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Date:   Wed, 26 Jan 2022 14:10:37 -0600 (CST)
From:   Ariadne Conill <>
To:     Kees Cook <>
cc:     Ariadne Conill <>,
        Michael Kerrisk <>,
        Matthew Wilcox <>,
        Christian Brauner <>,
        Rich Felker <>,
        Eric Biederman <>,
        Alexander Viro <>,,,,
Subject: Re: [PATCH] fs/binfmt_elf: Add padding NULL when argc == 0


On Wed, 26 Jan 2022, Kees Cook 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.
> [1]
> [2]
> [3]
> [4]*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0
> Reported-by: Ariadne Conill <>
> Reported-by: Michael Kerrisk <>
> Cc: Matthew Wilcox <>
> Cc: Christian Brauner <>
> Cc: Rich Felker <>
> Cc: Eric Biederman <>
> Cc: Alexander Viro <>
> Cc:
> Cc:
> Signed-off-by: Kees Cook <>

Tested-by: Ariadne Conill <>

It seems to work, but I still think bailing early with -EINVAL is a more 
reasonable position to take.  For example, the following code, when used 
with BusyBox applets results in a segfault, as the multicall stub does not 
support scenarios where argc < 1:

#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>

int main(int argc, const char **argv) {
         if (syscall(SYS_execve, "/bin/date", NULL, NULL) < 0)
         return 0;


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