[PATCH] powerpc/fault: fix wrong KUAP fault for IO_URING

[Zorro Lang]

On Thu, Jan 28, 2021 at 10:18:07AM +1000, Nicholas Piggin wrote:
> Excerpts from Jens Axboe's message of January 28, 2021 5:29 am:
> > On 1/27/21 9:38 AM, Christophe Leroy wrote:
> >> 
> >> 
> >> Le 27/01/2021 à 15:56, Zorro Lang a écrit :
> >>> On powerpc, io_uring test hit below KUAP fault on __do_page_fault.
> >>> The fail source line is:
> >>>
> >>>    if (unlikely(!is_user && bad_kernel_fault(regs, error_code, address, is_write)))
> >>>        return SIGSEGV;
> >>>
> >>> The is_user() is based on user_mod(regs) only. This's not suit for
> >>> io_uring, where the helper thread can assume the user app identity
> >>> and could perform this fault just fine. So turn to use mm to decide
> >>> if this is valid or not.
> >> 
> >> I don't understand why testing is_user would be an issue. KUAP purpose
> >> it to block any unallowed access from kernel to user memory
> >> (Equivalent to SMAP on x86). So it really must be based on MSR_PR bit,
> >> that is what is_user provides.
> >> 
> >> If the kernel access is legitimate, kernel should have opened
> >> userspace access then you shouldn't get this "Bug: Read fault blocked
> >> by KUAP!".
> >> 
> >> As far as I understand, the fault occurs in
> >> iov_iter_fault_in_readable() which calls fault_in_pages_readable() And
> >> fault_in_pages_readable() uses __get_user() so it is a legitimate
> >> access and you really should get a KUAP fault.
> >> 
> >> So the problem is somewhere else, I think you proposed patch just
> >> hides the problem, it doesn't fix it.
> > 
> > If we do kthread_use_mm(), can we agree that the user access is valid?
> 
> Yeah the io uring code is fine, provided it uses the uaccess primitives 
> like any other kernel code. It's looking more like a an arch/powerpc bug.
> 
> > We should be able to copy to/from user space, and including faults, if
> > that's been done and the new mm assigned. Because it really should be.
> > If SMAP was a problem on x86, we would have seen it long ago.
> > 
> > I'm assuming this may be breakage related to the recent uaccess changes
> > related to set_fs and friends? Or maybe recent changes on the powerpc
> > side?
> > 
> > Zorro, did 5.10 work?
> 
> Would be interesting to know.

Sure Nick and Jens, which 5.10 rc? version do you want to know ? Or any git
commit(be the HEAD) in 5.10 phase?

Thanks,
Zorro

> 
> Thanks,
> Nick
>