Re: [PATCH] exec: fix linux_binprm::exec in transfer_args_to_stack()

[Kees Cook]

On Thu, Mar 21, 2024 at 12:52:16PM -0700, Max Filippov wrote:
> On Thu, Mar 21, 2024 at 10:05 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >
> > On Wed, Mar 20, 2024 at 11:26:07AM -0700, Max Filippov wrote:
> > > In NUMMU kernel the value of linux_binprm::p is the offset inside the
> > > temporary program arguments array maintained in separate pages in the
> > > linux_binprm::page. linux_binprm::exec being a copy of linux_binprm::p
> > > thus must be adjusted when that array is copied to the user stack.
> > > Without that adjustment the value passed by the NOMMU kernel to the ELF
> > > program in the AT_EXECFN entry of the aux array doesn't make any sense
> > > and it may break programs that try to access memory pointed to by that
> > > entry.
> > >
> > > Adjust linux_binprm::exec before the successful return from the
> > > transfer_args_to_stack().
> >
> > What's the best way to test this? (Is there a qemu setup I can use to
> > see the before/after of AT_EXECFN?)
> 
> I put a readme with the steps to build such system here:
>   http://jcmvbkbc.org/~dumb/tmp/202403211236/README
> it uses a prebuilt rootfs image and a 6.8 kernel branch with two
> patches on top of it: one adds a dts and a defconfig and the other
> is this fix. The rootfs boots successfully with this fix, but panics
> if this fix is removed.

Ah, perfect! Thanks for this.

> The easiest way to actually see the AT_EXECFN is, I guess, to
> do something like that:
> ---8<---
> diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
> index fefc642541cb..22d34272a570 100644
> --- a/fs/binfmt_elf_fdpic.c
> +++ b/fs/binfmt_elf_fdpic.c
> @@ -659,6 +659,7 @@ static int create_elf_fdpic_tables(struct
> linux_binprm *bprm,
>        NEW_AUX_ENT(AT_EGID,    (elf_addr_t)
> from_kgid_munged(cred->user_ns, cred->egid));
>        NEW_AUX_ENT(AT_SECURE,  bprm->secureexec);
>        NEW_AUX_ENT(AT_EXECFN,  bprm->exec);
> +       pr_info("%s: AT_EXECFN = %#lx\n", __func__, bprm->exec);
> 
> #ifdef ARCH_DLINFO
>        nr = 0;
> ---8<---

Does musl have something like the LD_SHOW_AUXV env variable. With glibc,
I usually explore auxv like so:

$ LD_SHOW_AUXV=1 uname -a | grep EXECFN
AT_EXECFN:            /usr/bin/uname

> > How did you encounter the problem?
> 
> I'm doing xtensa FDPIC port of musl libc and this issue popped up when
> I began testing it on qemu-system-xtensa with the real linux kernel.
> Related post to the musl ML:
>   https://www.openwall.com/lists/musl/2024/03/20/2

Thanks!

-- 
Kees Cook