[PATCH v2] powerpc/64s: Fix pte update for kernel memory on radix

[Naveen N. Rao]

Hi Jordan,

On 2021/02/04 10:59AM, Jordan Niethe wrote:
> When adding a pte a ptesync is needed to order the update of the pte
> with subsequent accesses otherwise a spurious fault may be raised.
> 
> radix__set_pte_at() does not do this for performance gains. For
> non-kernel memory this is not an issue as any faults of this kind are
> corrected by the page fault handler.  For kernel memory these faults are
> not handled.  The current solution is that there is a ptesync in
> flush_cache_vmap() which should be called when mapping from the vmalloc
> region.
> 
> However, map_kernel_page() does not call flush_cache_vmap(). This is
> troublesome in particular for code patching with Strict RWX on radix. In
> do_patch_instruction() the page frame that contains the instruction to
> be patched is mapped and then immediately patched. With no ordering or
> synchronization between setting up the pte and writing to the page it is
> possible for faults.
> 
> As the code patching is done using __put_user_asm_goto() the resulting
> fault is obscured - but using a normal store instead it can be seen:
> 
> [  418.498768][  T757] BUG: Unable to handle kernel data access on write at 0xc008000008f24a3c
> [  418.498790][  T757] Faulting instruction address: 0xc00000000008bd74
> [  418.498805][  T757] Oops: Kernel access of bad area, sig: 11 [#1]
> [  418.498828][  T757] LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA PowerNV
> [  418.498843][  T757] Modules linked in: nop_module(PO+) [last unloaded: nop_module]
> [  418.498872][  T757] CPU: 4 PID: 757 Comm: sh Tainted: P           O      5.10.0-rc5-01361-ge3c1b78c8440-dirty #43
> [  418.498936][  T757] NIP:  c00000000008bd74 LR: c00000000008bd50 CTR: c000000000025810
> [  418.498979][  T757] REGS: c000000016f634a0 TRAP: 0300   Tainted: P           O       (5.10.0-rc5-01361-ge3c1b78c8440-dirty)
> [  418.499033][  T757] MSR:  9000000000009033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 44002884  XER: 00000000
> [  418.499084][  T757] CFAR: c00000000007c68c DAR: c008000008f24a3c DSISR: 42000000 IRQMASK: 1
> 
> This results in the kind of issue reported here:
> https://lore.kernel.org/linuxppc-dev/15AC5B0E-A221-4B8C-9039-FA96B8EF7C88@lca.pw/
> 

Thanks for fixing this!

- Naveen