From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-3.2 required=5.0 tests=MAILING_LIST_MULTI, RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL, SUBJ_OBFU_PUNCT_FEW autolearn=ham autolearn_force=no version=3.4.4 Received: (qmail 22569 invoked from network); 31 Aug 2020 01:42:00 -0000 Received: from mother.openwall.net (195.42.179.200) by inbox.vuxu.org with ESMTPUTF8; 31 Aug 2020 01:42:00 -0000 Received: (qmail 32146 invoked by uid 550); 31 Aug 2020 01:41:58 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 32128 invoked from network); 31 Aug 2020 01:41:57 -0000 Date: Sun, 30 Aug 2020 21:41:45 -0400 From: Rich Felker To: musl@lists.openwall.com Cc: Theodore Dubois Message-ID: <20200831014145.GI3265@brightrain.aerifal.cx> References: <3C00D395-838B-4DB0-99FC-3947F1BCF054@icloud.com> <20200831010710.GH3265@brightrain.aerifal.cx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200831010710.GH3265@brightrain.aerifal.cx> User-Agent: Mutt/1.5.21 (2010-09-15) Subject: Re: [musl] i386 __set_thread_area will crash if the syscall fails On Sun, Aug 30, 2020 at 09:07:10PM -0400, Rich Felker wrote: > On Sun, Aug 30, 2020 at 05:34:09PM -0700, Theodore Dubois wrote: > > Found a (small) bug in this file: > > https://git.musl-libc.org/cgit/musl/tree/src/thread/i386/__set_thread_area..s > > > > If the syscall fails, the branch on line 20 is taken and %eax will > > be a small negative number. Then "mov $123,%al" will make syscall > > 0xffffff7b instead of 0x7b, since overwriting %al only overwrites > > the low byte of %eax. So the modify_ldt fallback has apparently > > never worked. > > Thanks! Indeed, systems where the first syscall fails are outside the > actually-supported version range (before 2.6) so it's likely that this > was never actually tested. Nowadays SECCOMP_FILTER makes it easy to > test, so we should actually try to test some of these things. Have you > checked if it works adding xor %eax,%eax before the byte mov or > changing it to a 32-bit mov? OK, I just confirmed it works after this change. I'll push a fix soon along with a bunch of other pending commits. Thanks again for the report. Rich