Re: [PATCH] vsprintf: Do not break early boot with probing addresses

Sergey Senozhatsky
2019-05-10T04:32:00Z

On (05/09/19 14:19), Petr Mladek wrote:
> 1. Report on Power:
>
> Kernel crashes very early during boot with with CONFIG_PPC_KUAP and
> CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG
>
> The problem is the combination of some new code called via printk(),
> check_pointer() which calls probe_kernel_read(). That then calls
> allow_user_access() (PPC_KUAP) and that uses mmu_has_feature() too early
> (before we've patched features). With the JUMP_LABEL debug enabled that
> causes us to call printk() & dump_stack() and we end up recursing and
> overflowing the stack.

Hmm... hmm... PPC does an .opd-based symbol dereference, which
eventually probe_kernel_read()-s. So early printk(%pS) will do

printk(%pS)
dereference_function_descriptor()
probe_kernel_address()
dump_stack()
printk(%pS)
dereference_function_descriptor()
probe_kernel_address()
dump_stack()
printk(%pS)
...

I'd say... that it's not vsprintf that we want to fix, it's
the idea that probe_kernel_address() can dump_stack() on any
platform. On some archs probe_kernel_address()->dump_stack()
is going nowhere:
dump_stack() does probe_kernel_address(), which calls dump_stack(),
which calls printk(%pS)->probe_kernel_address() again and again,
and again.

-ss