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package/util-linux: add patch for CVE-2026-27456

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- CVE-2026-27456: mount(8) TOCTOU symlink attack via loop device. The SUID
    mount follows symlinks when resolving loop backing file paths. On
    systems where non-root users are permitted to mount loop devices (via
    'user' option in fstab), this allows access to arbitrary files.

For more information, see:

- https://nvd.nist.gov/vuln/detail/CVE-2026-27456
- 5e390467b2

Signed-off-by: Titouan Christophe <titouan.christophe@mind.be>
(cherry picked from commit cd52acda14)
Signed-off-by: Thomas Perale <thomas.perale@mind.be>
This commit is contained in:
Titouan Christophe
2026-04-27 12:12:06 +02:00
committed by Thomas Perale
parent dabbb7caa4
commit eeb65cf0f6
2 changed files with 114 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files

111
package/util-linux/0006-add-loopdev-fl-nofollow.patch Normal file
View File

@@ -0,0 +1,111 @@
From 5e390467b26a3cf3fecc04e1a0d482dff3162fc4 Mon Sep 17 00:00:00 2001
From: Karel Zak <kzak@redhat.com>
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 <kzak@redhat.com>
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 <titouan.christophe@mind.be>
---
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)) {

3
package/util-linux/util-linux.mk
View File

@@ -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 \