Re: [PATCH] fs/binfmt_elf: Add padding NULL when argc == 0

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Hi,

On Wed, 26 Jan 2022, Jann Horn wrote:

On Wed, Jan 26, 2022 at 6:58 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
Quoting Ariadne Conill:

"In several other operating systems, it is a hard requirement that the
first argument to execve(2) be the name of a program, thus prohibiting
a scenario where argc < 1. POSIX 2017 also recommends this behaviour,
but it is not an explicit requirement[1]:

    The argument arg0 should point to a filename string that is
    associated with the process being started by one of the exec
    functions.
...
Interestingly, Michael Kerrisk opened an issue about this in 2008[2],
but there was no consensus to support fixing this issue then.
Hopefully now that CVE-2021-4034 shows practical exploitative use[3]
of this bug in a shellcode, we can reconsider."

An examination of existing[4] users of execve(..., NULL, NULL) shows
mostly test code, or example rootkit code. While rejecting a NULL argv
would be preferred, it looks like the main cause of userspace confusion
is an assumption that argc >= 1, and buggy programs may skip argv[0]
when iterating. To protect against userspace bugs of this nature, insert
an extra NULL pointer in argv when argc == 0, so that argv[1] != envp[0].

Note that this is only done in the argc == 0 case because some userspace
programs expect to find envp at exactly argv[argc]. The overlap of these
two misguided assumptions is believed to be zero.

Will this result in the executed program being told that argc==0 but
having an extra NULL pointer on the stack?
If so, I believe this breaks the x86-64 ABI documented at
https://refspecs.linuxbase.org/elf/x86_64-abi-0.99.pdf - page 29,
figure 3.9 describes the layout of the initial process stack.

I'm presently compiling a kernel with the patch to see if it works or not.

Actually, does this even work? Can a program still properly access its
environment variables when invoked with argc==0 with this patch
applied? AFAIU the way userspace locates envv on x86-64 is by
calculating 8*(argc+1)?

In the other thread, it was suggested that perhaps we should set up an argv of {"", NULL}. In that case, it seems like it would be safe to claim argc == 1.

What do you think?

Ariadne



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