Messages in this thread | | | Date | Fri, 10 Apr 2015 20:04:25 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86/uaccess: Implement get_kernel() |
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Fri, Apr 10, 2015 at 4:14 AM, Ingo Molnar <mingo@kernel.org> wrote: > > > >> > >> The next patch will implement efficient > >> __copy_from_kernel_inatomic() for x86. > > > > The patch below does that. Note, for simplicity I've changed the > > interface to 'get_kernel()' (will propagate this through the other > > patches as well). > > So I think this needs a couple of changes: > > - That "get_kernel()" name is not clear enough about what the issue > is. I think it should make it clearer that it's an unsafe access > that could fault, and that we don't want a user access. > > So maybe "get_kernel_stalepointer()" or something like that.
Ok.
> - you're just re-implementing "__get_user_asm_ex()" afaik. Try to > share the code, renaming it to be something common.
Ok, will try that.
> - I think we should look at sharing the code for __get_user(). Could > we do something like this: > > (a) implement the basic "load with exceptions" as __get_with_exception() > (b) #define get_kernel_stalepointer() __get_with_exception > (c) make "__get_user()" be "stac(); __get_with_exception(); clac()"
Will try.
The only possible complication there might be the way we don't recover the error code in the _ex() variants, that's actually a pretty important aspect to making this zero cost. Since the error code comes back from assembly code in some cases we cannot make it go away in the _ex() case. So I'm not sure we can share code between _ex() and the normal methods - but we can certainly share with the _ex() variants.
> - finally, I wonder what the exact semantics of > "get_kernel_stalepointer()" should be. I could well imagine that > what we should do is > > #ifdef CONFIG_DEBUG_PAGEALLOC > #define get_kernel_stalepointer(x,ptr) ((x)=READ_ONCE(*(ptr)), 0) > #else > #define get_kernel_stalepointer(x,ptr) __get_with_exception(x,ptr) > #endif
I guess you meant that to be the other way around?
> because I think it's reasonable to require that the kernel pointer > is _stale_, and not "invalid". [...]
Absolutely, and I think this is a hard requirement: we don't (ever) want to dereference random addresses, due to possible mmio side effects.
> [...] IOW, guarantee that it *has* been a kernel pointer, and that > the only reason it would trap is for DEBUG_PAGEALLOC.
Yes.
> That last point might need to be verified with hotplug memory. I > think hotplug memory does a stop_machine() or similar, but I'm not > sure.
So memory hotplug does it in a pretty simple fashion IIRC: only such zones are movable and hot-unpluggable which don't contain kmalloc()-able of gfp()-able memory - they are limited purpose memory pools only usable for user pages and the page cache.
So stale pointers should never point to hot-unpluggable memory.
Thanks,
Ingo
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