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Wed, 04 Jun 2025 05:18:37 -0700 (PDT) Message-ID: <67be78c8-d276-4edc-a101-51f168e200da@gmail.com> Date: Wed, 4 Jun 2025 15:18:36 +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] x86/its: Always return a writable page for dynamic thunks To: Nikolay Borisov , x86@kernel.org Cc: pawan.kumar.gupta@linux.intel.com, peterz@infradead.org, linux-kernel@vger.kernel.org, dave.hansen@linux.intel.com References: <20250604104715.700149-1-nik.borisov@suse.com> From: Nikolay Borisov Content-Language: en-US In-Reply-To: <20250604104715.700149-1-nik.borisov@suse.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 6/4/25 13:47, Nikolay Borisov wrote: > its_alloc unconditionally allocates and return a ROX page, but setting > it RW is predicated on its_mod being set. This can cause a #GP on the > memset in its_allocate_thunk() if its_mod is not set. > > Since the function always returns a page, ensure it's always set > writable so it can be initialized properly. > > Fixes: 872df34d7c51 ("x86/its: Use dynamic thunks for indirect branches") > Signed-off-by: Nikolay Borisov > --- > arch/x86/kernel/alternative.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c > index ecfe7b497cad..191e909636a5 100644 > --- a/arch/x86/kernel/alternative.c > +++ b/arch/x86/kernel/alternative.c > @@ -199,6 +199,8 @@ static void *its_alloc(void) > if (!page) > return NULL; > > + execmem_make_temp_rw(page, PAGE_SIZE); > + > #ifdef CONFIG_MODULES > if (its_mod) { > void *tmp = krealloc(its_mod->its_page_array, > @@ -210,7 +212,6 @@ static void *its_alloc(void) > its_mod->its_page_array = tmp; > its_mod->its_page_array[its_mod->its_num_pages++] = page; > > - execmem_make_temp_rw(page, PAGE_SIZE); > } > #endif /* CONFIG_MODULES */ > Ok, this actually works in the upstream kernel thanks to d6d1e3e6580ca35071ad474381f053cbf1fb6414 meaning at the time retpolines are processed the execmem cache is actually writable...