Mauricio Faria de Oliveira <mauricfo@xxxxxxxxxxxxxxxxxx> writes:
> From: Nicholas Piggin <npiggin@xxxxxxxxx>
>
> commit eeb715c3e995fbdda0cc05e61216c6c5609bce66 upstream
>
> 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: 371b80447ff3 ("powerpc/64s: Initialize ISAv3 MMU registers before setting partition table")
> Cc: stable@xxxxxxxxxxxxxxx # v4.15+
> Reported-by: Florian Weimer <fweimer@xxxxxxxxxx>
> Tested-by: Mauricio Faria de Oliveira <mauricfo@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> [mauricfo: backport to v4.15.7 (context line updates only) and re-test]
> Signed-off-by: Mauricio Faria de Oliveira <mauricfo@xxxxxxxxxxxxxxxxxx>
Backport looks good, thanks very much for doing it.
Acked-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
cheers