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

[Max Filippov]

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.
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<---

> 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.
-- Max