[PATCH v2] drivers: char: mem: Fix thinkos in kmem address checks

[Robin Murphy]

When borrowing the pfn_valid() check from mmap_kmem(), somebody managed
to get physical and virtual addresses spectacularly muddled up, such
that we've ended up with checks for one being the other. Whilst this
does indeed prevent out-of-bounds accesses crashing, on most systems
it also prevents the more desirable use-case of working at all ever.

Check the *virtual* offset correctly for what it is. Furthermore, do
so in the right place - a read or write may span multiple pages, so a
single up-front check is insufficient. High memory accesses already
have a similar validity check just before the copy_to_user() call, so
just make the low memory path fully consistent with that.

Reported-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
CC: stable@xxxxxxxxxxxxxxx
Fixes: 148a1bc84398 ("drivers: char: mem: Check {read,write}_kmem() addresses")
Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
---

Third time lucky... And if there's some other problem with this one then
I guess we may as well just go ahead with Jason's revert, forget the whole
thing, and let 'cat /dev/kmem' go back to crashing on non-x86 :)

Robin.

 drivers/char/mem.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 5bb1985ec484..6d9cc2d39d22 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -381,9 +381,6 @@ 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;
@@ -412,6 +409,8 @@ static ssize_t read_kmem(struct file *file, char __user *buf,
 			 * by the kernel or data corruption may occur
 			 */
 			kbuf = xlate_dev_kmem_ptr((void *)p);
+			if (!virt_addr_valid(kbuf))
+				return -ENXIO;
 
 			if (copy_to_user(buf, kbuf, sz))
 				return -EFAULT;
@@ -482,6 +481,8 @@ static ssize_t do_write_kmem(unsigned long p, const char __user *buf,
 		 * corruption may occur.
 		 */
 		ptr = xlate_dev_kmem_ptr((void *)p);
+		if (!virt_addr_valid(ptr))
+			return -ENXIO;
 
 		copied = copy_from_user(ptr, buf, sz);
 		if (copied) {
@@ -512,9 +513,6 @@ 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);
-- 
2.10.2.dirty