linux-kernel - Re: [tip:x86/urgent] x86/boot/compressed/64: Fix boot on machines with broken E820 table

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Message-ID: <20190826071539.GB27636@zn.tnic>
Date:   Mon, 26 Aug 2019 09:15:39 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     tglx@...utronix.de, hpa@...or.com, linux-kernel@...r.kernel.org,
        mingo@...hat.com, x86@...nel.org, mingo@...nel.org,
        kirill.shutemov@...ux.intel.com, kirill@...temov.name,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86/boot/compressed/64: Fix boot on machines
 with broken E820 table

On Sun, Aug 25, 2019 at 10:33:15PM -0500, Gustavo A. R. Silva wrote:
> Hi all,
> 
> On 8/19/19 9:16 AM, tip-bot for Kirill A. Shutemov wrote:
> [..]
> > 
> > diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
> > index 5f2d03067ae5..2faddeb0398a 100644
> > --- a/arch/x86/boot/compressed/pgtable_64.c
> > +++ b/arch/x86/boot/compressed/pgtable_64.c
> > @@ -72,6 +72,8 @@ static unsigned long find_trampoline_placement(void)
> >  
> >  	/* Find the first usable memory region under bios_start. */
> >  	for (i = boot_params->e820_entries - 1; i >= 0; i--) {
> > +		unsigned long new;
> > +
> >  		entry = &boot_params->e820_table[i];
> >  
> >  		/* Skip all entries above bios_start. */
> > @@ -84,15 +86,20 @@ static unsigned long find_trampoline_placement(void)
> >  
> >  		/* Adjust bios_start to the end of the entry if needed. */
> >  		if (bios_start > entry->addr + entry->size)
> 
> Notice that if this condition happens to be false, we end up with an
> uninitialized variable *new*.

Yap, good catch.

> What would be the right value to assign to *new* at declaration under
> this condition?

Looking at the changed flow of the loop, how we use new instead of
bios_start and how we assign new back to bios_start, I think we should
do:

		unsigned long new = bios_start;

at the beginning...

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.