From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-qk1-f169.google.com (mail-qk1-f169.google.com [209.85.222.169]) (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 BCDFB2C0F93 for ; Thu, 11 Jun 2026 12:32:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.222.169 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781181139; cv=none; b=Su3dWSWGjiAJAf8/Ncq59nVotHF3zYUZNxsb112bTcCdSIZvtLabFMsStEY/RgkgSMXMqS9f6/GXpTp3MzYn6lbVJ0qUjta38vY/bEVBBxsaASVt/P0Ot+oKRxXBd7fgk8VlM1h1WeoU5q4b3utbkDEU6u65xXKlof5sF6myOxI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781181139; c=relaxed/simple; bh=BSx2REIDdYh6rBcLzILSd3dV8BAOIWvSwTx3IBfTmHI=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=I4dF0u8kumkwznXh+qFcnXi0hxnKA1y/A/S5GvwEGjs5ZDYZI0lAT3tOA7V2bLqxF7ZuqsyVL/EB7oUET9yo0YYA8B1hU/tESx0MW8NmhE1/xjhwJEeaL8BD48LsqnqRIR0RMDzKqpTfp+iCUJ+cWhI76fDM5XO/oq5KEkcd1Ik= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=fusS4jyO; arc=none smtp.client-ip=209.85.222.169 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="fusS4jyO" Received: by mail-qk1-f169.google.com with SMTP id af79cd13be357-9159f631656so128904785a.1 for ; Thu, 11 Jun 2026 05:32:17 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1781181137; x=1781785937; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=mLymrEZazNmQKknDX1QeqtCgPzdIbUvDepEeTIb67zo=; b=fusS4jyOa5zHNNzrdMeHlzjDsolfIbVNkICGkyClIIWsCwESgBqYtsHIEEU3x6HhGO 0JPnrpiybvQ4cWf4I4/S9UO3IcosKCI3tCMZbOWEsZay2gGu4ra3Fy4Li/ImZIHagM6x FUlMWvSW/pjSYWc0cD9h0O7i8l9V6Woydhq4bG6BP6hXkifYqt5kNhmUpHI5uR+aoaT6 oyregi29XXuDRDQG+KJEmyfIWqzYYK1bQrfeDJUemX3S53sqGS7T5j2MsBpYIxlfTpy4 ndHOD4LMCD1+YoH45+iawFoZlNkbcay+eo70O/X0LGtmPDTT3pCIweHfVi5Ioe4WCsjM xaaQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1781181137; x=1781785937; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-gg:x-gm-message-state:from :to:cc:subject:date:message-id:reply-to; bh=mLymrEZazNmQKknDX1QeqtCgPzdIbUvDepEeTIb67zo=; b=nYnXUgJ/iPLm1eUzoSEcivaxihhPNZD4nzY1tZr0vbJ3wEwC4WlCp6e9ZceAIm2UFG soS2/IHXYn1h+t83oCAvI6nYjZ7Tf0+UKfkfkaN69NclgZ3mnsqM6DFhnJ6PSX0d2Oti /27wY7KlPbdlWTJPXEhvFyiq+UhYJqwutFp9PuLAYhPIAQI63PJy+y0pjHUfftjNVKFZ MsF5OZJR/U2zGMkV+/uUb3uMOZmRCogWbdzeGL74zbijXFiUMxNyuBQ0KGxXVtji5w6p cvG87xrZ6637iLqQfmc3+bgVXfpt53zrISxtIAZ5CcH4MbVDWkZWfPA0F8DMQCdvBAAd IuHw== X-Forwarded-Encrypted: i=1; AFNElJ8rPUtrkFQ4frV9CYt0XjJT+vna5Up9qBEzoi2MbzZXYvHEfaXg0oD7Bf6/hGZNMY5kcRmbECLYbnke0VM=@vger.kernel.org X-Gm-Message-State: AOJu0YyqI4O4XUHRtpA46UaLNee7kP/IeDu3va0zsKFhu6X9YCQI33z8 r3NwoFAsGyMdmF1wU1mNODkrBpfbkWDbeq+Pw0QTlwkrPGMVjFalkmk0DQsuDXR/EMs= X-Gm-Gg: Acq92OGZj61NGIgzht5S4gPwVY6TQLcvEqvROah1r6H5OVMivBDfXZlxyMA2dsxSXXN rd8H/ZHYZcVEqrmpD77dWxyzA+nR5adCSMPV71oCmeYpAK05HTJeqg9QStu3UFrp+lzeW0aoaoO +13mXkfNPkOSY98+pZ6aibHxfWPCLig1F+yZetGck63RtoJ2F/Xx7ivrzGw/1uaqqkoe4D5zkhB kSYE8LUSX+POldyWWXLSrlLqm0MyDE+AvgYUAFrZDtwh7tgRDbppPW2U8GSNYweQ29JLSt6HQdI Valco772YiHi3EWpWsweRd64MOIgZIvSxteWFMcjQ6AJx/0pqB6XFBwtBtoiDZreN3gS0FJ7Htn 6N7exaYTUEAj6/pI6Oh5PKus817LicLUo+Iv7/Bmv14t79AgRb79lWMaIOS6Fh91wDE7Q6khIYs 0whpvUrOGTkFLihdF75SGTgU7F9Tt+KFi59ZOapme3BPCMyposN9Qbi2YxojaME50ogpXWd+JnM 4VCCB37azCrnnvJUK/jBKzPBt4SrRw= X-Received: by 2002:a05:620a:440c:b0:910:c1ba:91d3 with SMTP id af79cd13be357-9160a841f7fmr283346285a.45.1781181136610; Thu, 11 Jun 2026 05:32:16 -0700 (PDT) Received: from server0 (c-68-48-65-54.hsd1.mi.comcast.net. [68.48.65.54]) by smtp.gmail.com with ESMTPSA id af79cd13be357-9160b02f758sm171220685a.36.2026.06.11.05.32.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 11 Jun 2026 05:32:15 -0700 (PDT) From: Michael Bommarito To: Juergen Gross , Stefano Stabellini , Oleksandr Tyshchenko Cc: xen-devel@lists.xenproject.org, linux-scsi@vger.kernel.org, stable@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH 1/2] xen/scsiback: free unsubmitted command instead of double-putting it Date: Thu, 11 Jun 2026 08:30:45 -0400 Message-ID: <20260611123046.2323342-2-michael.bommarito@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260611123046.2323342-1-michael.bommarito@gmail.com> References: <20260611123046.2323342-1-michael.bommarito@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 7bit scsiback_get_pend_req() obtains a command tag and returns a vscsibk_pend whose embedded se_cmd has only been memset to 0, so its cmd_kref is 0; the se_cmd is initialised (kref_init() via target_init_cmd()) only later, in scsiback_cmd_exec(), on the successful VSCSIIF_ACT_SCSI_CDB path. The two error paths in scsiback_do_cmd_fn() taken before the command is submitted -- a failed scsiback_gnttab_data_map() and an unknown ring_req.act -- call transport_generic_free_cmd(&pending_req->se_cmd, 0), which kref_put()s a refcount of 0. That underflows it ("refcount_t: underflow; use-after-free") and, as the release function is not run, leaks the command tag. Impact: a pvSCSI guest can leak every command tag of a LUN's session, stopping the LUN, by submitting requests with a bad grant reference or an unknown request type; under panic_on_warn the refcount underflow panics the host. Add a helper that just returns the tag with target_free_tag() and sends the error response. It frees the tag while the v2p reference still pins the session, and snapshots the response fields beforehand because freeing the tag can let another ring reuse the pending_req slot. Fixes: 2dbcdf33dbf6 ("xen-scsiback: Convert to percpu_ida tag allocation") Cc: stable@vger.kernel.org Assisted-by: Claude:claude-opus-4-8 Signed-off-by: Michael Bommarito --- Reproduced on a Xen dom0 (Linux 6.1.y) exporting a pvSCSI LUN to a guest. A frontend that sends a single ring request with an unknown action type drives scsiback_do_cmd_fn() into transport_generic_free_cmd() on the never-initialised command and logs refcount_t: underflow; use-after-free WARNING: ... refcount_warn_saturate transport_generic_free_cmd+0x... [target_core_mod] scsiback_do_cmd_fn+0x... [xen_scsiback] scsiback_irq_fn+0x... [xen_scsiback] from the vscsiif IRQ thread, and panics the dom0 under panic_on_warn. The failed grant-map path reaches the same free. With this patch the same request is answered with DID_ERROR and the tag is returned, with no underflow. These error paths are unchanged since 2dbcdf33dbf6, so mainline is affected identically. drivers/xen/xen-scsiback.c | 28 ++++++++++++++++++++++------ 1 file changed, 22 insertions(+), 6 deletions(-) diff --git a/drivers/xen/xen-scsiback.c b/drivers/xen/xen-scsiback.c index e33f95c91b096..f324732eba7f8 100644 --- a/drivers/xen/xen-scsiback.c +++ b/drivers/xen/xen-scsiback.c @@ -611,6 +611,25 @@ static void scsiback_disconnect(struct vscsibk_info *info) xenbus_unmap_ring_vfree(info->dev, info->ring.sring); } +/* + * Send the error response for a request that did not reach the target core + * and return its tag. Free the tag before the response drops the v2p + * reference that keeps the session alive, and snapshot what the response + * needs since returning the tag can let the slot be reused. + */ +static void scsiback_resp_and_free(struct vscsibk_pend *pending_req, + int32_t result) +{ + struct vscsibk_info *info = pending_req->info; + struct v2p_entry *v2p = pending_req->v2p; + struct se_session *se_sess = v2p->tpg->tpg_nexus->tvn_se_sess; + u16 rqid = pending_req->rqid; + + target_free_tag(se_sess, &pending_req->se_cmd); + scsiback_send_response(info, NULL, result, 0, rqid); + kref_put(&v2p->kref, scsiback_free_translation_entry); +} + static void scsiback_device_action(struct vscsibk_pend *pending_req, enum tcm_tmreq_table act, int tag) { @@ -792,9 +811,8 @@ static int scsiback_do_cmd_fn(struct vscsibk_info *info, case VSCSIIF_ACT_SCSI_CDB: if (scsiback_gnttab_data_map(&ring_req, pending_req)) { scsiback_fast_flush_area(pending_req); - scsiback_do_resp_with_sense(NULL, - DID_ERROR << 16, 0, pending_req); - transport_generic_free_cmd(&pending_req->se_cmd, 0); + scsiback_resp_and_free(pending_req, + DID_ERROR << 16); } else { scsiback_cmd_exec(pending_req); } @@ -808,9 +826,7 @@ static int scsiback_do_cmd_fn(struct vscsibk_info *info, break; default: pr_err_ratelimited("invalid request\n"); - scsiback_do_resp_with_sense(NULL, DID_ERROR << 16, 0, - pending_req); - transport_generic_free_cmd(&pending_req->se_cmd, 0); + scsiback_resp_and_free(pending_req, DID_ERROR << 16); break; } -- 2.53.0