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Date: Mon, 31 Jan 2022 19:00:46 -0600 (CST) From: Ariadne Conill <ariadne@...eferenced.org> To: Kees Cook <keescook@...omium.org> cc: Andrew Morton <akpm@...ux-foundation.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] exec: Force single empty string when argv is empty Hi, On Mon, 31 Jan 2022, Kees Cook wrote: > Quoting[1] Ariadne Conill: > > "In several other operating systems, it is a hard requirement that the > second 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[2]: > > 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[3], > but there was no consensus to support fixing this issue then. > Hopefully now that CVE-2021-4034 shows practical exploitative use[4] > of this bug in a shellcode, we can reconsider. > > This issue is being tracked in the KSPP issue tracker[5]." > > While the initial code searches[6][7] turned up what appeared to be > mostly corner case tests, trying to that just reject argv == NULL > (or an immediately terminated pointer list) quickly started tripping[8] > existing userspace programs. Yes, it's a shame this is the case, but we do what we have to do, I guess :) > > The next best approach is forcing a single empty string into argv and > adjusting argc to match. The number of programs depending on argc == 0 > seems a smaller set than those calling execve with a NULL argv. > > Account for the additional stack space in bprm_stack_limits(). Inject an > empty string when argc == 0 (and set argc = 1). Warn about the case so > userspace has some notice about the change: > > process './argc0' launched './argc0' with NULL argv: empty string added > > Additionally WARN() and reject NULL argv usage for kernel threads. > > [1] https://lore.kernel.org/lkml/20220127000724.15106-1-ariadne@dereferenced.org/ > [2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html > [3] https://bugzilla.kernel.org/show_bug.cgi?id=8408 > [4] https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt > [5] https://github.com/KSPP/linux/issues/176 > [6] https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0 > [7] https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0 > [8] https://lore.kernel.org/lkml/20220131144352.GE16385@xsang-OptiPlex-9020/ > > 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> In terms of going with this approach as an alternative verses my original patch, Acked-by: Ariadne Conill <ariadne@...eferenced.org> Ariadne
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