Re: powerpc/64s/radix: kernel boot-time NULL pointer protection using a guard-PID

[Michael Ellerman <patch-notifications@ellerman.id.au>]

On Wed, 2018-02-07 at 01:20:02 UTC, Nicholas Piggin wrote:
> This change restores and formalises the behaviour that access to NULL or
> other user addresses by the kernel during boot should fault rather than
> succeed and modify memory. This was inadvertently broken when fixing
> another bug, because it was previously not well defined and only worked
> by chance.
> 
> powerpc/64s/radix uses high address bits to select an address space
> "quadrant", which determines which PID and LPID are used to translate
> the rest of the address (effective PID, effective LPID). The kernel
> mapping at 0xC... selects quadrant 3, which uses PID=0 and LPID=0. So
> the kernel page tables are installed in the PID 0 process table entry.
> 
> An address at 0x0... selects quadrant 0, which uses PID=PIDR for
> translating the rest of the address (that is, it uses the value of the
> PIDR register as the effective PID). If PIDR=0, then the translation is
> performed with the PID 0 process table entry page tables. This is the
> kernel mapping, so we effectively get another copy of the kernel address
> space at 0. A NULL pointer access will access physical memory address 0.
> 
> To prevent duplicating the kernel address space in quadrant 0, this
> patch allocates a guard PID containing no translations, and initializes
> PIDR with this during boot, before the MMU is switched on.  Any kernel
> access to quadrant 0 will use this guard PID for translation and find no
> valid mappings, and therefore fault.
> 
> After boot, this PID will be switchd away to user context PIDs, but
> those contain user mappings (and usually NULL pointer protection) rather
> than kernel mapping, which is much safer (and by design). It may be in
> future this is tightened further, which the guard PID could be used for.
> 
> Commit 371b8044 ("powerpc/64s: Initialize ISAv3 MMU registers before
> setting partition table"), introduced this problem because it zeroes
> PIDR at boot. However previously the value was inherited from firmware
> or kexec, which is not robust and can be zero (e.g., mambo).
> 
> Fixes: 371b8044 ("powerpc/64s: Initialize ISAv3 MMU registers before setting partition table")
> Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> Reported-by: Florian Weimer <fweimer@redhat.com>
> Tested-by: Mauricio Faria de Oliveira <mauricfo@linux.vnet.ibm.com>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/eeb715c3e995fbdda0cc05e61216c6

Added:
  Cc: stable@vger.kernel.org # v4.15+

cheers