From 08abe46b2cfcf5f815cd4961b1bf9e10b1714c6d Mon Sep 17 00:00:00 2001 From: Ondrej Mosnacek Date: Wed, 13 Jan 2021 13:38:02 +0100 Subject: [PATCH] selinux: fall back to SECURITY_FS_USE_GENFS if no xattr support When a superblock is assigned the SECURITY_FS_USE_XATTR behavior by the policy yet it lacks xattr support, try to fall back to genfs rather than rejecting the mount. If a genfscon rule is found for the filesystem, then change the behavior to SECURITY_FS_USE_GENFS, otherwise reject the mount as before. A similar fallback is already done in security_fs_use() if no behavior specification is found for the given filesystem. This is needed e.g. for virtiofs, which may or may not support xattrs depending on the backing host filesystem. Example: # seinfo --genfs | grep ' ramfs' genfscon ramfs / system_u:object_r:ramfs_t:s0 # echo '(fsuse xattr ramfs (system_u object_r fs_t ((s0) (s0))))' >ramfs_xattr.cil # semodule -i ramfs_xattr.cil # mount -t ramfs none /mnt Before: mount: /mnt: mount(2) system call failed: Operation not supported. After: (mount succeeds) # ls -Zd /mnt system_u:object_r:ramfs_t:s0 /mnt See also: https://lore.kernel.org/selinux/20210105142148.GA3200@redhat.com/T/ https://github.com/fedora-selinux/selinux-policy/pull/478 Cc: Vivek Goyal Signed-off-by: Ondrej Mosnacek Signed-off-by: Paul Moore --- security/selinux/hooks.c | 77 +++++++++++++++++++++++++++------------- 1 file changed, 52 insertions(+), 25 deletions(-) diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c index 7fb7ff03eaa9..f2b272c9eaf3 100644 --- a/security/selinux/hooks.c +++ b/security/selinux/hooks.c @@ -484,6 +484,55 @@ static int selinux_is_sblabel_mnt(struct super_block *sb) } } +static int sb_check_xattr_support(struct super_block *sb) +{ + struct superblock_security_struct *sbsec = sb->s_security; + struct dentry *root = sb->s_root; + struct inode *root_inode = d_backing_inode(root); + u32 sid; + int rc; + + /* + * Make sure that the xattr handler exists and that no + * error other than -ENODATA is returned by getxattr on + * the root directory. -ENODATA is ok, as this may be + * the first boot of the SELinux kernel before we have + * assigned xattr values to the filesystem. + */ + if (!(root_inode->i_opflags & IOP_XATTR)) { + pr_warn("SELinux: (dev %s, type %s) has no xattr support\n", + sb->s_id, sb->s_type->name); + goto fallback; + } + + rc = __vfs_getxattr(root, root_inode, XATTR_NAME_SELINUX, NULL, 0); + if (rc < 0 && rc != -ENODATA) { + if (rc == -EOPNOTSUPP) { + pr_warn("SELinux: (dev %s, type %s) has no security xattr handler\n", + sb->s_id, sb->s_type->name); + goto fallback; + } else { + pr_warn("SELinux: (dev %s, type %s) getxattr errno %d\n", + sb->s_id, sb->s_type->name, -rc); + return rc; + } + } + return 0; + +fallback: + /* No xattr support - try to fallback to genfs if possible. */ + rc = security_genfs_sid(&selinux_state, sb->s_type->name, "/", + SECCLASS_DIR, &sid); + if (rc) + return -EOPNOTSUPP; + + pr_warn("SELinux: (dev %s, type %s) falling back to genfs\n", + sb->s_id, sb->s_type->name); + sbsec->behavior = SECURITY_FS_USE_GENFS; + sbsec->sid = sid; + return 0; +} + static int sb_finish_set_opts(struct super_block *sb) { struct superblock_security_struct *sbsec = sb->s_security; @@ -492,30 +541,9 @@ static int sb_finish_set_opts(struct super_block *sb) int rc = 0; if (sbsec->behavior == SECURITY_FS_USE_XATTR) { - /* Make sure that the xattr handler exists and that no - error other than -ENODATA is returned by getxattr on - the root directory. -ENODATA is ok, as this may be - the first boot of the SELinux kernel before we have - assigned xattr values to the filesystem. */ - if (!(root_inode->i_opflags & IOP_XATTR)) { - pr_warn("SELinux: (dev %s, type %s) has no " - "xattr support\n", sb->s_id, sb->s_type->name); - rc = -EOPNOTSUPP; - goto out; - } - - rc = __vfs_getxattr(root, root_inode, XATTR_NAME_SELINUX, NULL, 0); - if (rc < 0 && rc != -ENODATA) { - if (rc == -EOPNOTSUPP) - pr_warn("SELinux: (dev %s, type " - "%s) has no security xattr handler\n", - sb->s_id, sb->s_type->name); - else - pr_warn("SELinux: (dev %s, type " - "%s) getxattr errno %d\n", sb->s_id, - sb->s_type->name, -rc); - goto out; - } + rc = sb_check_xattr_support(sb); + if (rc) + return rc; } sbsec->flags |= SE_SBINITIALIZED; @@ -554,7 +582,6 @@ static int sb_finish_set_opts(struct super_block *sb) spin_lock(&sbsec->isec_lock); } spin_unlock(&sbsec->isec_lock); -out: return rc; }