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h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=h9u9GPdvNR0ElftakLufpf9mHKlErVrcyDDV2jsss4vCqrBJJ/AIfIxyUtYqYki+B 5AZMMVzgViW/+KZiiqGc/KKV8U+77/iH6qmxeRAq6U20prPu27LZsDDlWPKTR6fWu9 6ICr1JBdXyiW8nq88GGwv6L1w17v4sowt29eoig22ISkBf3NqKvOs2IrOpWvIZA14g CXSGTVlJ7PpGgWY2rxcOL3tUhBpe2fGKhJ+GQNYR/0is/sjnsCjt0QIhyuDFHuuXxf IwS+0P69F+Ar6m0NcKw5Rw18lYqb/JyXr3qzF3BSbVgCep537ppLEasuG9YprogKP4 FvveROTHv4WJA== Message-ID: Date: Mon, 11 Aug 2025 15:48:47 -0700 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 v3 1/1] x86/fred: Remove ENDBR64 from FRED entry points From: Xin Li To: linux-kernel@vger.kernel.org Cc: luto@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, jmill@asu.edu, peterz@infradead.org, andrew.cooper3@citrix.com, stable@vger.kernel.org References: <20250716063320.1337818-1-xin@zytor.com> Content-Language: en-US Autocrypt: addr=xin@zytor.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 7/15/2025 11:33 PM, Xin Li (Intel) wrote: > The FRED specification has been changed in v9.0 to state that there > is no need for FRED event handlers to begin with ENDBR64, because > in the presence of supervisor indirect branch tracking, FRED event > delivery does not enter the WAIT_FOR_ENDBRANCH state. > > As a result, remove ENDBR64 from FRED entry points. > > Then add ANNOTATE_NOENDBR to indicate that FRED entry points will > never be used for indirect calls to suppress an objtool warning. > > This change implies that any indirect CALL/JMP to FRED entry points > causes #CP in the presence of supervisor indirect branch tracking. > > Credit goes to Jennifer Miller and other contributors > from Arizona State University whose research shows that placing ENDBR > at entry points has negative value thus led to this change. > > Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code") > Link: https://lore.kernel.org/linux-hardening/Z60NwR4w%2F28Z7XUa@ubun/ > Reviewed-by: H. Peter Anvin (Intel) > Reviewed-by: Andrew Cooper > Signed-off-by: Xin Li (Intel) > Cc: Jennifer Miller > Cc: Peter Zijlstra > Cc: Andrew Cooper > Cc: H. Peter Anvin > Cc: stable@vger.kernel.org # v6.9+ > --- > > Change in v3: > *) Revise the FRED spec change description to clearly indicate that it > deviates from previous versions and is based on new research showing > that placing ENDBR at entry points has negative value (Andrew Cooper). > > Change in v2: > *) CC stable and add a fixes tag (PeterZ). > --- > arch/x86/entry/entry_64_fred.S | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S > index 29c5c32c16c3..907bd233c6c1 100644 > --- a/arch/x86/entry/entry_64_fred.S > +++ b/arch/x86/entry/entry_64_fred.S > @@ -16,7 +16,7 @@ > > .macro FRED_ENTER > UNWIND_HINT_END_OF_STACK > - ENDBR > + ANNOTATE_NOENDBR > PUSH_AND_CLEAR_REGS > movq %rsp, %rdi /* %rdi -> pt_regs */ > .endm Ping :)