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; (aka I like my bikesheds painted in Toasty Eggshell)
