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[209.85.220.65]) by mx.google.com with SMTPS id k37sor20237533pgb.78.2019.02.12.10.23.38 for (Google Transport Security); Tue, 12 Feb 2019 10:23:38 -0800 (PST) Received-SPF: pass (google.com: domain of nadav.amit@gmail.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b="PwWWi/6z"; spf=pass (google.com: domain of nadav.amit@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=nadav.amit@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:subject:from:in-reply-to:date:cc :content-transfer-encoding:message-id:references:to; bh=nxsq3mYQfT8TSMKFYDwSPeWmp3gy8uwIjz9pCZKxPZU=; b=PwWWi/6z/KLx8kp9wjrAUlk1y9SQcQBpzqLI155xbQuh2NASaJrqCN5SSi//wUVc1t m3IZF6bQ0tcP181OqJfOC4PP5/jFvvVbmxqDrfbignDy8hOUrbxl+8jnwbZeoolbLdL4 XEz0GAUuJKq9evlAyfWx2172mzsxDQi2dKwmA/IQe/S8JkBksQoSmUGVq1jtWhmSgJc0 S24uEzPuL1T0dRYazHcnumWpjkpZRALizgsdThj7fKm4E8IDJyI3bRVvVr7yy3kI1ZNG wlVU/cnXIyVdyyBk6OVe0QBKrhyRc6Vl+YvVzbEghLWC6V5qmdIl7zdcaTSyrxQzEDjh Jqzw== X-Google-Smtp-Source: AHgI3IaRR3twVY25CJj/D8rBGyc9tBMWkA30qmXX+UKb5QbpMaxH737L2/uz8wJqnvc1p4BG6pttIw== X-Received: by 2002:a63:d052:: with SMTP id s18mr4726523pgi.11.1549995817579; Tue, 12 Feb 2019 10:23:37 -0800 (PST) Received: from [10.33.115.182] ([66.170.99.1]) by smtp.gmail.com with ESMTPSA id n72sm22979040pfg.13.2019.02.12.10.23.35 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 12 Feb 2019 10:23:36 -0800 (PST) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 12.2 \(3445.102.3\)) Subject: Re: [PATCH v2 05/20] x86/alternative: initializing temporary mm for patching From: Nadav Amit In-Reply-To: Date: Tue, 12 Feb 2019 10:23:35 -0800 Cc: Rick Edgecombe , Ingo Molnar , LKML , X86 ML , "H. Peter Anvin" , Thomas Gleixner , Borislav Petkov , Dave Hansen , Peter Zijlstra , Damian Tometzki , linux-integrity , LSM List , Andrew Morton , Kernel Hardening , Linux-MM , Will Deacon , Ard Biesheuvel , Kristen Carlson Accardi , "Dock, Deneen T" , Kees Cook , Dave Hansen Content-Transfer-Encoding: quoted-printable Message-Id: <570C76C6-DDD2-49C4-8DAF-E8CFEAA21081@gmail.com> References: <20190129003422.9328-1-rick.p.edgecombe@intel.com> <20190129003422.9328-6-rick.p.edgecombe@intel.com> <162C6C29-CD81-46FE-9A54-6ED05A93A9CB@gmail.com> <00649AE8-69C0-4CD2-A916-B8C8F0F5DAC3@amacapital.net> <6FE10C97-25FF-4E99-A96A-465CBACA935B@gmail.com> <3EA322C6-5645-4900-AEC6-97FC05716F75@gmail.com> To: Andy Lutomirski X-Mailer: Apple Mail (2.3445.102.3) 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 Feb 11, 2019, at 2:47 PM, Andy Lutomirski wrote: >=20 > On Mon, Feb 11, 2019 at 11:18 AM Nadav Amit = wrote: >>=20 >> + >> + /* >> + * If breakpoints are enabled, disable them while the = temporary mm is >> + * used - they do not belong and might cause wrong signals or = crashes. >> + */ >=20 > Maybe clarify this? Add some mention that the specific problem is > that user code could set a watchpoint on an address that is also used > in the temporary mm. >=20 > Arguably we should not disable *kernel* breakpoints a la perf, but > that seems like quite a minor issue, at least as long as > use_temporary_mm() doesn't get wider use. But a comment that this > also disables perf breakpoints and that this could be undesirable > might be in order as well. I think that in the future there may also be security benefits for = disabling breakpoints when you are in a sensitive code-block, for instance when = you poke text, to prevent the control flow from being hijacked (by = exploiting a bug in the debug exception handler). Some additional steps need to be = taken for that to be beneficial so I leave it out of the comment for now. Anyhow, how about this: -- >8 -- From: Nadav Amit Date: Mon, 11 Feb 2019 03:07:08 -0800 Subject: [PATCH] x86/mm: Save DRs when loading a temporary mm 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 =20 extern atomic64_t last_mm_ctx_id; =20 @@ -358,6 +359,7 @@ static inline unsigned long = __get_current_cr3_fast(void) =20 typedef struct { struct mm_struct *prev; + unsigned short bp_enabled : 1; } temp_mm_state_t; =20 /* @@ -380,6 +382,22 @@ static inline temp_mm_state_t = use_temporary_mm(struct mm_struct *mm) lockdep_assert_irqs_disabled(); state.prev =3D 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 =3D hw_breakpoint_active(); + if (state.bp_enabled) + hw_breakpoint_disable(); + return state; } =20 @@ -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(); } =20 #endif /* _ASM_X86_MMU_CONTEXT_H */ --=20 2.17.1=