From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by sourceware.org (Postfix) with ESMTP id 329C23857C40 for ; Mon, 8 Feb 2021 14:53:39 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 329C23857C40 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A9BF71FB; Mon, 8 Feb 2021 06:53:38 -0800 (PST) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8DC043F719; Mon, 8 Feb 2021 06:53:37 -0800 (PST) Date: Mon, 8 Feb 2021 14:53:16 +0000 From: Dave Martin To: Mark Brown Cc: Catalin Marinas , Will Deacon , Mark Rutland , libc-alpha@sourceware.org, Kees Cook , Jeremy Linton , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] arm64: bti: Set PROT_BTI on all BTI executables mapped by the kernel Message-ID: <20210208145315.GE21837@arm.com> References: <20210205173837.39315-1-broonie@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210205173837.39315-1-broonie@kernel.org> User-Agent: Mutt/1.5.23 (2014-03-12) X-Spam-Status: No, score=-11.9 required=5.0 tests=BAYES_00, GIT_PATCH_0, KAM_DMARC_STATUS, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 08 Feb 2021 14:53:40 -0000 On Fri, Feb 05, 2021 at 05:38:37PM +0000, Mark Brown via Libc-alpha wrote: > Currently for dynamically linked executables the kernel only enables > PROT_BTI for the interpreter, the interpreter is responsible for > enabling it for everything else including the main executable. > Unfortunately this interacts poorly with systemd's > MemoryDenyWriteExecute feature which uses a seccomp filter to prevent > setting PROT_EXEC on already mapped memory via mprotect(), it lacks the > context to detect that PROT_EXEC is already set and so refuses to allow > the mprotect() on the main executable which the kernel has already > mapped. > > Since we don't want to force users to choose between having MDWX and BTI > as these are othogonal features have the kernel enable PROT_BTI for all > the ELF objects it loads, not just the dynamic linker. This means that > if there is a problem with BTI it will be harder to disable at the > executable level but we currently have no conditional support for this > in any libc anyway so that would be new development. Ideally we would > have interfaces that allowed us to more clearly specify what is enabled > and disabled by a given syscall but this would be a far more difficult > change to deploy. > > Reported-by: Jeremy Linton > Suggested-by: Catalin Marinas > Signed-off-by: Mark Brown > Cc: Mark Rutland > Cc: Szabolcs Nagy > Cc: Dave Martin > Cc: Kees Cook > Cc: libc-alpha@sourceware.org > --- > > This solution was proposed by Catalin, I'm just writing it up into a > patch since it looks to be what we've converged on as the most practical > solution and but things seemed to have stalled out. > > arch/arm64/kernel/process.c | 8 -------- > 1 file changed, 8 deletions(-) > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > index 71c8265b9139..0967f9e1f9fd 100644 > --- a/arch/arm64/kernel/process.c > +++ b/arch/arm64/kernel/process.c > @@ -717,14 +717,6 @@ asmlinkage void __sched arm64_preempt_schedule_irq(void) > int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state, > bool has_interp, bool is_interp) > { > - /* > - * For dynamically linked executables the interpreter is > - * responsible for setting PROT_BTI on everything except > - * itself. > - */ > - if (is_interp != has_interp) > - return prot; > - Reviewed-by: Dave Martin > if (!(state->flags & ARM64_ELF_BTI)) > return prot; The original idea was to interfere with userspace as little as possible, and leave this until/unless there was a clear need for it and a clear understanding that it wouldn't break anything. Looks like we have both of those now -- I'll leave it to Szabolcs to confirm the userspace view of this. Question: will this change prevent BTI executables from working under a non-BTI-aware ld.so? And do we care? (I think "probably not" for both, but I'd be interested in others' views.) Cheers ---Dave