From patchwork Mon Apr 20 19:07:49 2026 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ross Burton X-Patchwork-Id: 86513 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from aws-us-west-2-korg-lkml-1.web.codeaurora.org (localhost.localdomain [127.0.0.1]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1E191F5A8B1 for ; Mon, 20 Apr 2026 19:08:04 +0000 (UTC) Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by mx.groups.io with SMTP id smtpd.msgproc02-g2.1311.1776712076881667769 for ; Mon, 20 Apr 2026 12:07:57 -0700 Authentication-Results: mx.groups.io; dkim=fail reason="dkim: body hash did not verify" header.i=@arm.com header.s=foss header.b=CyxAZvnm; spf=pass (domain: arm.com, ip: 217.140.110.172, mailfrom: ross.burton@arm.com) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E26DF22E6 for ; Mon, 20 Apr 2026 12:07:50 -0700 (PDT) Received: from cesw-amp-gbt-1s-m12830-04.lab.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 314293F915 for ; Mon, 20 Apr 2026 12:07:56 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1776712076; bh=/dkCmsLUpPZR7uyV3smlAtVH/CrACEo+KHJHtzPXMzc=; h=From:To:Subject:Date:In-Reply-To:References:From; b=CyxAZvnmVvUn5IDPnvbNwD1POMKB3Lo24MMfo7zwFL2o5QTLSEA9SfEgWgx3kHCvk +qmYcBH4NASnuT4JACVTUZfv0YufMmlElPTRx0xRp3pCdcRREhv/FjURHMzn/BvDwE ofT4/v1EJxQMMDDfsTpx1QdOqVYJ3rDlcTL8Y4X8= From: Ross Burton To: openembedded-core@lists.openembedded.org Subject: [PATCH 5/5] util-linux: fix CVE-2026-27456 Date: Mon, 20 Apr 2026 20:07:49 +0100 Message-ID: <20260420190749.1280090-5-ross.burton@arm.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260420190749.1280090-1-ross.burton@arm.com> References: <20260420190749.1280090-1-ross.burton@arm.com> MIME-Version: 1.0 List-Id: X-Webhook-Received: from 45-33-107-173.ip.linodeusercontent.com [45.33.107.173] by aws-us-west-2-korg-lkml-1.web.codeaurora.org with HTTPS for ; Mon, 20 Apr 2026 19:08:04 -0000 X-Groupsio-URL: https://lists.openembedded.org/g/openembedded-core/message/235586 Backport a patch from upstream to fix CVE-2026-27456: Prior to version 2.41.4, a TOCTOU (Time-of-Check-Time-of-Use) vulnerability has been identified in the SUID binary /usr/bin/mount from util-linux. The mount binary, when setting up loop devices, validates the source file path with user privileges via fork() + setuid() + realpath(), but subsequently re-canonicalizes and opens it with root privileges (euid=0) without verifying that the path has not been replaced between both operations. Neither O_NOFOLLOW, nor inode comparison, nor post-open fstat() are employed. This allows a local unprivileged user to replace the source file with a symlink pointing to any root-owned file or device during the race window, causing the SUID binary to open and mount it as root. Exploitation requires an /etc/fstab entry with user,loop options whose path points to a directory where the attacker has write permission, and that /usr/bin/mount has the SUID bit set (the default configuration on virtually all Linux distributions). The impact is unauthorized read access to root-protected files and block devices, including backup images, disk volumes, and any file containing a valid filesystem. Signed-off-by: Ross Burton --- meta/recipes-core/util-linux/util-linux.inc | 1 + ...DEV_FL_NOFOLLOW-to-prevent-symlink-a.patch | 114 ++++++++++++++++++ 2 files changed, 115 insertions(+) create mode 100644 meta/recipes-core/util-linux/util-linux/0001-loopdev-add-LOOPDEV_FL_NOFOLLOW-to-prevent-symlink-a.patch diff --git a/meta/recipes-core/util-linux/util-linux.inc b/meta/recipes-core/util-linux/util-linux.inc index deb9bfd0644..02358626669 100644 --- a/meta/recipes-core/util-linux/util-linux.inc +++ b/meta/recipes-core/util-linux/util-linux.inc @@ -20,6 +20,7 @@ SRC_URI = "${KERNELORG_MIRROR}/linux/utils/util-linux/v${MAJOR_VERSION}/util-lin file://0001-lsfd-mkfds-foreign-sockets-skip-when-lacking-sock_di.patch \ file://0001-ts-kill-decode-use-RTMIN-from-kill-L-instead-of-hard.patch \ file://0001-tests-script-Disable-size-option-test.patch \ + file://0001-loopdev-add-LOOPDEV_FL_NOFOLLOW-to-prevent-symlink-a.patch \ " SRC_URI[sha256sum] = "3330d873f0fceb5560b89a7dc14e4f3288bbd880e96903ed9b50ec2b5799e58b" diff --git a/meta/recipes-core/util-linux/util-linux/0001-loopdev-add-LOOPDEV_FL_NOFOLLOW-to-prevent-symlink-a.patch b/meta/recipes-core/util-linux/util-linux/0001-loopdev-add-LOOPDEV_FL_NOFOLLOW-to-prevent-symlink-a.patch new file mode 100644 index 00000000000..0951c9f5fbe --- /dev/null +++ b/meta/recipes-core/util-linux/util-linux/0001-loopdev-add-LOOPDEV_FL_NOFOLLOW-to-prevent-symlink-a.patch @@ -0,0 +1,114 @@ +From f55f9906b4f6eeb2b4a4120317df9de935253c10 Mon Sep 17 00:00:00 2001 +From: Karel Zak +Date: Thu, 19 Feb 2026 13:59:46 +0100 +Subject: [PATCH] loopdev: add LOOPDEV_FL_NOFOLLOW to prevent symlink attacks + +Add a new LOOPDEV_FL_NOFOLLOW flag for loop device context that +prevents symlink following in both path canonicalization and file open. + +When set: +- loopcxt_set_backing_file() uses strdup() instead of + ul_canonicalize_path() (which calls realpath() and follows symlinks) +- loopcxt_setup_device() adds O_NOFOLLOW to open() flags + +The flag is set for non-root (restricted) mount operations in +libmount's loop device hook. This prevents a TOCTOU race condition +where an attacker could replace the backing file (specified in +/etc/fstab) with a symlink to an arbitrary root-owned file between +path resolution and open(). + +Vulnerable Code Flow: + + mount /mnt/point (non-root, SUID) + mount.c: sanitize_paths() on user args (mountpoint only) + mnt_context_mount() + mnt_context_prepare_mount() + mnt_context_apply_fstab() <-- source path from fstab + hooks run at MNT_STAGE_PREP_SOURCE + hook_loopdev.c: setup_loopdev() + backing_file = fstab source path ("/home/user/disk.img") + loopcxt_set_backing_file() <-- calls realpath() as ROOT + ul_canonicalize_path() <-- follows symlinks! + loopcxt_setup_device() + open(lc->filename, O_RDWR|O_CLOEXEC) <-- no O_NOFOLLOW + +Two vulnerabilities in the path: + +1) loopcxt_set_backing_file() calls ul_canonicalize_path() which uses + realpath() -- this follows symlinks as euid=0. If the attacker swaps + the file to a symlink before this call, lc->filename becomes the + resolved target path (e.g., /root/secret.img). + +2) loopcxt_setup_device() opens lc->filename without O_NOFOLLOW. Even + if canonicalization happened correctly, the file can be swapped to a + symlink between canonicalize and open. + +Addresses: https://github.com/util-linux/util-linux/security/advisories/GHSA-qq4x-vfq4-9h9g +Signed-off-by: Karel Zak +(cherry picked from commit 5e390467b26a3cf3fecc04e1a0d482dff3162fc4) + +CVE: CVE-2026-27456 +Upstream-Status: Backport +Signed-off-by: Ross Burton +--- + include/loopdev.h | 3 ++- + lib/loopdev.c | 7 ++++++- + libmount/src/hook_loopdev.c | 3 ++- + 3 files changed, 10 insertions(+), 3 deletions(-) + +diff --git a/include/loopdev.h b/include/loopdev.h +index e5ec1c98a..6bdb1393a 100644 +--- a/include/loopdev.h ++++ b/include/loopdev.h +@@ -140,7 +140,8 @@ enum { + LOOPDEV_FL_NOIOCTL = (1 << 6), + LOOPDEV_FL_DEVSUBDIR = (1 << 7), + LOOPDEV_FL_CONTROL = (1 << 8), /* system with /dev/loop-control */ +- LOOPDEV_FL_SIZELIMIT = (1 << 9) ++ LOOPDEV_FL_SIZELIMIT = (1 << 9), ++ LOOPDEV_FL_NOFOLLOW = (1 << 10) /* O_NOFOLLOW, don't follow symlinks */ + }; + + /* +diff --git a/lib/loopdev.c b/lib/loopdev.c +index 2359bf781..76685be70 100644 +--- a/lib/loopdev.c ++++ b/lib/loopdev.c +@@ -1267,7 +1267,10 @@ int loopcxt_set_backing_file(struct loopdev_cxt *lc, const char *filename) + if (!lc) + return -EINVAL; + +- lc->filename = canonicalize_path(filename); ++ if (lc->flags & LOOPDEV_FL_NOFOLLOW) ++ lc->filename = strdup(filename); ++ else ++ lc->filename = canonicalize_path(filename); + if (!lc->filename) + return -errno; + +@@ -1408,6 +1411,8 @@ int loopcxt_setup_device(struct loopdev_cxt *lc) + + if (lc->config.info.lo_flags & LO_FLAGS_DIRECT_IO) + flags |= O_DIRECT; ++ if (lc->flags & LOOPDEV_FL_NOFOLLOW) ++ flags |= O_NOFOLLOW; + + if ((file_fd = open(lc->filename, mode | flags)) < 0) { + if (mode != O_RDONLY && (errno == EROFS || errno == EACCES)) +diff --git a/libmount/src/hook_loopdev.c b/libmount/src/hook_loopdev.c +index 444d69d6f..34351116c 100644 +--- a/libmount/src/hook_loopdev.c ++++ b/libmount/src/hook_loopdev.c +@@ -272,7 +272,8 @@ static int setup_loopdev(struct libmnt_context *cxt, + } + + DBG(LOOP, ul_debugobj(cxt, "not found; create a new loop device")); +- rc = loopcxt_init(&lc, 0); ++ rc = loopcxt_init(&lc, ++ mnt_context_is_restricted(cxt) ? LOOPDEV_FL_NOFOLLOW : 0); + if (rc) + goto done_no_deinit; + if (mnt_opt_has_value(loopopt)) { +-- +2.43.0 +