Hi,
On Wed, 26 Jan 2022, Ariadne Conill wrote:
Hi,
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] https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
[2] https://bugzilla.kernel.org/show_bug.cgi?id=8408
[3] https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt
[4]
https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0
Reported-by: Ariadne Conill <ariadne@xxxxxxxxxxxxxxxx>
Reported-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Rich Felker <dalias@xxxxxxxx>
Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Tested-by: Ariadne Conill <ariadne@xxxxxxxxxxxxxxxx>
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)
perror("execve");
return 0;
}
Further testing indicates that while things *mostly* work, it results in
memory corruption in various tasks, for example, trying to build a new
kernel hung, and the gcc process's name was a bunch of uninitialized
memory. So, I don't think { NULL, NULL } is a good way to go.
Ariadne
