On Wed, Jan 26, 2022 at 08:50:39PM +0100, Jann Horn wrote: > On Wed, Jan 26, 2022 at 7:42 PM Ariadne Conill <ariadne@xxxxxxxxxxxxxxxx> wrote: > > On Wed, 26 Jan 2022, 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. > > > > 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? > > Sounds good to me, since that's something that could also happen > normally if userspace calls execve(..., {"", NULL}, ...). > > (I'd like it even better if we could just bail out with an error code, > but I guess the risk of breakage might be too high with that > approach?) We can't mutate argc; it'll turn at least some userspace into an infinite loop: https://sources.debian.org/src/valgrind/1:3.18.1-1/none/tests/execve.c/?hl=22#L22 -- Kees Cook
