MAX) {
+		ret = -EFBIG;
+		goto out;
+	}
+	/* The entire file cannot be read in one buffer. */
+	if (!file_size && offset == 0 && i_size > buf_size) {
+		ret = -EFBIG;
+		goto out;
+	}
+
+	whole_file = (offset == 0 && i_size <= buf_size);
+	ret = security_kernel_read_file(file, id, whole_file);
+	if (ret)
+		goto out;
+
+	if (file_size)
+		*file_size = i_size;
+
+	if (!*buf)
+		*buf = allocated = vmalloc(i_size);
+	if (!*buf) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	pos = offset;
+	copied = 0;
+	while (copied < buf_size) {
+		ssize_t bytes;
+		size_t wanted = min_t(size_t, buf_size - copied,
+					      i_size - pos);
+
+		bytes = kernel_read(file, *buf + copied, wanted, &pos);
+		if (bytes < 0) {
+			ret = bytes;
+			goto out_free;
+		}
+
+		if (bytes == 0)
+			break;
+		copied += bytes;
+	}
+
+	if (whole_file) {
+		if (pos != i_size) {
+			ret = -EIO;
+			goto out_free;
+		}
+
+		ret = security_kernel_post_read_file(file, *buf, i_size, id);
+	}
+
+out_free:
+	if (ret < 0) {
+		if (allocated) {
+			vfree(*buf);
+			*buf = NULL;
+		}
+	}
+
+out:
+	allow_write_access(file);
+	return ret == 0 ? copied : ret;
+}
+EXPORT_SYMBOL_GPL(kernel_read_file);
+
+ssize_t kernel_read_file_from_path(const char *path, loff_t offset, void **buf,
+				   size_t buf_size, size_t *file_size,
+				   enum kernel_read_file_id id)
+{
+	struct file *file;
+	ssize_t ret;
+
+	if (!path || !*path)
+		return -EINVAL;
+
+	file = filp_open(path, O_RDONLY, 0);
+	if (IS_ERR(file))
+		return PTR_ERR(file);
+
+	ret = kernel_read_file(file, offset, buf, buf_size, file_size, id);
+	fput(file);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(kernel_read_file_from_path);
+
+ssize_t kernel_read_file_from_path_initns(const char *path, loff_t offset,
+					  void **buf, size_t buf_size,
+					  size_t *file_size,
+					  enum kernel_read_file_id id)
+{
+	struct file *file;
+	struct path root;
+	ssize_t ret;
+
+	if (!path || !*path)
+		return -EINVAL;
+
+	task_lock(&init_task);
+	get_fs_root(init_task.fs, &root);
+	task_unlock(&init_task);
+
+	file = file_open_root(&root, path, O_RDONLY, 0);
+	path_put(&root);
+	if (IS_ERR(file))
+		return PTR_ERR(file);
+
+	ret = kernel_read_file(file, offset, buf, buf_size, file_size, id);
+	fput(file);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(kernel_read_file_from_path_initns);
+
+ssize_t kernel_read_file_from_fd(int fd, loff_t offset, void **buf,
+				 size_t buf_size, size_t *file_size,
+				 enum kernel_read_file_id id)
+{
+	CLASS(fd, f)(fd);
+
+	if (fd_empty(f) || !(fd_file(f)->f_mode & FMODE_READ))
+		return -EBADF;
+
+	return kernel_read_file(fd_file(f), offset, buf, buf_size, file_size, id);
+}
+EXPORT_SYMBOL_GPL(kernel_read_file_from_fd);