* FAILED: patch "[PATCH] exec: Force single empty string when argv is empty" failed to apply to 4.19-stable tree
@ 2022-04-02 12:43 gregkh
2022-06-01 10:09 ` [PATCH 4.19.y] exec: Force single empty string when argv is empty Vegard Nossum
0 siblings, 1 reply; 2+ messages in thread
From: gregkh @ 2022-04-02 12:43 UTC (permalink / raw)
To: keescook, ariadne, brauner, dalias, ebiederm, luto, mtk.manpages,
viro, willy
Cc: stable
The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From dcd46d897adb70d63e025f175a00a89797d31a43 Mon Sep 17 00:00:00 2001
From: Kees Cook <keescook@chromium.org>
Date: Mon, 31 Jan 2022 16:09:47 -0800
Subject: [PATCH] exec: Force single empty string when argv is empty
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 <ariadne@dereferenced.org>
Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Rich Felker <dalias@libc.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Christian Brauner <brauner@kernel.org>
Acked-by: Ariadne Conill <ariadne@dereferenced.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20220201000947.2453721-1-keescook@chromium.org
diff --git a/fs/exec.c b/fs/exec.c
index 79f2c9483302..40b1008fb0f7 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 = (max(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;
^ permalink raw reply related [flat|nested] 2+ messages in thread
* [PATCH 4.19.y] exec: Force single empty string when argv is empty
2022-04-02 12:43 FAILED: patch "[PATCH] exec: Force single empty string when argv is empty" failed to apply to 4.19-stable tree gregkh
@ 2022-06-01 10:09 ` Vegard Nossum
0 siblings, 0 replies; 2+ messages in thread
From: Vegard Nossum @ 2022-06-01 10:09 UTC (permalink / raw)
To: stable
Cc: Kees Cook, Ariadne Conill, Michael Kerrisk, Matthew Wilcox,
Christian Brauner, Rich Felker, Eric Biederman, Alexander Viro,
linux-fsdevel, Andy Lutomirski, Vegard Nossum
From: Kees Cook <keescook@chromium.org>
commit dcd46d897adb70d63e025f175a00a89797d31a43 upstream.
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 <ariadne@dereferenced.org>
Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Rich Felker <dalias@libc.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Christian Brauner <brauner@kernel.org>
Acked-by: Ariadne Conill <ariadne@dereferenced.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20220201000947.2453721-1-keescook@chromium.org
[vegard: fixed conflicts due to missing
886d7de631da71e30909980fdbf318f7caade262^- and
3950e975431bc914f7e81b8f2a2dbdf2064acb0f^- and
655c16a8ce9c15842547f40ce23fd148aeccc074]
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
---
fs/exec.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)
This has been tested in both argc == 0 and argc >= 1 cases, but I would
still appreciate a review given the differences with mainline. If it's
considered too risky I'm also fine with dropping it -- just wanted to
make sure this didn't fall through the cracks, as it does block a real
(albeit old by now) exploit.
diff --git a/fs/exec.c b/fs/exec.c
index e87e3c020c61e..28e3b5eb2f4a1 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1805,6 +1805,9 @@ static int __do_execve_file(int fd, struct filename *filename,
goto out_unmark;
bprm->argc = count(argv, MAX_ARG_STRINGS);
+ if (bprm->argc == 0)
+ pr_warn_once("process '%s' launched '%s' with NULL argv: empty string added\n",
+ current->comm, bprm->filename);
if ((retval = bprm->argc) < 0)
goto out;
@@ -1829,6 +1832,20 @@ static int __do_execve_file(int fd, struct filename *filename,
if (retval < 0)
goto out;
+ /*
+ * 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) {
+ const char *argv[] = { "", NULL };
+ retval = copy_strings_kernel(1, argv, bprm);
+ if (retval < 0)
+ goto out;
+ bprm->argc = 1;
+ }
+
retval = exec_binprm(bprm);
if (retval < 0)
goto out;
--
2.35.1.46.g38062e73e0
^ permalink raw reply related [flat|nested] 2+ messages in thread
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2022-04-02 12:43 FAILED: patch "[PATCH] exec: Force single empty string when argv is empty" failed to apply to 4.19-stable tree gregkh
2022-06-01 10:09 ` [PATCH 4.19.y] exec: Force single empty string when argv is empty Vegard Nossum
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