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

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Date:   Thu, 9 May 2019 15:38:29 +0200
From:   Michal Suchánek <msuchanek@...e.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-arch@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        linux-s390@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Michal Hocko <mhocko@...e.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Stephen Rothwell <sfr@...abs.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        "Tobin C . Harding" <me@...in.cc>
Subject: Re: [PATCH] vsprintf: Do not break early boot with probing
 addresses

On Thu,  9 May 2019 14:19:23 +0200
Petr Mladek <pmladek@...e.com> 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