Mauricio Faria de Oliveira mauricfo@linux.vnet.ibm.com writes:
From: Nicholas Piggin npiggin@gmail.com
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@vger.kernel.org # v4.15+ 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 Signed-off-by: Michael Ellerman mpe@ellerman.id.au [mauricfo: backport to v4.15.7 (context line updates only) and re-test] Signed-off-by: Mauricio Faria de Oliveira mauricfo@linux.vnet.ibm.com
Backport looks good, thanks very much for doing it.
Acked-by: Michael Ellerman mpe@ellerman.id.au
cheers