Hi, On Wed, 26 Jan 2022, Matthew Wilcox wrote:
On Wed, Jan 26, 2022 at 11:44:47AM +0000, Ariadne Conill wrote:In several other operating systems, it is a hard requirement that the first 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[0]: The argument arg0 should point to a filename string that is associated with the process being started by one of the exec functions. To ensure that execve(2) with argc < 1 is not a useful gadget for shellcode to use, we can validate this in do_execveat_common() and fail for this scenario, effectively blocking successful exploitation of CVE-2021-4034 and similar bugs which depend on this gadget. The use of -EFAULT for this case is similar to other systems, such as FreeBSD, OpenBSD and Solaris. QNX uses -EINVAL for this case. Interestingly, Michael Kerrisk opened an issue about this in 2008[1], but there was no consensus to support fixing this issue then. Hopefully now that CVE-2021-4034 shows practical exploitative use of this bug in a shellcode, we can reconsider. [0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html [1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408 Changes from v1: - Rework commit message significantly. - Make the argv[0] check explicit rather than hijacking the error-check for count(). Signed-off-by: Ariadne Conill <ariadne@xxxxxxxxxxxxxxxx> --- fs/exec.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/fs/exec.c b/fs/exec.c index 79f2c9483302..e52c41991aab 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1899,6 +1899,10 @@ static int do_execveat_common(int fd, struct filename *filename, retval = count(argv, MAX_ARG_STRINGS); if (retval < 0) goto out_free; + if (retval == 0) { + retval = -EFAULT; + goto out_free; + }I don't object to the concept, but it's a more common pattern in Linux to do this: retval = count(argv, MAX_ARG_STRINGS); + if (retval == 0) + retval = -EFAULT; if (retval < 0) goto out_free;
Yeah, that seems fine. We can of course do it that way, which I will revise the patch to do if we decide to stick with denial over making a "safe" argv instead.
(aka I like my bikesheds painted in Toasty Eggshell)
Toasty Eggshell is a nice color for a bikeshed :) Ariadne
