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Wed, 02 Jul 2025 05:16:16 -0700 (PDT) Message-ID: Date: Wed, 2 Jul 2025 14:16:09 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: [PATCH v7 2/7] x86/alternatives: serialize after (self-)modifying code From: Jan Beulich To: "xen-devel@lists.xenproject.org" Cc: Andrew Cooper , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= References: <0b57c6f0-9fd0-4f8a-93f8-dc494a1e952c@suse.com> Content-Language: en-US Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: <0b57c6f0-9fd0-4f8a-93f8-dc494a1e952c@suse.com> Content-Transfer-Encoding: quoted-printable X-ZohoMail-DKIM: pass (identity @suse.com) X-ZM-MESSAGEID: 1751458595345116600 Content-Type: text/plain; charset="utf-8" While supposedly safe via enforcing a control flow change when modifying already prefetched code, it may not really be. Afaik a request is pending to drop the first of the two options in the SDM's "Handling Self- and Cross-Modifying Code" section (still present there as of version 087). Insert a serializing instruction there, and remove the "noinline" in exchange. Signed-off-by: Jan Beulich Reviewed-by: Jason Andryuk --- Strictly speaking with LIVEPATCH=3Dn the if() (but not its body) is dead code. Adding IS_ENABLED(CONFIG_LIVEPATCH) would feel quite a bit like extra clutter, though. Plus Misra rule 2.2 is globally deviated anyway. --- v6: Split off. Extend commentary. Load deterministic value into CR2. Drop "noinline". --- a/xen/arch/x86/alternative.c +++ b/xen/arch/x86/alternative.c @@ -194,13 +194,18 @@ void *place_ret(void *ptr) * You should run this with interrupts disabled or on code that is not * executing. * - * "noinline" to cause control flow change and thus invalidate I$ and - * cause refetch after modification. + * While the SDM continues to suggest using "noinline" would be sufficient= , it + * may not be, e.g. due to errata. Issue a serializing insn afterwards, u= nless + * this is for live-patching, where we modify code before it goes live. I= ssue + * a serializing insn which is unlikely to be intercepted by a hypervisor,= in + * case we run virtualized ourselves. */ -static void init_or_livepatch noinline +static void init_or_livepatch text_poke(void *addr, const void *opcode, size_t len) { memcpy(addr, opcode, len); + if ( system_state < SYS_STATE_active ) + asm volatile ( "mov %0, %%cr2" :: "r" (0L) : "memory" ); } =20 extern void *const __initdata_cf_clobber_start[];