On Tue, May 5, 2020 at 11:33 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > Enable to forbid access to files open with O_MAYEXEC. Thanks to the > noexec option from the underlying VFS mount, or to the file execute > permission, userspace can enforce these execution policies. This may > allow script interpreters to check execution permission before reading > commands from a file, or dynamic linkers to allow shared object loading. > > Add a new sysctl fs.open_mayexec_enforce to enable system administrators > to enforce two complementary security policies according to the > installed system: enforce the noexec mount option, and enforce > executable file permission. Indeed, because of compatibility with > installed systems, only system administrators are able to check that > this new enforcement is in line with the system mount points and file > permissions. A following patch adds documentation. > > For tailored Linux distributions, it is possible to enforce such > restriction at build time thanks to the CONFIG_OMAYEXEC_STATIC option. > The policy can then be configured with CONFIG_OMAYEXEC_ENFORCE_MOUNT and > CONFIG_OMAYEXEC_ENFORCE_FILE. > > Being able to restrict execution also enables to protect the kernel by > restricting arbitrary syscalls that an attacker could perform with a > crafted binary or certain script languages. It also improves multilevel > isolation by reducing the ability of an attacker to use side channels > with specific code. These restrictions can natively be enforced for ELF > binaries (with the noexec mount option) but require this kernel > extension to properly handle scripts (e.g., Python, Perl). To get a > consistent execution policy, additional memory restrictions should also > be enforced (e.g. thanks to SELinux). > > Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx> > Reviewed-by: Thibaut Sautereau <thibaut.sautereau@xxxxxxxxxxx> > Cc: Aleksa Sarai <cyphar@xxxxxxxxxx> > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > --- > diff --git a/fs/namei.c b/fs/namei.c > index 33b6d372e74a..70f179f6bc6c 100644 > --- a/fs/namei.c > +++ b/fs/namei.c > @@ -411,10 +412,90 @@ static int sb_permission(struct super_block *sb, struct inode *inode, int mask) <snip> > +#if defined(CONFIG_SYSCTL) && !defined(CONFIG_OMAYEXEC_STATIC) > +int proc_omayexec(struct ctl_table *table, int write, void __user *buffer, > + size_t *lenp, loff_t *ppos) > +{ > + int error; > + > + if (write) { > + struct ctl_table table_copy; > + int tmp_mayexec_enforce; > + > + if (!capable(CAP_MAC_ADMIN)) > + return -EPERM; Not fond of using CAP_MAC_ADMIN here (or elsewhere outside of security modules). The ability to set this sysctl is not equivalent to being able to load a MAC policy, set arbitrary MAC labels on processes/files, etc. > + * omayexec_inode_permission - Check O_MAYEXEC before accessing an inode > + * > + * @inode: Inode to check permission on > + * @mask: Right to check for (%MAY_OPENEXEC, %MAY_EXECMOUNT, %MAY_EXEC) > + * > + * Returns 0 if access is permitted, -EACCES otherwise. > + */ > +static inline int omayexec_inode_permission(struct inode *inode, int mask) > +{ > + if (!(mask & MAY_OPENEXEC)) > + return 0; > + > + if ((sysctl_omayexec_enforce & OMAYEXEC_ENFORCE_MOUNT) && > + !(mask & MAY_EXECMOUNT)) > + return -EACCES; > + > + if (sysctl_omayexec_enforce & OMAYEXEC_ENFORCE_FILE) > + return generic_permission(inode, MAY_EXEC); > + > + return 0; > +} I'm wondering if this is being done at the wrong level. I would think that OMAYEXEC_ENFORCE_FILE would mean to check file execute permission with respect to all mechanisms/policies, including DAC, filesystem-specific checking (inode->i_op->permission), security modules, etc. That requires more than just calling generic_permission() with MAY_EXEC, which only covers the default DAC/ACL logic; you'd need to take the handling up a level to inode_permission() and re-map MAY_OPENEXEC to MAY_EXEC for do_inode_permission() and security_inode_permission() at least. Alternatively, we can modify each individual filesystem (that implements its own i_op->permission) and security module to start handling MAY_OPENEXEC and have them choose to remap it to a file execute check (or not) independent of the sysctl. Not sure of your intent. As it stands, selinux_inode_permission() will ignore the new MAY_OPENEXEC flag until someone updates it. Likewise for Smack. AppArmor/TOMOYO would probably need to check and handle FMODE_EXEC in their file_open hooks since they don't implement inode_permission().