On Wed, Jan 26, 2022 at 07:07:20PM +0100, Jann Horn wrote: > On Wed, Jan 26, 2022 at 6:58 PM Kees Cook <keescook@xxxxxxxxxxxx> 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. > > 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)? Hrm, yeah, I guess it's libc providing the envp pointer; it's not passes separately. Hrm. -- Kees Cook
