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

[Robin Murphy]

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-space 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 userspace fixup handler and a chain of events transpires
leading to returning the expected -EFAULT. On other architectures,
though, the read is not covered by the fixup entry for the write, and we
get a straightforward "Unable to hande kernel paging request..." dump.

The more typical use-case of mmap_kmem() already validates the address
with pfn_valid() as one might expect, so let's make that consistent
across {read,write}_kem() too.

Reported-by: Kefeng Wang <wangkefeng.wang at huawei.com>
Signed-off-by: Robin Murphy <robin.murphy at arm.com>
---

I'm not sure if this warrants going to stable or not, as it's really
just making an existing failure case more graceful and less confusing.

 drivers/char/mem.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 71025c2f6bbb..64c766023b15 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -384,6 +384,9 @@ 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 -EFAULT;
+
 	read = 0;
 	if (p < (unsigned long) high_memory) {
 		low_count = count;
@@ -512,6 +515,9 @@ 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 -EFAULT;
+
 	if (p < (unsigned long) high_memory) {
 		unsigned long to_write = min_t(unsigned long, count,
 					       (unsigned long)high_memory - p);
-- 
2.8.1.dirty