drivers: char: mem: Check {read,write}_kmem() addresses

Arriving at read_kmem() with an offset representing a bogus kernel
address (e.g. 0 from a simple "cat /dev/kmem") leads to copy_to_user
faulting on the kernel-side read.

x86_64 happens to get away with this since the optimised implementation
uses "rep movs*", thus the user write (which is allowed to fault) and
the kernel read are the same instruction, the kernel-side fault falls
into the user-side fixup handler and the chain of events which
transpires ends up returning an error as one might expect, even if it's
an inappropriate -EFAULT. On other architectures, though, the read is
not covered by the fixup entry for the write, and we get a big scary
"Unable to hande kernel paging request..." dump.

The more typical use-case of mmap_kmem() has always (within living
memory at least) returned -EIO for addresses which don't satisfy
pfn_valid(), so let's make that consistent across {read,write}_kem()
too.
Reported-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NRobin Murphy <robin.murphy@arm.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/mem.c

@@ -381,6 +381,9 @@ static ssize_t read_kmem(struct file *file, char __user *buf,
 	char *kbuf; /* k-addr because vread() takes vmlist_lock rwlock */
 	int err = 0;
 
+	if (!pfn_valid(PFN_DOWN(p)))
+		return -EIO;
+
 	read = 0;
 	if (p < (unsigned long) high_memory) {
 		low_count = count;
@@ -509,6 +512,9 @@ static ssize_t write_kmem(struct file *file, const char __user *buf,
 	char *kbuf; /* k-addr because vwrite() takes vmlist_lock rwlock */
 	int err = 0;
 
+	if (!pfn_valid(PFN_DOWN(p)))
+		return -EIO;
+
 	if (p < (unsigned long) high_memory) {
 		unsigned long to_write = min_t(unsigned long, count,
 					       (unsigned long)high_memory - p);