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Tue, 22 Apr 2025 06:38:06 -0700 (PDT) Message-ID: <20eaf26f-90e0-44b4-aa4a-ca4aa06be1dd@suse.com> Date: Tue, 22 Apr 2025 16:38:05 +0300 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 2/2] x86/devmem: Drop /dev/mem access for confidential guests To: Dan Williams , dave.hansen@linux.intel.com Cc: x86@kernel.org, Kees Cook , Ingo Molnar , Naveen N Rao , Vishal Annapurve , Kirill Shutemov , stable@vger.kernel.org, linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org References: <174491712829.1395340.5054725417641299524.stgit@dwillia2-xfh.jf.intel.com> <174500659632.1583227.11220240508166521765.stgit@dwillia2-xfh.jf.intel.com> From: Nikolay Borisov Content-Language: en-US Autocrypt: addr=nik.borisov@suse.com; keydata= xsFNBGcrpvIBEAD5cAR5+qu30GnmPrK9veWX5RVzzbgtkk9C/EESHy9Yz0+HWgCVRoNyRQsZ 7DW7vE1KhioDLXjDmeu8/0A8u5nFMqv6d1Gt1lb7XzSAYw7uSWXLPEjFBtz9+fBJJLgbYU7G OpTKy6gRr6GaItZze+r04PGWjeyVUuHZuncTO7B2huxcwIk9tFtRX21gVSOOC96HcxSVVA7X N/LLM2EOL7kg4/yDWEhAdLQDChswhmdpHkp5g6ytj9TM8bNlq9I41hl/3cBEeAkxtb/eS5YR 88LBb/2FkcGnhxkGJPNB+4Siku7K8Mk2Y6elnkOctJcDvk29DajYbQnnW4nhfelZuLNupb1O M0912EvzOVI0dIVgR+xtosp66bYTOpX4Xb0fylED9kYGiuEAeoQZaDQ2eICDcHPiaLzh+6cc pkVTB0sXkWHUsPamtPum6/PgWLE9vGI5s+FaqBaqBYDKyvtJfLK4BdZng0Uc3ijycPs3bpbQ bOnK9LD8TYmYaeTenoNILQ7Ut54CCEXkP446skUMKrEo/HabvkykyWqWiIE/UlAYAx9+Ckho TT1d2QsmsAiYYWwjU8igXBecIbC0uRtF/cTfelNGrQwbICUT6kJjcOTpQDaVyIgRSlUMrlNZ XPVEQ6Zq3/aENA8ObhFxE5PLJPizJH6SC89BMKF3zg6SKx0qzQARAQABzSZOaWtvbGF5IEJv cmlzb3YgPG5pay5ib3Jpc292QHN1c2UuY29tPsLBkQQTAQoAOxYhBDuWB8EJLBUZCPjT3SRn XZEnyhfsBQJnK6byAhsDBQsJCAcCAiICBhUKCQgLAgQWAgMBAh4HAheAAAoJECRnXZEnyhfs XbIQAJxuUnelGdXbSbtovBNm+HF3LtT0XnZ0+DoR0DemUGuA1bZAlaOXGr5mvVbTgaoGUQIJ 3Ejx3UBEG7ZSJcfJobB34w1qHEDO0pN9orGIFT9Bic3lqhawD2r85QMcWwjsZH5FhyRx7P2o DTuUClLMO95GuHYQngBF2rHHl8QMJPVKsR18w4IWAhALpEApxa3luyV7pAAqKllfCNt7tmed uKmclf/Sz6qoP75CvEtRbfAOqYgG1Uk9A62C51iAPe35neMre3WGLsdgyMj4/15jPYi+tOUX Tc7AAWgc95LXyPJo8069MOU73htZmgH4OYy+S7f+ArXD7h8lTLT1niff2bCPi6eiAQq6b5CJ Ka4/27IiZo8tm1XjLYmoBmaCovqx5y5Xt2koibIWG3ZGD2I+qRwZ0UohKRH6kKVHGcrmCv0J YO8yIprxgoYmA7gq21BpTqw3D4+8xujn/6LgndLKmGESM1FuY3ymXgj5983eqaxicKpT9iq8 /a1j31tms4azR7+6Dt8H4SagfN6VbJ0luPzobrrNFxUgpjR4ZyQQ++G7oSRdwjfIh1wuCF6/ mDUNcb6/kA0JS9otiC3omfht47yQnvod+MxFk1lTNUu3hePJUwg1vT1te3vO5oln8lkUo9BU knlYpQ7QA2rDEKs+YWqUstr4pDtHzwQ6mo0rqP+zzsFNBGcrpvIBEADGYTFkNVttZkt6e7yA LNkv3Q39zQCt8qe7qkPdlj3CqygVXfw+h7GlcT9fuc4kd7YxFys4/Wd9icj9ZatGMwffONmi LnUotIq2N7+xvc4Xu76wv+QJpiuGEfCDB+VdZOmOzUPlmMkcJc/EDSH4qGogIYRu72uweKEq VfBI43PZIGpGJ7TjS3THX5WVI2YNSmuwqxnQF/iVqDtD2N72ObkBwIf9GnrOgxEyJ/SQq2R0 g7hd6IYk7SOKt1a8ZGCN6hXXKzmM6gHRC8fyWeTqJcK4BKSdX8PzEuYmAJjSfx4w6DoxdK5/ 9sVrNzaVgDHS0ThH/5kNkZ65KNR7K2nk45LT5Crjbg7w5/kKDY6/XiXDx7v/BOR/a+Ryo+lM MffN3XSnAex8cmIhNINl5Z8CAvDLUtItLcbDOv7hdXt6DSyb65CdyY8JwOt6CWno1tdjyDEG 5ANwVPYY878IFkOJLRTJuUd5ltybaSWjKIwjYJfIXuoyzE7OL63856MC/Os8PcLfY7vYY2LB cvKH1qOcs+an86DWX17+dkcKD/YLrpzwvRMur5+kTgVfXcC0TAl39N4YtaCKM/3ugAaVS1Mw MrbyGnGqVMqlCpjnpYREzapSk8XxbO2kYRsZQd8J9ei98OSqgPf8xM7NCULd/xaZLJUydql1 JdSREId2C15jut21aQARAQABwsF2BBgBCgAgFiEEO5YHwQksFRkI+NPdJGddkSfKF+wFAmcr pvICGwwACgkQJGddkSfKF+xuuxAA4F9iQc61wvAOAidktv4Rztn4QKy8TAyGN3M8zYf/A5Zx VcGgX4J4MhRUoPQNrzmVlrrtE2KILHxQZx5eQyPgixPXri42oG5ePEXZoLU5GFRYSPjjTYmP ypyTPN7uoWLfw4TxJqWCGRLsjnkwvyN3R4161Dty4Uhzqp1IkNhl3ifTDYEvbnmHaNvlvvna 7+9jjEBDEFYDMuO/CA8UtoVQXjy5gtOhZZkEsptfwQYc+E9U99yxGofDul7xH41VdXGpIhUj 4wjd3IbgaCiHxxj/M9eM99ybu5asvHyMo3EFPkyWxZsBlUN/riFXGspG4sT0cwOUhG2ZnExv XXhOGKs/y3VGhjZeCDWZ+0ZQHPCL3HUebLxW49wwLxvXU6sLNfYnTJxdqn58Aq4sBXW5Un0Q vfbd9VFV/bKFfvUscYk2UKPi9vgn1hY38IfmsnoS8b0uwDq75IBvup9pYFyNyPf5SutxhFfP JDjakbdjBoYDWVoaPbp5KAQ2VQRiR54lir/inyqGX+dwzPX/F4OHfB5RTiAFLJliCxniKFsM d8eHe88jWjm6/ilx4IlLl9/MdVUGjLpBi18X7ejLz3U2quYD8DBAGzCjy49wJ4Di4qQjblb2 pTXoEyM2L6E604NbDu0VDvHg7EXh1WwmijEu28c/hEB6DwtzslLpBSsJV0s1/jE= In-Reply-To: <174500659632.1583227.11220240508166521765.stgit@dwillia2-xfh.jf.intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 18.04.25 г. 23:04 ч., Dan Williams wrote: > Nikolay reports [1] that accessing BIOS data (first 1MB of the physical > address space) via /dev/mem results in an SEPT violation. > > The cause is ioremap() (via xlate_dev_mem_ptr()) establishes an > unencrypted mapping where the kernel had established an encrypted > mapping previously. > > Linux traps read(2) access to the BIOS data area, and returns zero. > However, it turns out the kernel fails to enforce the same via mmap(2). > This is a hole, and unfortunately userspace has learned to exploit it > [2]. > > This means the kernel either needs a mechanism to ensure consistent > "encrypted" mappings of this /dev/mem mmap() hole, close the hole by > mapping the zero page in the mmap(2) path, block only BIOS data access > and let typical STRICT_DEVMEM protect the rest, or disable /dev/mem > altogether. > > The simplest option for now is arrange for /dev/mem to always behave as > if lockdown is enabled for confidential guests. Require confidential > guest userspace to jettison legacy dependencies on /dev/mem similar to > how other legacy mechanisms are jettisoned for confidential operation. > Recall that modern methods for BIOS data access are available like > /sys/firmware/dmi/tables. > > Cc: > Cc: Kees Cook > Cc: Ingo Molnar > Cc: "Naveen N Rao" > Cc: Vishal Annapurve > Cc: Kirill Shutemov > Link: https://sources.debian.org/src/libdebian-installer/0.125/src/system/subarch-x86-linux.c/?hl=113#L93 [2] > Reported-by: Nikolay Borisov > Closes: http://lore.kernel.org/20250318113604.297726-1-nik.borisov@suse.com [1] > Fixes: 9aa6ea69852c ("x86/tdx: Make pages shared in ioremap()") > Cc: > Acked-by: Dave Hansen > Signed-off-by: Dan Williams > --- > Changes since v3 > * Fix a 0day kbuild robot report about missing cc_platform.h include. > > arch/x86/Kconfig | 4 ++++ > drivers/char/mem.c | 10 ++++++++++ > 2 files changed, 14 insertions(+) > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig > index 4b9f378e05f6..bf4528d9fd0a 100644 > --- a/arch/x86/Kconfig > +++ b/arch/x86/Kconfig > @@ -891,6 +891,8 @@ config INTEL_TDX_GUEST > depends on X86_X2APIC > depends on EFI_STUB > depends on PARAVIRT > + depends on STRICT_DEVMEM > + depends on IO_STRICT_DEVMEM > select ARCH_HAS_CC_PLATFORM > select X86_MEM_ENCRYPT > select X86_MCE > @@ -1510,6 +1512,8 @@ config AMD_MEM_ENCRYPT > bool "AMD Secure Memory Encryption (SME) support" > depends on X86_64 && CPU_SUP_AMD > depends on EFI_STUB > + depends on STRICT_DEVMEM > + depends on IO_STRICT_DEVMEM > select DMA_COHERENT_POOL > select ARCH_USE_MEMREMAP_PROT > select INSTRUCTION_DECODER > diff --git a/drivers/char/mem.c b/drivers/char/mem.c > index 48839958b0b1..47729606b817 100644 > --- a/drivers/char/mem.c > +++ b/drivers/char/mem.c > @@ -30,6 +30,7 @@ > #include > #include > #include > +#include > > #define DEVMEM_MINOR 1 > #define DEVPORT_MINOR 4 > @@ -595,6 +596,15 @@ static int open_port(struct inode *inode, struct file *filp) > if (rc) > return rc; > > + /* > + * Enforce encrypted mapping consistency and avoid unaccepted > + * memory conflicts, "lockdown" /dev/mem for confidential > + * guests. > + */ > + if (IS_ENABLED(CONFIG_STRICT_DEVMEM) && > + cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) > + return -EPERM; Just confirming - the STRIC_DEVMEM check here is needed in case other CoCo technologies i.e ARM's CCA or risc-v tvm doesn't depend on it? Because for the x86 world it's redundant since both implementations imply STRICT_DEVMEM. > + > if (iminor(inode) != DEVMEM_MINOR) > return 0; > >