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Message-ID: <d3f61d89-77de-42e7-b16b-e5b1031ac8bc@lucifer.local>
Date: Wed, 13 Nov 2024 20:47:23 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: jeffxu@...omium.org
Cc: akpm@...ux-foundation.org, keescook@...omium.org, jannh@...gle.com,
torvalds@...ux-foundation.org, adhemerval.zanella@...aro.org,
oleg@...hat.com, linux-kernel@...r.kernel.org,
linux-hardening@...r.kernel.org, linux-mm@...ck.org,
jorgelo@...omium.org, sroettger@...gle.com, ojeda@...nel.org,
adobriyan@...il.com, anna-maria@...utronix.de, mark.rutland@....com,
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gerg@...nel.org, dave.hansen@...ux.intel.com, mingo@...nel.org,
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42.hyeyoo@...il.com, peterz@...radead.org, ardb@...gle.com,
enh@...gle.com, rientjes@...gle.com, groeck@...omium.org
Subject: Re: [PATCH v3 1/1] exec: seal system mappings
I'd prefer not to move forward with this until we have confirmation that
adequate testing has been performed, given how invasive this change is,
even if behind a flag (unless we explicitly mention it is untested in the
Kconfig).
We are touching arch-specific stuff with VDSO, VVAR, etc. so we need to be
cautious when we're in effect hooking an arch-specific function in mm.
Other than that, the actual patch isn't too crazy overall.
I think a sensible approach might be to only enable on known-good arches.
On Wed, Nov 13, 2024 at 07:16:02PM +0000, jeffxu@...omium.org wrote:
> From: Jeff Xu <jeffxu@...omium.org>
>
> Seal vdso, vvar, sigpage, uprobes and vsyscall.
>
> Those mappings are readonly or executable only, sealing can protect
> them from ever changing or unmapped during the life time of the process.
> For complete descriptions of memory sealing, please see mseal.rst [1].
>
> System mappings such as vdso, vvar, and sigpage (for arm) are
> generated by the kernel during program initialization, and are
> sealed after creation.
>
> Unlike the aforementioned mappings, the uprobe mapping is not
> established during program startup. However, its lifetime is the same
> as the process's lifetime [1]. It is sealed from creation.
>
> The vdso, vvar, sigpage, and uprobe mappings all invoke the
> _install_special_mapping() function. As no other mappings utilize this
> function, it is logical to incorporate sealing logic within
> _install_special_mapping(). This approach avoids the necessity of
> modifying code across various architecture-specific implementations.
>
> The vsyscall mapping, which has its own initialization function, is
> sealed in the XONLY case, it seems to be the most common and secure
> case of using vsyscall.
>
> It is important to note that the CHECKPOINT_RESTORE feature (CRIU) may
> alter the mapping of vdso, vvar, and sigpage during restore
> operations. Consequently, this feature cannot be universally enabled
> across all systems. To address this, a kernel configuration option has
> been introduced to enable or disable this functionality.
>
> [1] Documentation/userspace-api/mseal.rst
It'd be nice to explicitly refer to this in the docs, it's not quite urgent
though would be nice to be part of this series.
> [2] https://lore.kernel.org/all/CABi2SkU9BRUnqf70-nksuMCQ+yyiWjo3fM4XkRkL-NrCZxYAyg@mail.gmail.com/
> Signed-off-by: Jeff Xu <jeffxu@...omium.org>
> ---
> .../admin-guide/kernel-parameters.txt | 10 +++++
> arch/x86/entry/vsyscall/vsyscall_64.c | 9 ++++-
> include/linux/mm.h | 12 ++++++
> mm/mmap.c | 10 +++++
> mm/mseal.c | 39 +++++++++++++++++++
> security/Kconfig | 11 ++++++
> 6 files changed, 89 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index e7bfe1bde49e..469a65b3cf50 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1538,6 +1538,16 @@
> Permit 'security.evm' to be updated regardless of
> current integrity status.
>
> + exec.seal_system_mappings = [KNL]
> + Format: { no | yes }
> + Seal system mappings: vdso, vvar, sigpage, vsyscall,
> + uprobe.
> + This overwrites KCONFIG CONFIG_SEAL_SYSTEM_MAPPINGS
> + - 'no': do not seal system mappings.
> + - 'yes': seal system mappings.
> + If not specified or invalid, default is the KCONFIG value.
> + This option has no effect if CONFIG_64BIT=n
Or if CONFIG_CHECKPOINT_RESTORE is not set. Please update to reference this
also.
> +
> early_page_ext [KNL,EARLY] Enforces page_ext initialization to earlier
> stages so cover more early boot allocations.
> Please note that as side effect some optimizations
> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
> index 2fb7d53cf333..185553376f39 100644
> --- a/arch/x86/entry/vsyscall/vsyscall_64.c
> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
> @@ -366,8 +366,13 @@ void __init map_vsyscall(void)
> set_vsyscall_pgtable_user_bits(swapper_pg_dir);
> }
>
> - if (vsyscall_mode == XONLY)
> - vm_flags_init(&gate_vma, VM_EXEC);
> + if (vsyscall_mode == XONLY) {
> + unsigned long vm_flags = VM_EXEC;
> +
> + vm_flags |= seal_system_mappings();
> +
> + vm_flags_init(&gate_vma, vm_flags);
Nit: remove weird whitespace above. Also might be worth adding a comment as
to what we're doing here similar to the one in _install_special_mapping().
> + }
>
> BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
> (unsigned long)VSYSCALL_ADDR);
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index df0a5eac66b7..f787d6c85cbb 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4238,4 +4238,16 @@ int arch_get_shadow_stack_status(struct task_struct *t, unsigned long __user *st
> int arch_set_shadow_stack_status(struct task_struct *t, unsigned long status);
> int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
>
> +#ifdef CONFIG_64BIT
> +/*
> + * return VM_SEALED if seal system mapping is enabled.
> + */
> +unsigned long seal_system_mappings(void);
> +#else
> +static inline unsigned long seal_system_mappings(void)
> +{
> + return 0;
> +}
> +#endif
> +
> #endif /* _LINUX_MM_H */
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 57fd5ab2abe7..bc694c555805 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2133,6 +2133,16 @@ struct vm_area_struct *_install_special_mapping(
> unsigned long addr, unsigned long len,
> unsigned long vm_flags, const struct vm_special_mapping *spec)
> {
> + /*
> + * At present, all mappings (vdso, vvar, sigpage, and uprobe) that
> + * invoke the _install_special_mapping function can be sealed.
> + * Therefore, it is logical to call the seal_system_mappings_enabled()
> + * function here. In the future, if this is not the case, i.e. if certain
> + * mappings cannot be sealed, then it would be necessary to move this
> + * check to the calling function.
> + */
Nice comment!
> + vm_flags |= seal_system_mappings();
> +
> return __install_special_mapping(mm, addr, len, vm_flags, (void *)spec,
> &special_mapping_vmops);
> }
> diff --git a/mm/mseal.c b/mm/mseal.c
> index ece977bd21e1..0a9d1e9faa28 100644
> --- a/mm/mseal.c
> +++ b/mm/mseal.c
> @@ -7,6 +7,7 @@
> * Author: Jeff Xu <jeffxu@...omium.org>
> */
>
> +#include <linux/fs_parser.h>
> #include <linux/mempolicy.h>
> #include <linux/mman.h>
> #include <linux/mm.h>
> @@ -266,3 +267,41 @@ SYSCALL_DEFINE3(mseal, unsigned long, start, size_t, len, unsigned long,
> {
> return do_mseal(start, len, flags);
> }
> +
> +/*
> + * Kernel cmdline overwrite for CONFIG_SEAL_SYSTEM_MAPPINGS
> + */
> +enum seal_system_mappings_type {
> + SEAL_SYSTEM_MAPPINGS_DISABLED,
> + SEAL_SYSTEM_MAPPINGS_ENABLED
> +};
> +
> +static enum seal_system_mappings_type seal_system_mappings_v __ro_after_init =
> + IS_ENABLED(CONFIG_SEAL_SYSTEM_MAPPINGS) ? SEAL_SYSTEM_MAPPINGS_ENABLED :
> + SEAL_SYSTEM_MAPPINGS_DISABLED;
> +
> +static const struct constant_table value_table_sys_mapping[] __initconst = {
> + { "no", SEAL_SYSTEM_MAPPINGS_DISABLED},
> + { "yes", SEAL_SYSTEM_MAPPINGS_ENABLED},
> + { }
> +};
> +
> +static int __init early_seal_system_mappings_override(char *buf)
> +{
> + if (!buf)
> + return -EINVAL;
> +
> + seal_system_mappings_v = lookup_constant(value_table_sys_mapping,
> + buf, seal_system_mappings_v);
> + return 0;
> +}
> +
> +early_param("exec.seal_system_mappings", early_seal_system_mappings_override);
> +
> +unsigned long seal_system_mappings(void)
> +{
> + if (seal_system_mappings_v == SEAL_SYSTEM_MAPPINGS_ENABLED)
> + return VM_SEALED;
> +
> + return 0;
> +}
> diff --git a/security/Kconfig b/security/Kconfig
> index 28e685f53bd1..63b87a218943 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -51,6 +51,17 @@ config PROC_MEM_NO_FORCE
>
> endchoice
>
> +config SEAL_SYSTEM_MAPPINGS
> + bool "seal system mappings"
> + default n
> + depends on 64BIT
> + depends on !CHECKPOINT_RESTORE
Would prefer to depend on actually tested architectures only.
> + help
> + Seal system mappings such as vdso, vvar, sigpage, vsyscall, uprobes.
> + Note: CHECKPOINT_RESTORE might relocate vdso mapping during restore,
> + and remap will fail if the mapping is sealed, therefore
> + !CHECKPOINT_RESTORE is added as dependency.
> +
> config SECURITY
> bool "Enable different security models"
> depends on SYSFS
> --
> 2.47.0.277.g8800431eea-goog
>
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