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

On Thu,  9 May 2019 14:19:23 +0200
Petr Mladek [off-list ref] wrote:

The commit 3e5903eb9cff70730 ("vsprintf: Prevent crash when dereferencing
invalid pointers") broke boot on several architectures. The common
pattern is that probe_kernel_read() is not working during early
boot because userspace access framework is not ready.

The check is only the best effort. Let's not rush with it during
the early boot.

Details:

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).

There is early_mmu_has_feature for this case. mmu_has_feature does not
work before patching so parts of kernel that can run before patching
must use the early_ variant which actually runs code reading the
feature bitmap to determine the answer.

Thanks

Michal