From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from sfi-mx-4.v28.ch3.sourceforge.com ([172.29.28.124] helo=mx.sourceforge.net) by 235xhf1.ch3.sourceforge.com with esmtp (Exim 4.69) (envelope-from ) id 1MR1W0-0000b0-DD for ltp-list@lists.sourceforge.net; Wed, 15 Jul 2009 10:15:08 +0000 Received: from moutng.kundenserver.de ([212.227.17.9]) by 1b2kzd1.ch3.sourceforge.com with esmtp (Exim 4.69) id 1MR1Vu-0006Bq-0N for ltp-list@lists.sourceforge.net; Wed, 15 Jul 2009 10:15:08 +0000 From: Arnd Bergmann Date: Wed, 15 Jul 2009 12:14:52 +0200 References: <4A5C8068.6020203@monstr.eu> <200907141843.05629.arnd@arndb.de> <9e6f3dfd0907141811p512b4edp3f9dd0fdeae1123e@mail.gmail.com> In-Reply-To: <9e6f3dfd0907141811p512b4edp3f9dd0fdeae1123e@mail.gmail.com> MIME-Version: 1.0 Message-Id: <200907151214.52369.arnd@arndb.de> Subject: Re: [LTP] access_ok macor List-Id: Linux Test Project General Discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: ltp-list-bounces@lists.sourceforge.net To: John Williams Cc: LTP , monstr@monstr.eu, Linux Kernel list , Ralf Baechle On Wednesday 15 July 2009, John Williams wrote: > On Wed, Jul 15, 2009 at 2:43 AM, Arnd Bergmann wrote: > > The solution then is to handle fixups from the unaligned exception handler > > if you come from the kernel. That should fix the three text cases. > > > > I don't fully understand your exception handling there, but I think you > > also need to add code checking for __range_ok() to your unaligned handler, > > to prevent malicious user space code from accessing the kernel through > > unaligned pointers. > > > Just to try to clarify - are there any alignment rules in the ABI on > user-space pointers (which end up going to get/put_user)? The kernel normally expects aligned input from user space, but I guess it can't hurt to handle it anyway. arch/mips/kernel/alignment.c seems to handle that case. Maybe Ralf can give some more insight. > It seems the failure path is like this: > > 1. userspace passes unaligned pointer > 2. get_user attempts to access > 3. CPU raises unaligned exception (if only it would raise the segfault as > higher priority, before the unaligned!) > 4. unaligned exception handler attempts to simulate the unaligned access > with multiple partial read/write ops > 5. CPU raises MMU exception on the read/write by the unaligned handler > 6. kernel segfault handler looks up faulting address, it is in the unaligned > exception handler, which has no fixup. > 7. no fixup -> failure Right. > So, I suppose the question is - where in the sequence is the true failure? I think in step 4. AFIACT, the kernel must do a number of checks on accesses to random pointers. > Clearly LTP thinks it's ok to pass unaligned pointers to the kernel, > suggesting (1) is fine - thus my question about alignment rules in the ABI. No, LTP thinks it should get a -EFAULT error code for that access. It does specify whether it expects this because of an unaligned address or because of an invalid page. > Do we need fixups on the unaligned handler itself? This will be ugly ugly > ugly. That's what ARM does. You don't have to do it from assembly though, implementing it in C is probably easier. > Or, some way of tracing the segfault back through the unaligned > exception and to the root cause (the get/put-user), and call that fixup as > required? Yes, I guess that would have to look roughly like this: int emulate_insn(struct pt_regs *regs, unsigned long addr, unsigned long len) { /* use inline assembly with fixups here, return -EFAULT on bad addr */ } void alignment_exception(struct pt_regs *regs, unsigned long addr, unsigned long len) { const struct exception_table_entry *fixup; int err; if (user_mode(regs)) { if (!access_ok(addr, len)) goto segv; if (emulate_insn(regs) == -EFAULT)) goto segv; } else { if (!access_ok(addr, len)) goto fixup; if (emulate_insn(regs, addr, len) == -EFAULT)) goto fixup; return; fixup: fixup = search_exception_tables(regs->ip); if (!fixup) goto segv; regs->ip = fixup->fixup; return; segv: force_sig(SIGSEGV, current)); } ------------------------------------------------------------------------------ Enter the BlackBerry Developer Challenge This is your chance to win up to $100,000 in prizes! For a limited time, vendors submitting new applications to BlackBerry App World(TM) will have the opportunity to enter the BlackBerry Developer Challenge. See full prize details at: http://p.sf.net/sfu/Challenge _______________________________________________ Ltp-list mailing list Ltp-list@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/ltp-list