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SubjectRe: [PATCH v2] binfmt_elf: Fix PIE execution with randomization disabled (was Re: [RFC PATCH] binfmt_elf: Fix PIE execution with randomization disabled)
On Mon, 3 Oct 2011, Andrew Morton wrote:

> > From: Jiri Kosina <jkosina@suse.cz>
> > Subject: [PATCH] binfmt_elf: fix PIE execution with randomization disabled
> >
> > The case of address space randomization being disabled in runtime through
> > randomize_va_space sysctl is not treated properly in load_elf_binary(),
> > resulting in SIGKILL coming at exec() time for certain PIE-linked binaries
> > in case the randomization has been disabled at runtime prior to calling
> > exec().
> >
> > Handle the randomize_va_space == 0 case the same way as if we were not
> > supporting .text randomization at all.
> >
> > Based on original patch by H.J. Lu <hongjiu.lu@intel.com> and
> > Josh Boyer <jwboyer@redhat.com>
> >
> > Cc: Ingo Molnar <mingo@elte.hu>
> > Cc: Jiri Kosina <jkosina@suse.cz>
> > Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
> > Cc: Russell King <rmk@arm.linux.org.uk>
> > Signed-off-by: Jiri Kosina <jkosina@suse.cz>
> > ---
> > fs/binfmt_elf.c | 5 ++++-
> > 1 files changed, 4 insertions(+), 1 deletions(-)
> >
> > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> > index dd0fdfc..bb11fe4 100644
> > --- a/fs/binfmt_elf.c
> > +++ b/fs/binfmt_elf.c
> > @@ -795,7 +795,10 @@ static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs)
> > * might try to exec. This is because the brk will
> > * follow the loader, and is not movable. */
> > #if defined(CONFIG_X86) || defined(CONFIG_ARM)
> > - load_bias = 0;
> > + if (current->flags & PF_RANDOMIZE)
> > + load_bias = 0;
> > + else
> > + load_bias = ELF_PAGESTART(ELF_ET_DYN_BASE - vaddr);
> > #else
> > load_bias = ELF_PAGESTART(ELF_ET_DYN_BASE - vaddr);
> > #endif
>
> Guys, it took several people several days and 10+ emails to work out
> what's happening in there, and the first attempt to fix it was buggy.
> This is all a huuuuge signal that the code is unobvious, hard to
> understand, hard to maintain.

It's unfortunately true. Rewriting the ELF binary loader is on my list of
very-long-term TODOs. It has became quite a monster over decades.

> Please, let's get a good code comment in there while it's fresh in your
> minds. So the next person who comes along doesn't have the same amount
> of difficulty?

I have to run now, I will resend the patch to you with an explanatory
comment added in about 8 hours from now.

Thanks,

--
Jiri Kosina
SUSE Labs


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