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[157.230.128.187]) by smtp.gmail.com with ESMTPSA id fu12sm11110748pjb.20.2020.05.11.17.17.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 11 May 2020 17:17:03 -0700 (PDT) Received: by 42.do-not-panic.com (Postfix, from userid 1000) id 1579B40E88; Tue, 12 May 2020 00:17:03 +0000 (UTC) Date: Tue, 12 May 2020 00:17:03 +0000 From: Luis Chamberlain To: Rafael Aquini Cc: Tso Ted , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, keescook@chromium.org, yzaikin@google.com Subject: Re: [PATCH] kernel: sysctl: ignore invalid taint bits introduced via kernel.tainted and taint the kernel with TAINT_USER on writes Message-ID: <20200512001702.GW11244@42.do-not-panic.com> References: <20200511215904.719257-1-aquini@redhat.com> <20200511231045.GV11244@42.do-not-panic.com> <20200511235914.GF367616@optiplex-lnx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200511235914.GF367616@optiplex-lnx> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, May 11, 2020 at 07:59:14PM -0400, Rafael Aquini wrote: > On Mon, May 11, 2020 at 11:10:45PM +0000, Luis Chamberlain wrote: > > On Mon, May 11, 2020 at 05:59:04PM -0400, Rafael Aquini wrote: > > > diff --git a/kernel/sysctl.c b/kernel/sysctl.c > > > index 8a176d8727a3..f0a4fb38ac62 100644 > > > --- a/kernel/sysctl.c > > > +++ b/kernel/sysctl.c > > > @@ -2623,17 +2623,32 @@ static int proc_taint(struct ctl_table *table, int write, > > > return err; > > > > > > if (write) { > > > + int i; > > > + > > > + /* > > > + * Ignore user input that would make us committing > > > + * arbitrary invalid TAINT flags in the loop below. > > > + */ > > > + tmptaint &= (1UL << TAINT_FLAGS_COUNT) - 1; > > > > This looks good but we don't pr_warn() of information lost on intention. > > > > Are you thinking in sth like: > > + if (tmptaint > TAINT_FLAGS_MAX) { > + tmptaint &= TAINT_FLAGS_MAX; > + pr_warn("proc_taint: out-of-range invalid input ignored" > + " tainted_mask adjusted to 0x%x\n", tmptaint); > + } > ? Sure that would clarify this. > > > + > > > /* > > > * Poor man's atomic or. Not worth adding a primitive > > > * to everyone's atomic.h for this > > > */ > > > - int i; > > > for (i = 0; i < BITS_PER_LONG && tmptaint >> i; i++) { > > > if ((tmptaint >> i) & 1) > > > add_taint(i, LOCKDEP_STILL_OK); > > > } > > > + > > > + /* > > > + * Users with SYS_ADMIN capability can include any arbitrary > > > + * taint flag by writing to this interface. If that's the case, > > > + * we also need to mark the kernel "tainted by user". > > > + */ > > > + add_taint(TAINT_USER, LOCKDEP_STILL_OK); > > > > I'm in favor of this however I'd like to hear from Ted on if it meets > > the original intention. I would think he had a good reason not to add > > it here. > > > > Fair enough. The impression I got by reading Ted's original commit > message is that the intent was to have TAINT_USER as the flag set > via this interface, even though the code was allowing for any > arbitrary value. That wasn't my reading, it was that the user did something very odd with user input which we don't like as kernel developers, and it gives us a way to prove: hey you did something stupid, sorry but I cannot support your kernel panic. > I think it's OK to let the user fiddle with > the flags, as it's been allowed since the introduction of > this interface, but we need to reflect that fact in the > tainting itself. Since TAINT_USER is not used anywhere, I see users of TAINT_USER sprinkled around > this change perfectly communicates that fact without > the need for introducing yet another taint flag. I'd be happy if we don't have introduce yet-anothe flag as well. But since Ted introduced it, without using the flag on the proc_taint() I'd like confirmation we won't screw things up with existing test cases which assume proc_taint() won't set this up. We'd therefore regress userspace. This is why I'd like for us to be careful with this flag. Luis