From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-oa1-f72.google.com (mail-oa1-f72.google.com [209.85.160.72]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3792536CDF8 for ; Tue, 8 Sep 2026 04:34:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.160.72 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788842077; cv=none; b=R3wEtxLSifrixoULGDIdg4JW+9PoZihYT5hN1aYH6qRtiQEeM+wZk5R7DU8hza7pkGbu1GHYqORkoguNVP0p/4Dv4OmDll5xfW7mo3dDYOaf1JP0WPpItWmSwHceN0MiznW+O9nj4TD8YWaAvtio9IPcWOEdeDhFl0LKVf3+LGE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788842077; c=relaxed/simple; bh=9GI5Z4OXm6r2NqJVRrt9DakVj5uxrvUci+yMH6Y5JR0=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=u+QRUMTo0kFpL2nwIKBWFqAL7A+YrOMiK5Wt8+82lDSLws09fCEIWp/cHFUDazL7UJEEztv9epczYNzlRrKsOSUPnIb4SyVgYj17KQX8LeXaNlj6wHh+N9Gqn6XdoLAvxMnxxDfv5AuTSlR1HkW3f+rWN/SuyTxfz78bCKnBQYs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--avagin.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=m1kgJLwu; arc=none smtp.client-ip=209.85.160.72 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--avagin.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="m1kgJLwu" Received: by mail-oa1-f72.google.com with SMTP id 586e51a60fabf-464fb819ac5so3847447fac.1 for ; Mon, 07 Sep 2026 21:34:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1788842074; x=1789446874; darn=lists.linux.dev; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:from:to:cc:subject:date:message-id:reply-to :content-type; bh=E+3pXqdr3UPO/oCiF3HW/6/WAxnlQ8Fn84d6+8TUwB0=; b=m1kgJLwuaO345X9Q2OcrPURs3WabscKQP/VVtswSkJeri0Jgr9WiGhc9qpOT1Aio15 omhRETwtqDVVQedjsIkWac1UMoCP3ad+mng2JEj61oR5oZr7RvGXHaFVKfVhMn0XhAVu adtTYBcGlrqu0Gknbvp1uu4B8t2v+WhPM5Ys1y++AzyM72n0Sv3Zx8cOB4hz9b44Hxb0 kwk9f+8Q7Wda1OG3B8Qo50T351DcHnNSSQ92oCNx8DNrA9UNLvRi5vsv0ksB6S59Gcqs MawCrBvH9rtIvZplV8k7dHH3rlQNguFrj/a/SuWNxPB/Ul+ICSqIq0UQfO/aKlJ3JhZr a/bg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1788842074; x=1789446874; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to:content-type; bh=E+3pXqdr3UPO/oCiF3HW/6/WAxnlQ8Fn84d6+8TUwB0=; b=KRb4XGux84xyo+mJYjESX39T6C+ojgcoxdLOdQa1yTXhXukVdHVlHDeoliNEaD22Zb vyRVWG36YPjbd/P8oJhLa3YjHqusZlkHFJQ6JjWGR0nGleVfCBer3NzrWDa1SFP1feFY W9vWUx6LOj/hhJoqfktnVcxMgyds80YjaA4c21tO+DLm+cpQbac787wZRTtRCA2EFJwn z/xcyjrxJJUdmbXDga2y3R3OPHD/fCPpY0M7fUpHNyGwUJFH4C4wqfmQUPB+nzFTXb5j lEJV+DHzrYGP9SHJKDHME3OcUKh2DEH+Ty08A6N8wRJVOicU9FgWFDbtTsWgMgwWmu0z 1OqQ== X-Forwarded-Encrypted: i=1; AKwUvByBN9l0E4gYJm60d/ttAwycWvGNIRT6ipWxNttDVhQDrkmxHSfhy/Gz2ih5YG5LpxapYuZI@lists.linux.dev X-Gm-Message-State: AFuF++lz0gj6UH09+OeOrE/G1FStH6W5qVUZDss2yOqPADNl8yA5ZnSO HCY6e/FGF9AJzsNZal8JKTfTRQCzJn8bCCnCvHLnTxzwWhByhx8BD+E29hF+NMFeRE2xXSbzugi 3Lt1k3g== X-Received: from ilbbe4.prod.google.com ([2002:a05:6e02:3044:b0:509:a53c:7df3]) (user=avagin job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6820:1c9d:b0:6b7:46fa:1695 with SMTP id 006d021491bc7-6b746fa1a33mr14056827eaf.42.1788842073901; Mon, 07 Sep 2026 21:34:33 -0700 (PDT) Date: Tue, 8 Sep 2026 04:34:24 +0000 In-Reply-To: <20260908043427.1842515-1-avagin@google.com> Precedence: bulk X-Mailing-List: criu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260908043427.1842515-1-avagin@google.com> X-Mailer: git-send-email 2.55.0.979.g7e5102b832-goog Message-ID: <20260908043427.1842515-5-avagin@google.com> Subject: [PATCH 4/7] x86/fpu: Document reasoning of FX-only fallback From: Andrei Vagin To: Thomas Gleixner , Ingo Molnar , Borislav Petkov , "Chang S. Bae" Cc: linux-kernel@vger.kernel.org, criu@lists.linux.dev, Dave Hansen , x86@kernel.org, Andrei Vagin , Alexander Mikhalitsyn , "H. Peter Anvin" Content-Type: text/plain; charset="UTF-8" Add a comment to check_xstate_in_sigframe() to explain the reasoning behind falling back to the FX-only state when signal frame metadata is inconsistent. The fallback is intended to preserve backward compatibility with legacy user-space processes that are not aware of XSAVE states and might only fill or copy the legacy FP state. However, this fallback should be avoided whenever possible. If a process was actively using extended features, falling back to the FX-only state silently resets those extended registers to their initial state, which can lead to silent user-space state corruption. Reviewed-by: Alexander Mikhalitsyn Reviewed-by: Chang S. Bae Signed-off-by: Andrei Vagin --- arch/x86/kernel/fpu/signal.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c index 60063a7a44f8..d686d5f7f3d0 100644 --- a/arch/x86/kernel/fpu/signal.c +++ b/arch/x86/kernel/fpu/signal.c @@ -54,6 +54,14 @@ static inline bool check_xstate_in_sigframe(struct fxregs_state __user *buf_fx, if (likely(magic2 == FP_XSTATE_MAGIC2)) return true; err_setfx: + /* + * The fallback to FX-only state is used to preserve backward + * compatibility with user-space processes that are not aware of xsave + * states. + * + * In all other cases, returning false (to trigger SIGSEGV) is + * preferred to avoid silent user-space state corruption. + */ trace_x86_fpu_xstate_check_failed(x86_task_fpu(current)); /* Set the parameters for fx only state */ -- 2.55.0.979.g7e5102b832-goog