linux-kernel - Re: [PATCH] binfmt_elf: Do not move brk for INTERP-less ET

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Message-ID: <201909261136.780526BB@keescook>
Date:   Thu, 26 Sep 2019 11:37:03 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Richard Kojedzinszky <richard@...edz.in>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Ali Saidi <alisaidi@...zon.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] binfmt_elf: Do not move brk for INTERP-less ET_EXEC

On Thu, Sep 26, 2019 at 08:26:12PM +0200, Richard Kojedzinszky wrote:
> Thanks for the quick patch. It seems my binaries start up well, and work as
> expected, as before.
> 
> Thanks again for the quick response.

Awesome; thanks for the testing (and sorry for the breakage)! :)

-Kees

> 
> Regards,
> Richard Kojedzinszky
> 
> 2019-09-26 19:15 időpontban Kees Cook ezt írta:
> > When brk was moved for binaries without an interpreter, it should have
> > been limited to ET_DYN only. In other words, the special case was an
> > ET_DYN that lacks an INTERP, not just an executable that lacks INTERP.
> > The bug manifested for giant static executables, where the brk would end
> > up in the middle of the text area on 32-bit architectures.
> > 
> > Reported-by: Richard Kojedzinszky <richard@...edz.in>
> > Fixes: bbdc6076d2e5 ("binfmt_elf: move brk out of mmap when doing
> > direct loader exec")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> > Richard, are you able to test this? I'm able to run the gitea binary
> > with this change, and my INTERP-less ET_DYN tests (from the original
> > bug) continue to pass as well.
> > ---
> >  fs/binfmt_elf.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> > index cec3b4146440..ad4c6b1d5074 100644
> > --- a/fs/binfmt_elf.c
> > +++ b/fs/binfmt_elf.c
> > @@ -1121,7 +1121,8 @@ static int load_elf_binary(struct linux_binprm
> > *bprm)
> >  		 * (since it grows up, and may collide early with the stack
> >  		 * growing down), and into the unused ELF_ET_DYN_BASE region.
> >  		 */
> > -		if (IS_ENABLED(CONFIG_ARCH_HAS_ELF_RANDOMIZE) && !interpreter)
> > +		if (IS_ENABLED(CONFIG_ARCH_HAS_ELF_RANDOMIZE) &&
> > +		    loc->elf_ex.e_type == ET_DYN && !interpreter)
> >  			current->mm->brk = current->mm->start_brk =
> >  				ELF_ET_DYN_BASE;
> > 
> > --
> > 2.17.1

-- 
Kees Cook