diff --git a/package/util-linux/0006-add-loopdev-fl-nofollow.patch b/package/util-linux/0006-add-loopdev-fl-nofollow.patch new file mode 100644 index 0000000000..21b1e2596c --- /dev/null +++ b/package/util-linux/0006-add-loopdev-fl-nofollow.patch @@ -0,0 +1,111 @@ +From 5e390467b26a3cf3fecc04e1a0d482dff3162fc4 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 + +CVE: CVE-2026-27456 +Upstream: https://github.com/util-linux/util-linux/commit/5e390467b26a3cf3fecc04e1a0d482dff3162fc4 +[Titouan: Adapt patch to apply cleanly onto util-linux 2.40] +Signed-off-by: Titouan Christophe +--- + 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 d10bf7f37..0f85dd254 100644 +--- a/include/loopdev.h ++++ b/include/loopdev.h +@@ -139,7 +139,8 @@ 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 c72fb2c40..3d2274693 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 = ul_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 597b9339a..4df1915a6 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)) { diff --git a/package/util-linux/util-linux.mk b/package/util-linux/util-linux.mk index 5d761e01c9..d30c26deb5 100644 --- a/package/util-linux/util-linux.mk +++ b/package/util-linux/util-linux.mk @@ -36,6 +36,9 @@ UTIL_LINUX_CPE_ID_VENDOR = kernel # 0001-libmount-ifdef-statx-call.patch UTIL_LINUX_AUTORECONF = YES +# 0006-add-loopdev-fl-nofollow.patch +UTIL_LINUX_IGNORE_CVES += CVE-2026-27456 + UTIL_LINUX_INSTALL_STAGING = YES UTIL_LINUX_DEPENDENCIES = \ host-pkgconf \