Why can user virtual addresses sometimes be dereferenced in the kernel?

[Timur Tabi <timur.tabi@xxxxxxxxxxx>]

I recently discovered in a driver I'm working on that there are several 
places where a user-space virtual address is being dereferenced 
successfully.  I was under the impression that that should not work - 
user pointers should not be usuable in kernel space.  The only reason I 
discovered this is because in one particular kernel (Fedora Core 2), one 
specific dereference did cause an oops.  Why this kernel is special is 
another question altogether.

So my question is: why did this ever work?  My theory is that the 
process' page tables are still in the MMU when the driver is called, so 
the virtual address space is technically still valid, even if the kernel 
won't admit it.  Am I right?  If so, are the page tables not flushed 
because it would affect performance?  Is there a way I can manual flush 
the page tables in my driver in order to find all the places in my code 
where user virtual addresses are used?

--
Timur Tabi
Staff Software Engineer
timur.tabi@xxxxxxxxxxx

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