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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id c18sm2021208pfp.181.2022.01.31.16.09.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 31 Jan 2022 16:09:51 -0800 (PST) From: Kees Cook To: Andrew Morton Cc: Kees Cook , Ariadne Conill , Michael Kerrisk , Matthew Wilcox , Christian Brauner , Rich Felker , Eric Biederman , Alexander Viro , linux-fsdevel@vger.kernel.org, stable@vger.kernel.org, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org Subject: [PATCH] exec: Force single empty string when argv is empty Date: Mon, 31 Jan 2022 16:09:47 -0800 Message-Id: <20220201000947.2453721-1-keescook@chromium.org> X-Mailer: git-send-email 2.30.2 MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=4766; h=from:subject; bh=gaATF+4EbtmlAe3Nasp9+LtjYlIXpznex7XqhlI1JLg=; b=owEBbQKS/ZANAwAKAYly9N/cbcAmAcsmYgBh+HpKpuM0Z7w1vc/DKue58FoVRYdahszqKEUikyKs LkopZsyJAjMEAAEKAB0WIQSlw/aPIp3WD3I+bhOJcvTf3G3AJgUCYfh6SgAKCRCJcvTf3G3AJkQwD/ 9gaH8XIr+oEzkggBsQWnCKXTzVmALVe4Zgbuh4C1rNxscXB+aeb4/KQKr3K6vvJ2l/fkh70QW+Pn7u gIJ269Ql3ilTvYMn4HXVyAftwmN3ReDuZJeWNXj4y8gu7ACcVdP4DJh0sfA83KqkhOsgAf9y4DJ8iP OCGZIYAtkcZ+f/KxXM2BDfRb9ddLfwTJn9jHsMftdAK+Yn1zDj17kaeVxZ0W/d65OjCPd6tGgZ3s1c PlXZkeb4CHB7Oav2IKW6lx+21mDFSdm39f9LFCxm+vkzRsLN7rNIJXtdYnvmDgp3BiiEN59pAnk4LM b3tUHI2SsF2nwRpgeVXNUvPYyQvhSiEwC+oY7A16TzgvXU/bF78NmoWUa++albTJtNIeclGpUY63jE v/gCI2FvzFJ6AQBKO5k4pYdUY2/9xPW0b5P6lBvZJyiNFq29tSe1Hiug56G71uztL142LyKBpY1kLG MNpkEp5mybjnAWfXuO5CJNWOaqu3nWYwWqp+3C3AoJP1ftEZo4D4d8jYJLvL/1oyLriS3Y8T5p+LNE zjwsDh7xTQmmr/+sCcnGH3Elw7u2oOGxfZoP24E+ZhWsr4d3NzZ56dxWxYswOkMITolnGYZsne/Zj9 RHyBi5T2aGJ2C2EM0GgBZxw0rpfMuR5J/mTN8WYHLei3px/8432HbTf+5lkQ== X-Developer-Key: i=keescook@chromium.org; a=openpgp; fpr=A5C3F68F229DD60F723E6E138972F4DFDC6DC026 Precedence: bulk List-ID: X-Mailing-List: linux-hardening@vger.kernel.org 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/ Reported-by: Ariadne Conill Reported-by: Michael Kerrisk Cc: Matthew Wilcox Cc: Christian Brauner Cc: Rich Felker Cc: Eric Biederman Cc: Alexander Viro Cc: linux-fsdevel@vger.kernel.org Cc: stable@vger.kernel.org Signed-off-by: Kees Cook Acked-by: Ariadne Conill Acked-by: Andy Lutomirski Acked-by: Christian Brauner --- 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;