Messages in this thread | | | Date | Wed, 26 Jan 2022 14:10:37 -0600 (CST) | From | Ariadne Conill <> | Subject | Re: [PATCH] fs/binfmt_elf: Add padding NULL when argc == 0 |
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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@dereferenced.org> > Reported-by: Michael Kerrisk <mtk.manpages@gmail.com> > Cc: Matthew Wilcox <willy@infradead.org> > Cc: Christian Brauner <brauner@kernel.org> > Cc: Rich Felker <dalias@libc.org> > Cc: Eric Biederman <ebiederm@xmission.com> > Cc: Alexander Viro <viro@zeniv.linux.org.uk> > Cc: linux-fsdevel@vger.kernel.org > Cc: stable@vger.kernel.org > Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Ariadne Conill <ariadne@dereferenced.org>
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; }
Ariadne
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