| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <234f8cb0-8f9c-0caf-c169-cf9355b33075@kernel.org>
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;
Powered by blists - more mailing lists