linux-kernel - Re: [PATCH] pstore: Convert buf

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Date:   Tue, 4 Dec 2018 09:23:13 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        linux-efi <linux-efi@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pstore: Convert buf_lock to semaphore

On Tue, Dec 4, 2018 at 7:41 AM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> On 2018-11-30 14:47:36 [-0800], Kees Cook wrote:
> > diff --git a/drivers/firmware/efi/efi-pstore.c b/drivers/firmware/efi/efi-pstore.c
> > index cfe87b465819..0f7d97917197 100644
> > --- a/drivers/firmware/efi/efi-pstore.c
> > +++ b/drivers/firmware/efi/efi-pstore.c
> > @@ -259,8 +259,7 @@ static int efi_pstore_write(struct pstore_record *record)
> >               efi_name[i] = name[i];
> >
> >       ret = efivar_entry_set_safe(efi_name, vendor, PSTORE_EFI_ATTRIBUTES,
> > -                           !pstore_cannot_block_path(record->reason),
> > -                           record->size, record->psi->buf);
> > +                           preemptible(), record->size, record->psi->buf);
>
> Well. Better I think.
> might_sleep() / preempt_count_equals() checks for preemptible() + rcu_preempt_depth().
> kmsg_dump() starts with rcu_read_lock() which means with this patch applied I
> got:

Okay, so, if kmsg_dump() uses rcu_read_lock(), that means efi-pstore
can _never_ sleep, and it's nothing to do with pstore internals. :( I
guess we just hard-code it, then? And efi-pstore should probably only
attach to pstore if it has a nonblock implementation (and warn if one
isn't available).

-Kees

-- 
Kees Cook