[PATCH] powerpc/ptdump: Fix generic ptdump for 64-bit

[Nathan Chancellor]

On Tue, Aug 31, 2021 at 11:51:51PM +1000, Michael Ellerman wrote:
> Since the conversion to generic ptdump we see crashes on 64-bit:
> 
>   BUG: Unable to handle kernel data access on read at 0xc0eeff7f00000000
>   Faulting instruction address: 0xc00000000045e5fc
>   Oops: Kernel access of bad area, sig: 11 [#1]
>   ...
>   NIP __walk_page_range+0x2bc/0xce0
>   LR  __walk_page_range+0x240/0xce0
>   Call Trace:
>     __walk_page_range+0x240/0xce0 (unreliable)
>     walk_page_range_novma+0x74/0xb0
>     ptdump_walk_pgd+0x98/0x170
>     ptdump_check_wx+0x88/0xd0
>     mark_rodata_ro+0x48/0x80
>     kernel_init+0x74/0x1a0
>     ret_from_kernel_thread+0x5c/0x64
> 
> What's happening is that have walked off the end of the kernel page
> tables, and started dereferencing junk values.
> 
> That happens because we initialised the ptdump_range to span all the way
> up to 0xffffffffffffffff:
> 
> static struct ptdump_range ptdump_range[] __ro_after_init = {
> 	{TASK_SIZE_MAX, ~0UL},
> 
> But the kernel page tables don't span that far. So on 64-bit set the end
> of the range to be the address immediately past the end of the kernel
> page tables, to limit the page table walk to valid addresses.
> 
> Fixes: e084728393a5 ("powerpc/ptdump: Convert powerpc to GENERIC_PTDUMP")
> Reported-by: Nathan Chancellor <nathan at kernel.org>
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>

Tested-by: Nathan Chancellor <nathan at kernel.org>

> ---
>  arch/powerpc/mm/ptdump/ptdump.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/powerpc/mm/ptdump/ptdump.c b/arch/powerpc/mm/ptdump/ptdump.c
> index 2d80d775d15e..bf251191e78d 100644
> --- a/arch/powerpc/mm/ptdump/ptdump.c
> +++ b/arch/powerpc/mm/ptdump/ptdump.c
> @@ -359,6 +359,8 @@ static int __init ptdump_init(void)
>  		ptdump_range[0].start = KERN_VIRT_START;
>  	else
>  		ptdump_range[0].start = PAGE_OFFSET;
> +
> +	ptdump_range[0].end = PAGE_OFFSET + (PGDIR_SIZE * PTRS_PER_PGD);
>  #endif
>  
>  	populate_markers();
> 
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> -- 
> 2.25.1