Re: [PATCH] vsprintf: Do not break early boot with probing addresses

On Fri 2019-05-10 12:40:58, Steven Rostedt wrote:

On Fri, 10 May 2019 18:32:58 +0200
Martin Schwidefsky [off-list ref] wrote:

quoted

On Fri, 10 May 2019 12:24:01 -0400
Steven Rostedt [off-list ref] wrote:

quoted

On Fri, 10 May 2019 10:42:13 +0200
Petr Mladek [off-list ref] wrote:
  

quoted

 static const char *check_pointer_msg(const void *ptr)
 {
-	char byte;
-
 	if (!ptr)
 		return "(null)";
 
-	if (probe_kernel_address(ptr, byte))
+	if ((unsigned long)ptr < PAGE_SIZE || IS_ERR_VALUE(ptr))
 		return "(efault)";
     

	< PAGE_SIZE ?

do you mean: < TASK_SIZE ?  

The check with < TASK_SIZE would break on s390. The 'ptr' is
in the kernel address space, *not* in the user address space.
Remember s390 has two separate address spaces for kernel/user
the check < TASK_SIZE only makes sense with a __user pointer.

So we allow this to read user addresses? Can't that cause a fault?

I did some quick check and did not found anywhere a user pointer
being dereferenced via vsprintf().

In each case, %s did the check (ptr < PAGE_SIZE) even before this
patchset. The other checks are in %p format modifiers that are
used to print various kernel structures.

Finally, it accesses the pointer directly. I am not completely sure
but I think that it would not work that easily with an address
from the user address space.

Best Regards,
Petr