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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=KxGzQ00fjj4NMMFTj0STMg35dePQevrF3uI1L3l8h6Ch6qMpUVknic1Pa7Ho6tdpq kJRqyNg9eBE1FumsT2aDoih3OASlw6943L0Ly/+rGM/rqafDgW1aj0bs3zy8wCENvt zaBHeJ99hJLOgGO55gQZngzYR7PomAo6uK5Pi/NMX2DXYxdmqpUxHCG0CMsHNiKfqJ sm/B8BshZR8VdjI7NitHPFbI2uIGRqZA+DaB/DIjEnobALNK0OR3FdCM7I816GCG95 6DXEFloZrSATNt1W0lEQjiwwgwO4kQa/U5efLvOr2CuwZ78fZyQbekjuDamfs5cHhD qiInNBCzs0kqg== Message-ID: Date: Tue, 18 Feb 2025 23:55:21 -0800 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 1/1] x86/ia32: Leave NULL selector values 0~3 as is To: Andrew Cooper , linux-kernel@vger.kernel.org Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, brgerst@gmail.com, ebiederm@xmission.com References: <20241126184529.1607334-1-xin@zytor.com> <36970ddb-c0d8-43e4-a94e-0d9ea3d55ced@zytor.com> Content-Language: en-US From: Xin Li Autocrypt: addr=xin@zytor.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2/14/2025 6:01 AM, Andrew Cooper wrote: > On 14/02/2025 6:56 am, Xin Li wrote: >> On 12/12/2024 10:44 AM, Xin Li wrote: >>> On 11/26/2024 10:45 AM, Xin Li (Intel) wrote: >>>> The first GDT descriptor is reserved as 'NULL descriptor'.  As bits 0 >>>> and 1 of a segment selector, i.e., the RPL bits, are NOT used to index >>>> GDT, selector values 0~3 all point to the NULL descriptor, thus values >>>> 0, 1, 2 and 3 are all valid NULL selector values. >>>> >>>> When a NULL selector value is to be loaded into a segment register, >>>> reload_segments() sets its RPL bits.  Later IRET zeros ES, FS, GS, and >>>> DS segment registers if any of them is found to have any nonzero NULL >>>> selector value.  The two operations offset each other to actually >>>> effect >>>> a nop. >>>> >>>> Besides, zeroing of RPL in NULL selector values is an information leak >>>> in pre-FRED systems as userspace can spot any interrupt/exception by >>>> loading a nonzero NULL selector, and waiting for it to become zero. >>>> But there is nothing software can do to prevent it before FRED. >>>> >>>> ERETU, the only legit instruction to return to userspace from kernel >>>> under FRED, by design does NOT zero any segment register to avoid this >>>> problem behavior. >>>> >>>> As such, leave NULL selector values 0~3 as is. >>> >>> Hi Andrew, >>> >>> Do you have any more comments? >> >> Hi Andrew, >> >> Are you okay to give a review-by to this patch? > > Apologies. > > Reviewed-by: Andrew Cooper Thank you very much! Thanks! Xin