From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-oi1-f199.google.com (mail-oi1-f199.google.com [209.85.167.199]) (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 6B1BB4E2F33 for ; Wed, 16 Sep 2026 23:23:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.167.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789601007; cv=none; b=SYCCL34Epqj1YYQlnOo1vxwtv54lAbE6Tko54avPtZOS2xukt1N0nfkAMNumYpx9gBOyzXpOlxu3euecfXkeCEVIBawH0jcy4Vl5xBbFSVZ48t5NIKNNuHNMtH9f8PTYGYpCvDPP7q/AXg1XBntVyLq+5jS62ZPteApAf9b4wE4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789601007; c=relaxed/simple; bh=ebQ3kkBOnMbGpYQizGxh7N6zJHu9JIKJfF0XRRz/hPs=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=aX855rilJgckCVN2GN7jmT1k6dkI/xtQWF6gdXS4jqA+y4pL4wWPJQoYXZFAx5/f/JSPzdmUa9Ow/6qM8+F2mFMcib/wkRbhD5JPlvoVfYGObXy1usjLXIVOoCWpemizXmN0dE6mVae5r2BAmqnId6ny0LwpK5J+ynid5l4cfgs= 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=XygavY3w; arc=none smtp.client-ip=209.85.167.199 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="XygavY3w" Received: by mail-oi1-f199.google.com with SMTP id 5614622812f47-4b26d8d2498so446480b6e.2 for ; Wed, 16 Sep 2026 16:23:25 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1789601004; x=1790205804; 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=xSSmIBW07WEV1TvZ4Ek3lr4k1UmQn4uFX87eg6ruy0w=; b=XygavY3wO0X0zXIdQIwrKxNL3YRDgVpvolQ0rHePEnzwDy/NoZpu1HQjusy7Ed0cFV dLUEhiVMY/A1f0pGFml3smllSl5hQqOyRYc1ynXfXhSu9gC34UDosnhoEdI5kvVWdmyA tjPDEZ+NpyaP48GFdOdC2pg0S9zxsPX9ywcCqBQt5c3JVAH27iyCArCzIBeJF2Emkr3L 94QezNlWqGpgY9H7OrC4cZwopQocg2RD+SdtgA4PiKaJh+zJwGiqvetAh9WsBdSGFHQE ZcVLVtKj+TA/AEXwqmzD2kAxHwP0x/7P5I4gN1QWCiRiIc9BtxjUoc5WwDpSAiyBMi0I RkdA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20260707; t=1789601004; x=1790205804; 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=xSSmIBW07WEV1TvZ4Ek3lr4k1UmQn4uFX87eg6ruy0w=; b=1pl3WCrtcqqbF6MQ6wZnn1mhhNbCFHjFt16GKv3CiGfTw1VtjjjQf2AuaWutK27FwG S7cKzyggXQ+7XmU3Fzvk1OTOCxD6vmvO4wIVJUGBbE1sh6GJWRZKlCV9ymNdu7Q37TNb NdjKWvyzPXYECPZmy+27WDRWOKeImNWcnS5XE+t7RekXQAK1vb8roj4Wl8xqUl0EyTnD 6hqcnoob3uz4YS8V1QrhgC8YJrB8EWxSmZHR1LTeIYlb4WJYavlvafHpD+BCu0CtHZbS 4jcwZBt+RivpBuQuvi6ZmdhC7mkFVbDQg3ViWmh+QiDJtBMCSMwuO5kdggXOrI3gXAK3 v7cA== X-Forwarded-Encrypted: i=1; AKwUvBymZuX4sIiJggbcMx0KnLsJhxf98qW5r6xSx95WJMvBusHuf+vfEOFFzb/ugMFkyhAs0Ia0@lists.linux.dev X-Gm-Message-State: AFuF++kTeitulL6ZyhIckysK/AdsVkT9mNuNsMsXoV6mIMHZyhkP5kn9 mugdX92K9zK2ba0Cbx5tbLOqoYCEGcoVSEi+8MLdMgdTkgmRXJgxthPemKBOPxYXUYrVuOrUdKQ vJHEHfA== X-Received: from ilol10.prod.google.com ([2002:a92:d8ca:0:b0:509:9557:9554]) (user=avagin job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6808:c136:b0:4bb:ac05:9ebf with SMTP id 5614622812f47-4ca49ecc9b2mr4306460b6e.14.1789601003946; Wed, 16 Sep 2026 16:23:23 -0700 (PDT) Date: Wed, 16 Sep 2026 23:23:07 +0000 In-Reply-To: <20260916232310.490786-1-avagin@google.com> Precedence: bulk X-Mailing-List: criu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260916232310.490786-1-avagin@google.com> X-Mailer: git-send-email 2.55.0.1082.g2b9226bbc0-goog Message-ID: <20260916232310.490786-5-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 db069947bdb8..050e58691964 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 */ -- 2.55.0.1082.g2b9226bbc0-goog