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Date: Wed, 26 Jan 2022 14:46:15 -0600 (CST) From: Ariadne Conill <ariadne@...eferenced.org> To: Ariadne Conill <ariadne@...eferenced.org> cc: Kees Cook <keescook@...omium.org>, Michael Kerrisk <mtk.manpages@...il.com>, Matthew Wilcox <willy@...radead.org>, Christian Brauner <brauner@...nel.org>, Rich Felker <dalias@...c.org>, Eric Biederman <ebiederm@...ssion.com>, Alexander Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org, stable@...r.kernel.org, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org Subject: Re: [PATCH] fs/binfmt_elf: Add padding NULL when argc == 0 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@...eferenced.org> >> Reported-by: Michael Kerrisk <mtk.manpages@...il.com> >> Cc: Matthew Wilcox <willy@...radead.org> >> Cc: Christian Brauner <brauner@...nel.org> >> Cc: Rich Felker <dalias@...c.org> >> Cc: Eric Biederman <ebiederm@...ssion.com> >> Cc: Alexander Viro <viro@...iv.linux.org.uk> >> Cc: linux-fsdevel@...r.kernel.org >> Cc: stable@...r.kernel.org >> Signed-off-by: Kees Cook <keescook@...omium.org> > > Tested-by: Ariadne Conill <ariadne@...eferenced.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; > } > 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
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