[PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC
Kees Cook
keescook at chromium.org
Tue May 12 21:48:48 UTC 2020
On Tue, May 05, 2020 at 05:31:53PM +0200, Mickaël Salaün 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.
Some language tailoring. I might change the first sentence to:
Allow for the enforcement of the O_MAYEXEC openat2(2) flag.
> 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.
OMAYEXEC feels like the wrong name here. Maybe something closer to the
sysctl name? CONFIG_OPEN_MAYEXEC?
And I think it's not needed to have 3 configs for this. That's a lot of
mess for a corner case option. I think I would model this after other
sysctl CONFIGs, and just call this CONFIG_OPEN_MAYEXEC_DEFAULT.
Is _disabling_ the sysctl needed? This patch gets much smaller without
the ..._STATIC bit. (And can we avoid "static", it means different
things to different people. How about invert the logic and call it
CONFIG_OPEN_MAYEXEC_SYSCTL?)
Further notes below...
> [...]
> diff --git a/fs/namei.c b/fs/namei.c
> index 33b6d372e74a..70f179f6bc6c 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -39,6 +39,7 @@
> #include <linux/bitops.h>
> #include <linux/init_task.h>
> #include <linux/uaccess.h>
> +#include <linux/sysctl.h>
>
> #include "internal.h"
> #include "mount.h"
> @@ -411,10 +412,90 @@ static int sb_permission(struct super_block *sb, struct inode *inode, int mask)
> return 0;
> }
>
> +#define OMAYEXEC_ENFORCE_NONE 0
Like the CONFIG, I'd stay close to the sysctl, OPEN_MAYEXEC_ENFORCE_...
> +#define OMAYEXEC_ENFORCE_MOUNT (1 << 0)
> +#define OMAYEXEC_ENFORCE_FILE (1 << 1)
Please use BIT(0), BIT(1)...
> +#define _OMAYEXEC_LAST OMAYEXEC_ENFORCE_FILE
> +#define _OMAYEXEC_MASK ((_OMAYEXEC_LAST << 1) - 1)
> +
> +#ifdef CONFIG_OMAYEXEC_STATIC
> +const int sysctl_omayexec_enforce =
> +#ifdef CONFIG_OMAYEXEC_ENFORCE_MOUNT
> + OMAYEXEC_ENFORCE_MOUNT |
> +#endif
> +#ifdef CONFIG_OMAYEXEC_ENFORCE_FILE
> + OMAYEXEC_ENFORCE_FILE |
> +#endif
> + OMAYEXEC_ENFORCE_NONE;
> +#else /* CONFIG_OMAYEXEC_STATIC */
> +int sysctl_omayexec_enforce __read_mostly = OMAYEXEC_ENFORCE_NONE;
> +#endif /* CONFIG_OMAYEXEC_STATIC */
If you keep CONFIG_OPEN_MAYEXEC_SYSCTL, you could do this in namei.h:
#ifdef CONFIG_OPEN_MAYEXEC_SYSCTL
#define __sysctl_writable __read_mostly
#else
#define __sysctl_write const
#endif
Then with my proposed change to the enforce CONFIG, all of this is
reduced to simply:
int open_mayexec_enforce __sysctl_writable = CONFIG_OPEN_MAYEXEC_DEFAULT;
> +
> +/*
> + * Handle open_mayexec_enforce sysctl
> + */
> +#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;
> +
> + tmp_mayexec_enforce = *((int *)table->data);
> + table_copy = *table;
> + /* Do not erase sysctl_omayexec_enforce. */
> + table_copy.data = &tmp_mayexec_enforce;
> + error = proc_dointvec(&table_copy, write, buffer, lenp, ppos);
> + if (error)
> + return error;
> +
> + if ((tmp_mayexec_enforce | _OMAYEXEC_MASK) != _OMAYEXEC_MASK)
> + return -EINVAL;
> +
> + *((int *)table->data) = tmp_mayexec_enforce;
> + } else {
> + error = proc_dointvec(table, write, buffer, lenp, ppos);
> + if (error)
> + return error;
> + }
> + return 0;
> +}
> +#endif
I don't think any of this is needed. There are no complex bit field
interactions to check for. The sysctl is min=0, max=3. The only thing
special here is checking CAP_MAC_ADMIN. I would just add
proc_dointvec_minmax_macadmin(), like we have for ..._minmax_sysadmin().
> +
> +/**
> + * 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;
> +}
More naming nits: I think this should be called may_openexec() to match
the other may_*() functions.
> +
> /**
> * inode_permission - Check for access rights to a given inode
> * @inode: Inode to check permission on
> - * @mask: Right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
> + * @mask: Right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC, %MAY_OPENEXEC,
> + * %MAY_EXECMOUNT)
> *
> * Check for read/write/execute permissions on an inode. We use fs[ug]id for
> * this, letting us set arbitrary permissions for filesystem access without
> @@ -454,6 +535,10 @@ int inode_permission(struct inode *inode, int mask)
> if (retval)
> return retval;
>
> + retval = omayexec_inode_permission(inode, mask);
> + if (retval)
> + return retval;
> +
> return security_inode_permission(inode, mask);
> }
> EXPORT_SYMBOL(inode_permission);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 79435fca6c3e..39c80a64d054 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -83,6 +83,9 @@ extern int sysctl_protected_symlinks;
> extern int sysctl_protected_hardlinks;
> extern int sysctl_protected_fifos;
> extern int sysctl_protected_regular;
> +#ifndef CONFIG_OMAYEXEC_STATIC
> +extern int sysctl_omayexec_enforce;
> +#endif
Now there's no need to wrap this in ifdef.
>
> typedef __kernel_rwf_t rwf_t;
>
> @@ -3545,6 +3548,8 @@ int proc_nr_dentry(struct ctl_table *table, int write,
> void __user *buffer, size_t *lenp, loff_t *ppos);
> int proc_nr_inodes(struct ctl_table *table, int write,
> void __user *buffer, size_t *lenp, loff_t *ppos);
> +int proc_omayexec(struct ctl_table *table, int write, void __user *buffer,
> + size_t *lenp, loff_t *ppos);
> int __init get_filesystem_list(char *buf);
>
> #define __FMODE_EXEC ((__force int) FMODE_EXEC)
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 8a176d8727a3..29bbf79f444c 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1892,6 +1892,15 @@ static struct ctl_table fs_table[] = {
> .extra1 = SYSCTL_ZERO,
> .extra2 = &two,
> },
> +#ifndef CONFIG_OMAYEXEC_STATIC
> + {
> + .procname = "open_mayexec_enforce",
> + .data = &sysctl_omayexec_enforce,
> + .maxlen = sizeof(int),
> + .mode = 0600,
> + .proc_handler = proc_omayexec,
This can just be min/max of 0/3 with a new macadmin handler.
> + },
> +#endif
> #if defined(CONFIG_BINFMT_MISC) || defined(CONFIG_BINFMT_MISC_MODULE)
> {
> .procname = "binfmt_misc",
> diff --git a/security/Kconfig b/security/Kconfig
> index cd3cc7da3a55..d8fac9240d14 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -230,6 +230,32 @@ config STATIC_USERMODEHELPER_PATH
> If you wish for all usermode helper programs to be disabled,
> specify an empty string here (i.e. "").
>
> +menuconfig OMAYEXEC_STATIC
> + tristate "Configure O_MAYEXEC behavior at build time"
> + ---help---
> + Enable to enforce O_MAYEXEC at build time, and disable the dedicated
> + fs.open_mayexec_enforce sysctl.
> +
> + See Documentation/admin-guide/sysctl/fs.rst for more details.
> +
> +if OMAYEXEC_STATIC
> +
> +config OMAYEXEC_ENFORCE_MOUNT
> + bool "Mount restriction"
> + default y
> + ---help---
> + Forbid opening files with the O_MAYEXEC option if their underlying VFS is
> + mounted with the noexec option or if their superblock forbids execution
> + of its content (e.g., /proc).
> +
> +config OMAYEXEC_ENFORCE_FILE
> + bool "File permission restriction"
> + ---help---
> + Forbid opening files with the O_MAYEXEC option if they are not marked as
> + executable for the current process (e.g., POSIX permissions).
> +
> +endif # OMAYEXEC_STATIC
> +
> source "security/selinux/Kconfig"
> source "security/smack/Kconfig"
> source "security/tomoyo/Kconfig"
> --
> 2.26.2
>
Otherwise, yeah, the intent here looks good to me.
--
Kees Cook
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