On Wed, Jan 26, 2022 at 7:42 PM Ariadne Conill ariadne@dereferenced.org wrote:
On Wed, 26 Jan 2022, Jann Horn wrote:
On Wed, Jan 26, 2022 at 6:58 PM Kees Cook keescook@chromium.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?
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?)