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[134.134.136.100]) by mx.google.com with ESMTPS id s2si226478pfm.289.2019.02.21.16.07.32 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 21 Feb 2019 16:07:33 -0800 (PST) Received-SPF: pass (google.com: domain of sean.j.christopherson@intel.com designates 134.134.136.100 as permitted sender) client-ip=134.134.136.100; Authentication-Results: mx.google.com; spf=pass (google.com: domain of sean.j.christopherson@intel.com designates 134.134.136.100 as permitted sender) smtp.mailfrom=sean.j.christopherson@intel.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=intel.com X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by orsmga105.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 21 Feb 2019 16:07:32 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.58,397,1544515200"; d="scan'208";a="145517897" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.181]) by fmsmga002.fm.intel.com with ESMTP; 21 Feb 2019 16:07:31 -0800 Date: Thu, 21 Feb 2019 16:07:31 -0800 From: Sean Christopherson To: Rick Edgecombe Cc: Andy Lutomirski , Ingo Molnar , linux-kernel@vger.kernel.org, x86@kernel.org, hpa@zytor.com, Thomas Gleixner , Borislav Petkov , Nadav Amit , Dave Hansen , Peter Zijlstra , linux_dti@icloud.com, linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org, akpm@linux-foundation.org, kernel-hardening@lists.openwall.com, linux-mm@kvack.org, will.deacon@arm.com, ard.biesheuvel@linaro.org, kristen@linux.intel.com, deneen.t.dock@intel.com, Nadav Amit Subject: Re: [PATCH v3 03/20] x86/mm: Save DRs when loading a temporary mm Message-ID: <20190222000731.GE7224@linux.intel.com> References: <20190221234451.17632-1-rick.p.edgecombe@intel.com> <20190221234451.17632-4-rick.p.edgecombe@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190221234451.17632-4-rick.p.edgecombe@intel.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Feb 21, 2019 at 03:44:34PM -0800, Rick Edgecombe wrote: > From: Nadav Amit > > Prevent user watchpoints from mistakenly firing while the temporary mm > is being used. As the addresses that of the temporary mm might overlap > those of the user-process, this is necessary to prevent wrong signals > or worse things from happening. > > Cc: Andy Lutomirski > Signed-off-by: Nadav Amit > --- > arch/x86/include/asm/mmu_context.h | 25 +++++++++++++++++++++++++ > 1 file changed, 25 insertions(+) > > diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h > index d684b954f3c0..0d6c72ece750 100644 > --- a/arch/x86/include/asm/mmu_context.h > +++ b/arch/x86/include/asm/mmu_context.h > @@ -13,6 +13,7 @@ > #include > #include > #include > +#include > > extern atomic64_t last_mm_ctx_id; > > @@ -358,6 +359,7 @@ static inline unsigned long __get_current_cr3_fast(void) > > typedef struct { > struct mm_struct *prev; > + unsigned short bp_enabled : 1; > } temp_mm_state_t; > > /* > @@ -380,6 +382,22 @@ static inline temp_mm_state_t use_temporary_mm(struct mm_struct *mm) > lockdep_assert_irqs_disabled(); > state.prev = this_cpu_read(cpu_tlbstate.loaded_mm); > switch_mm_irqs_off(NULL, mm, current); > + > + /* > + * If breakpoints are enabled, disable them while the temporary mm is > + * used. Userspace might set up watchpoints on addresses that are used > + * in the temporary mm, which would lead to wrong signals being sent or > + * crashes. > + * > + * Note that breakpoints are not disabled selectively, which also causes > + * kernel breakpoints (e.g., perf's) to be disabled. This might be > + * undesirable, but still seems reasonable as the code that runs in the > + * temporary mm should be short. > + */ > + state.bp_enabled = hw_breakpoint_active(); Pretty sure caching hw_breakpoint_active() is unnecessary. It queries a per-cpu value, not hardware's DR7 register, and that same value is consumed by hw_breakpoint_restore(). No idea if breakpoints can be disabled while using a temp mm, but even if that can happen, there's no need to restore breakpoints if they've all been disabled, i.e. if hw_breakpoint_active() returns false in unuse_temporary_mm(). > + if (state.bp_enabled) > + hw_breakpoint_disable(); > + > return state; > } > > @@ -387,6 +405,13 @@ static inline void unuse_temporary_mm(temp_mm_state_t prev) > { > lockdep_assert_irqs_disabled(); > switch_mm_irqs_off(NULL, prev.prev, current); > + > + /* > + * Restore the breakpoints if they were disabled before the temporary mm > + * was loaded. > + */ > + if (prev.bp_enabled) > + hw_breakpoint_restore(); > } > > #endif /* _ASM_X86_MMU_CONTEXT_H */ > -- > 2.17.1 >