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

[Greg KH]

On Thu, Jan 05, 2017 at 05:15:01PM +0000, Robin Murphy wrote:
> 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>

Jason, did this patch fix your issue?

thanks,

greg k-h

> ---
> 
> 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