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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id u17sm17431673pgi.14.2022.01.26.14.49.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 26 Jan 2022 14:49:14 -0800 (PST) Date: Wed, 26 Jan 2022 14:49:13 -0800 From: Kees Cook To: Ariadne Conill Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Eric Biederman , Alexander Viro Subject: Re: [PATCH v2] fs/exec: require argv[0] presence in do_execveat_common() Message-ID: <202201261440.0C13601104@keescook> References: <20220126114447.25776-1-ariadne@dereferenced.org> <202201261202.EC027EB@keescook> <202201261239.CB5D7C991A@keescook> <5e963fab-88d4-2039-1cf4-6661e9bd16b@dereferenced.org> <202201261323.9499FA51@keescook> <64e91dc2-7f5c-6e8-308e-414c82a8ae6b@dereferenced.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <64e91dc2-7f5c-6e8-308e-414c82a8ae6b@dereferenced.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 26, 2022 at 03:30:13PM -0600, Ariadne Conill wrote: > Hi, > > On Wed, 26 Jan 2022, Kees Cook wrote: > > > On Wed, Jan 26, 2022 at 03:13:10PM -0600, Ariadne Conill wrote: > > > Looks good to me, but I wonder if we shouldn't set an argv of > > > {bprm->filename, NULL} instead of {"", NULL}. Discussion in IRC led to the > > > realization that multicall programs will try to use argv[0] and might crash > > > in this scenario. If we're going to fake an argv, I guess we should try to > > > do it right. > > > > They're crashing currently, though, yes? I think the goal is to move > > toward making execve(..., NULL, NULL) just not work at all. Using the > > {"", NULL} injection just gets us closer to protecting a bad userspace > > program. I think things _should_ crash if they try to start depending > > on this work-around. > > Is there a reason to spawn a program, just to have it crash, rather than > just denying it to begin with, though? I think the correct behavior here is to unconditionally reject a NULL argv -- and I wish this had gotten fixed in 2008. :P Given the code we've found that depends on NULL argv, I think we likely can't make the change outright, so we're down this weird rabbit hole of trying to reject what we can and create work-around behaviors for the cases that currently exist. I think new users of the new work-around shouldn't be considered. We'd prefer they get a rejection, etc. > I mean, it all seems fine enough, and perhaps I'm just a bit picky on the > colors and flavors of my bikesheds, so if you want to go with this patch, > I'll be glad to carry it in the Alpine security update I am doing to make > sure the *other* GLib-using SUID programs people find don't get exploited in > the same way. They "don't break userspace" guideline is really "don't break userspace if someone notices". :P Since this is a mitigation (not strictly a security flaw fix), changes to userspace behavior tend to be very conservatively viewed by Linus. ;) My preference is the earlier very simple version to fix this: diff --git a/fs/exec.c b/fs/exec.c index 79f2c9483302..aabadcf4a525 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1897,6 +1897,8 @@ static int do_execveat_common(int fd, struct filename *filename, } retval = count(argv, MAX_ARG_STRINGS); + if (reval == 0) + retval = -EINVAL; if (retval < 0) goto out_free; bprm->argc = retval; So, I guess we should start there and send a patch to valgrind? -- Kees Cook