On 12/05/2020 23:48, Kees Cook wrote: > 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.
OK > >> 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. OK, I guess you mean to store the default integer value of the sysctl in this config option. > > 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?) I added this in response to James's comment: https://lore.kernel.org/lkml/alpine.lrh.2.21.2005020405210.5...@namei.org/ I'm fine to let the sysctl visible whatever the kernel config is. It makes the code simpler. I guess tailored security distros already protect sysctl entries anyway. > > 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; Except the position of the const, this is clearer indeed. > >> + >> +/* >> + * 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(). OK > >> + >> +/** >> + * 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. Other *_inode_permission() functions have a similar meaning and the same signature. The may_*() functions have various signatures. What do the filesystem folks prefer? > >> + >> /** >> * 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. Right, if the sysctl can't be disabled with a kernel configuration. > >> >> 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. OK > >> + }, >> +#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. >
