linux-kernel - Re: [PATCH] binfmt_elf.c : the BAD

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Date:	Mon, 21 Aug 2006 16:35:47 -0400
From:	Ernie Petrides <petrides@...hat.com>
To:	Willy Tarreau <w@....eu>
cc:	Solar Designer <solar@...nwall.com>,
	Chuck Ebbert <76306.1226@...puserve.com>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] binfmt_elf.c : the BAD_ADDR macro again

On Sunday, 20-Aug-2006 at 18:23 +0200, Willy Tarreau wrote:

> On Sun, Aug 20, 2006 at 07:51:22PM +0400, Solar Designer wrote:
> > On Sun, Aug 20, 2006 at 11:15:15AM +0200, Willy Tarreau wrote:
> > > The proper fix would then be :
> > [...]
> > > -#define BAD_ADDR(x)	((unsigned long)(x) > TASK_SIZE)
> > > +#define BAD_ADDR(x)	((unsigned long)(x) >= TASK_SIZE)
> > [...]
> > > -	    if (k > TASK_SIZE || eppnt->p_filesz > eppnt->p_memsz ||
> > > +	    if (BAD_ADDR(k) || eppnt->p_filesz > eppnt->p_memsz ||
> > [...]
> > > -		if (k > TASK_SIZE || elf_ppnt->p_filesz > elf_ppnt->p_memsz ||
> > > +		if (BAD_ADDR(k) || elf_ppnt->p_filesz > elf_ppnt->p_memsz ||
> > 
> > Looks OK to me.

These are all correct.

> > > And even then, I'm not happy with this test :
> > > 
> > >    TASK_SIZE - elf_ppnt->p_memsz < k
> > > 
> > > because it means that we only raise the error when
> > > 
> > >    k + elf_ppnt->p_memsz > TASK_SIZE
> > > 
> > > I really think that we want to check this instead :
> > > 
> > >    k + elf_ppnt->p_memsz >= TASK_SIZE
> > > 
> > > Otherwise we leave a window where this is undetected :
> > > 
> > >    load_addr + eppnt->p_vaddr == TASK_SIZE - eppnt->p_memsz

The reason I did not propose changing these is because these are
end-point checks (as opposed to starting address checks).  I think
that the following "equals" condition is conceptually valid:

	(starting-address + region-size == TASK_SIZE)

> > > This will later lead to last_bss getting readjusted to TASK_SIZE, which I
> > > don't think is expected at all :
> > > 
> > >             k = load_addr + eppnt->p_memsz + eppnt->p_vaddr;
> > >             if (k > last_bss)
> > >                 last_bss = k;

This is an interesting case, but I think the error checking works okay.

After the ELF phdr loop, the resulting "last_bss" is used as follows:

	/* Map the last of the bss segment */
	if (last_bss > elf_bss) {
		down_write(&current->mm->mmap_sem);
		error = do_brk(elf_bss, last_bss - elf_bss);
		up_write(&current->mm->mmap_sem);
		if (BAD_ADDR(error))
			goto out_close;
	}

The variable "last_bss" is used to compute the size argument in the
call to do_brk().  If the section extends beyond TASK_SIZE, then do_brk()
will return -EINVAL.  If the do_brk() call succeeds but "elf_bss" is itself
exactly at TASK_SIZE, then the BAD_ADDR() call above will catch it.

> [...]   But before this, I'd like to get comments from
> the people who discussed the subject recently.

Thus, I think that both 2.4.33 and 2.6.<latest> are okay without any
further changes.

Cheers.  -ernie
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