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Date: Wed, 26 Jan 2022 12:42:27 -0600 (CST) From: Ariadne Conill <ariadne@...eferenced.org> To: Jann Horn <jannh@...gle.com> cc: Kees Cook <keescook@...omium.org>, Ariadne Conill <ariadne@...eferenced.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, Jann Horn wrote: > On Wed, Jan 26, 2022 at 6:58 PM Kees Cook <keescook@...omium.org> 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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