From owner-freebsd-hackers@freebsd.org Wed Mar 31 11:21:31 2021 Return-Path: Delivered-To: freebsd-hackers@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id 84BEF5C43D6 for ; Wed, 31 Mar 2021 11:21:31 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from mailman.nyi.freebsd.org (unknown [127.0.1.3]) by mx1.freebsd.org (Postfix) with ESMTP id 4F9P5l1ybKz3CGl for ; Wed, 31 Mar 2021 11:21:31 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: by mailman.nyi.freebsd.org (Postfix) id 433575C43D5; Wed, 31 Mar 2021 11:21:31 +0000 (UTC) Delivered-To: hackers@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id 42FBB5C43D4 for ; Wed, 31 Mar 2021 11:21:31 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 4F9P5k6x8lz3C3M; Wed, 31 Mar 2021 11:21:30 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from tom.home (kib@localhost [127.0.0.1]) by kib.kiev.ua (8.16.1/8.16.1) with ESMTPS id 12VBLGKc077447 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NO); Wed, 31 Mar 2021 14:21:19 +0300 (EEST) (envelope-from kostikbel@gmail.com) DKIM-Filter: OpenDKIM Filter v2.10.3 kib.kiev.ua 12VBLGKc077447 Received: (from kostik@localhost) by tom.home (8.16.1/8.16.1/Submit) id 12VBLGlZ077446; Wed, 31 Mar 2021 14:21:16 +0300 (EEST) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Wed, 31 Mar 2021 14:21:16 +0300 From: Konstantin Belousov To: Alan Somers Cc: "freebsd-hackers@freebsd.org" Subject: Re: How does the stack's guard page work on amd64? Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Spam-Status: No, score=-1.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_ADSP_CUSTOM_MED,FORGED_GMAIL_RCVD,FREEMAIL_FROM, NML_ADSP_CUSTOM_MED autolearn=no autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on tom.home X-Rspamd-Queue-Id: 4F9P5k6x8lz3C3M X-Spamd-Bar: ---- Authentication-Results: mx1.freebsd.org; none X-Spamd-Result: default: False [-4.00 / 15.00]; REPLY(-4.00)[] X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: Technical discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 31 Mar 2021 11:21:31 -0000 On Tue, Mar 30, 2021 at 08:28:09PM -0600, Alan Somers wrote: > On Tue, Mar 30, 2021 at 3:35 AM Konstantin Belousov > wrote: > > > On Mon, Mar 29, 2021 at 11:06:36PM -0600, Alan Somers wrote: > > > Rust tries to detect stack overflow and handles it differently than other > > > segfaults, but it's currently broken on FreeBSD/amd64. I've got a patch > > > that fixes the problem, but I would like someone to confirm my reasoning. > > > > > > It seems like FreeBSD's main thread stacks include a guard page at the > > > bottom. However, when Rust tries to create its own guard page (by > > > re-mmap()ping and mprotect()ing it), it seems like FreeBSD's guard page > > > automatically moves up into the un-remapped region. At least, that's how > > > it behaves, based on the addresses that segfault. Is that correct? > > Show the facts. For instance, procstat -v (and a note which > > mapping was established by runtime for the 'guard') would tell the whole > > story. > > > > My guess would be that procctl(PROC_STACKGAP_CTL, &PROC_STACKGAP_DISABLE) > > would be enough. Cannot tell without specific data. > > > > > > > > For other threads, Rust doesn't try to remap the guard page, it just > > relies > > > on the guard page created by libthr in _thr_stack_alloc. > > > > > > Finally, what changed in between FreeBSD 10.3 and 11.4? Rust's stack > > > overflow detection worked in 10.3. > > > > > > -Alan > > > _______________________________________________ > > > freebsd-hackers@freebsd.org mailing list > > > https://lists.freebsd.org/mailman/listinfo/freebsd-hackers > > > To unsubscribe, send any mail to " > > freebsd-hackers-unsubscribe@freebsd.org" > > > > Here is the relevant portion of procstat -v for a test program built with > the buggy rustc: > 651 0x801554000 0x80155d000 rw- 0 17 3 0 ----- df > 651 0x801600000 0x801e00000 rw- 30 30 1 0 ----- df > 651 0x7fffdfffd000 0x7fffdfffe000 --- 0 0 0 0 ----- -- > 651 0x7fffdfffe000 0x7fffdffff000 --- 0 0 0 0 ----- -- > <--- What Rustc thinks is the guard page > 651 0x7fffdffff000 0x7fffe0000000 --- 0 0 0 0 ----- -- > <--- Where did this come from? This is the stack grow area, occupied by 'elastic' guard entry. It serves two purposes: 1. it keeps the space, preventing other non-fixed mappings from selecting the grow area for mapping. 2. it prevents stack from growing down to the next mapping below it, preventing issues like StackClash. See mmap(2) esp. MAP_STACK part of it. > 651 0x7fffe0000000 0x7fffe001e000 rw- 30 30 1 0 ---D- df > 651 0x7fffe001e000 0x7fffe003e000 rw- 32 32 1 0 ---D- df > > Rustc tries to create that guard page by finding the base address of the > stack, reallocating one page, then mprotect()ing it, like this: > mmap(0x7fffdfffe000,0x1000,0x3,0x1012,0xffffffff,0) > mprotect(0x7fffdfffe000,0x1000,0) > > If I patch rustc to not attempt to allocate a guard page, then its memory > map looks like this. Notice that 0x7fffdffff000 is now accessible It is accessible because stack grown down into this address. > 662 0x801531000 0x80155b000 rw- 3 17 3 0 ----- df > 662 0x801600000 0x801e00000 rw- 30 30 1 0 ----- df > 662 0x7fffdfffd000 0x7fffdfffe000 --- 0 0 0 0 ----- -- > 662 0x7fffdfffe000 0x7fffdffff000 --- 0 0 0 0 ----- -- > 662 0x7fffdffff000 0x7fffe001e000 rw- 31 31 1 0 ---D- df > 662 0x7fffe001e000 0x7fffe003e000 rw- 32 32 1 0 ---D- df > > So the real question is, why does 0x7fffdffff000 become protected when > rustc protects 0x7fffdfffe000 ? See above. As I said in earlier response, if you want fully shrinkable stack guard, set procctl(PROC_STACKGAP_CTL, &PROC_STACKGAP_DISABLE) during runtime initialization. Or better, do not create custom guard page at all, relying on system guard.