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Date: Mon, 31 Jan 2022 18:00:01 -0800 From: Andy Lutomirski <luto@...nel.org> To: Kees Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org> Cc: 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, Linux API <linux-api@...r.kernel.org> Subject: Re: [PATCH] exec: Force single empty string when argv is empty On 1/31/22 16:09, 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. > > 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/ Acked-by: Andy Lutomirski <luto@...nel.org> and cc-ing linux-api. I agree that this should be done regardless of any security context change. > > 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> > --- > fs/exec.c | 26 +++++++++++++++++++++++++- > 1 file changed, 25 insertions(+), 1 deletion(-) > > diff --git a/fs/exec.c b/fs/exec.c > index 79f2c9483302..bbf3aadf7ce1 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -495,8 +495,14 @@ static int bprm_stack_limits(struct linux_binprm *bprm) > * the stack. They aren't stored until much later when we can't > * signal to the parent that the child has run out of stack space. > * Instead, calculate it here so it's possible to fail gracefully. > + * > + * In the case of argc = 0, make sure there is space for adding a > + * empty string (which will bump argc to 1), to ensure confused > + * userspace programs don't start processing from argv[1], thinking > + * argc can never be 0, to keep them from walking envp by accident. > + * See do_execveat_common(). > */ > - ptr_size = (bprm->argc + bprm->envc) * sizeof(void *); > + ptr_size = (min(bprm->argc, 1) + bprm->envc) * sizeof(void *); > if (limit <= ptr_size) > return -E2BIG; > limit -= ptr_size; > @@ -1897,6 +1903,9 @@ static int do_execveat_common(int fd, struct filename *filename, > } > > retval = count(argv, MAX_ARG_STRINGS); > + if (retval == 0) > + pr_warn_once("process '%s' launched '%s' with NULL argv: empty string added\n", > + current->comm, bprm->filename); > if (retval < 0) > goto out_free; > bprm->argc = retval; > @@ -1923,6 +1932,19 @@ static int do_execveat_common(int fd, struct filename *filename, > if (retval < 0) > goto out_free; > > + /* > + * When argv is empty, add an empty string ("") as argv[0] to > + * ensure confused userspace programs that start processing > + * from argv[1] won't end up walking envp. See also > + * bprm_stack_limits(). > + */ > + if (bprm->argc == 0) { > + retval = copy_string_kernel("", bprm); > + if (retval < 0) > + goto out_free; > + bprm->argc = 1; > + } > + > retval = bprm_execve(bprm, fd, filename, flags); > out_free: > free_bprm(bprm); > @@ -1951,6 +1973,8 @@ int kernel_execve(const char *kernel_filename, > } > > retval = count_strings_kernel(argv); > + if (WARN_ON_ONCE(retval == 0)) > + retval = -EINVAL; > if (retval < 0) > goto out_free; > bprm->argc = retval;
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