[PATCH] powerpc: Fix invalid page flags in create TLB CAM path for PTE_64BIT

[Scott Wood]

On Thu, 23 Sep 2010 16:10:15 -0400
Paul Gortmaker <paul.gortmaker at windriver.com> wrote:

> So the possibility exists to wrongly assign the user MAS3_U<RWX> bits
> to kernel (PAGE_KERNEL_X) address space via the following code fragment:
> 
>         if (flags & _PAGE_USER) {
>            TLBCAM[index].MAS3 |= MAS3_UX | MAS3_UR;
>            TLBCAM[index].MAS3 |= ((flags & _PAGE_RW) ? MAS3_UW : 0);
>         }
> 
> Here is a dump of the TLB info from Simics with the above code present:
> ------
> L2 TLB1
>                                             GT                   SSS UUU V I
>  Row  Logical           Physical            SS TLPID  TID  WIMGE XWR XWR F P   V
> ----- ----------------- ------------------- -- ----- ----- ----- --- --- - -   -
>   0   c0000000-cfffffff 000000000-00fffffff 00     0     0   M   XWR XWR 0 1   1
>   1   d0000000-dfffffff 010000000-01fffffff 00     0     0   M   XWR XWR 0 1   1
>   2   e0000000-efffffff 020000000-02fffffff 00     0     0   M   XWR XWR 0 1   1
> 
> Actually this conditional code was only used for two legacy functions:
> 
>   1: support KGDB to set break point.
>      KGDB already dropped this; now uses its core write to set break point.
> 
>   2: io_block_mapping() to create TLB in segmentation size (not PAGE_SIZE)
>      for device IO space.
>      This use case is also removed from the latest PowerPC kernel.

io_block_mapping() went away, but the feature itself is still useful
and might come back with something like this:

http://www.mail-archive.com/linuxppc-dev@lists.ozlabs.org/msg33851.html

...though I'm not sure why such mappings would ever have user access.

This could end up being used for large user pages by something like
hugetlbfs or KVM, though.  I don't think we want to make large user
pages fail, especailly if it just happens with the 32-bit page table
format (which i may not what the person adding such a feature tests
with).

I don't see a generic accessor that can test PTE flags for user
access -- in the absence of one, I guess we need an ifdef here.  Or at
least put in a comment so anyone who adds a userspace use knows they
need to fix it.

-Scott