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selinux: fall back to SECURITY_FS_USE_GENFS if no xattr support

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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 <vgoyal@redhat.com>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
This commit is contained in:
Ondrej Mosnacek 2021-01-13 13:38:02 +01:00 committed by Paul Moore
parent e0de8a9aeb
commit 08abe46b2c
1 changed files with 52 additions and 25 deletions
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77
security/selinux/hooks.c
View File

@ -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) static int sb_finish_set_opts(struct super_block *sb)
{ {
struct superblock_security_struct *sbsec = sb->s_security; 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; int rc = 0;
if (sbsec->behavior == SECURITY_FS_USE_XATTR) { if (sbsec->behavior == SECURITY_FS_USE_XATTR) {
/* Make sure that the xattr handler exists and that no rc = sb_check_xattr_support(sb);
error other than -ENODATA is returned by getxattr on if (rc)
the root directory. -ENODATA is ok, as this may be return rc;
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;
}
} }
sbsec->flags |= SE_SBINITIALIZED; sbsec->flags |= SE_SBINITIALIZED;
@ -554,7 +582,6 @@ static int sb_finish_set_opts(struct super_block *sb)
spin_lock(&sbsec->isec_lock); spin_lock(&sbsec->isec_lock);
} }
spin_unlock(&sbsec->isec_lock); spin_unlock(&sbsec->isec_lock);
out:
return rc; return rc;
} }