Message-ID: <efc8bca3da13f7cc384c96b367e7f087aa5f5703.camel@sipsolutions.net>
Date: Wed, 04 Dec 2024 19:24:38 +0100
From: Benjamin Berg <benjamin@...solutions.net>
To: Jeff Xu <jeffxu@...omium.org>
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Subject: Re: [PATCH v4 1/1] exec: seal system mappings

Hi,

On Wed, 2024-12-04 at 09:43 -0800, Jeff Xu wrote:
> On Wed, Dec 4, 2024 at 6:04 AM Benjamin Berg <benjamin@...solutions.net> wrote:
> > On Mon, 2024-11-25 at 20:20 +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 [2]. 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.
> > 
> > I think that enabling this feature would break User Mode Linux (UML).
> > It uses a tiny static helper executable to create userspace MMs. This
> > executable just maps some "stub" data/code pages[1] for management and
> > after that all other memory has to be unmapped as it is managed by the
> > UML kernel.
> > This unmapping will not work if the vdso/vvar mappings are sealed.
> > 
> > Maybe nobody who enables the feature cares about UML. But wanted to
> > raise it as a potential issue in case you are not aware yet.
> > 
> Thank you for bringing this to attention, I will add this information
> to documentation/comments.
> 
> Do you think we need to add a KCONFIG check similar to
> !CHECKPOINT_RESTORE ? or this is something purely  in userspace and
> the kernel doesn't have a control.

UML is purely in userspace, so there is no need for any checks.

> > [1] Hmm, we should mseal() those stub pages.
> > 
> is this reference [1] correct ?

I think so. But it was off-topic to this thread. I just realized that
this is a possible improvement of the UML code.

Benjamin

> > > 
> > > Currently, memory sealing is only functional in a 64-bit kernel
> > > configuration.
> > > 
> > > To enable this feature, the architecture needs to be tested to
> > > confirm that it doesn't unmap/remap system mappings during the
> > > the life time of the process. After the architecture enables
> > > ARCH_HAS_SEAL_SYSTEM_MAPPINGS, a distribution can set
> > > CONFIG_SEAL_SYSTEM_MAPPING to manage access to the feature.
> > > Alternatively, kernel command line (exec.seal_system_mappings)
> > > enables this feature also.
> > > 
> > > This feature is tested using ChromeOS and Android on X86_64 and ARM64,
> > > therefore ARCH_HAS_SEAL_SYSTEM_MAPPINGS is set for X86_64 and ARM64.
> > > Other architectures can enable this after testing. No specific hardware
> > > features from the CPU are needed.
> > > 
> > > This feature's security enhancements will benefit ChromeOS, Android,
> > > and other secure-by-default systems.
> > > 
> > > [1] Documentation/userspace-api/mseal.rst
> > > [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         | 11 ++++++
> > >  Documentation/userspace-api/mseal.rst         |  4 ++
> > >  arch/arm64/Kconfig                            |  1 +
> > >  arch/x86/Kconfig                              |  1 +
> > >  arch/x86/entry/vsyscall/vsyscall_64.c         |  8 +++-
> > >  include/linux/mm.h                            | 12 ++++++
> > >  init/Kconfig                                  | 25 ++++++++++++
> > >  mm/mmap.c                                     | 10 +++++
> > >  mm/mseal.c                                    | 39 +++++++++++++++++++
> > >  security/Kconfig                              | 24 ++++++++++++
> > >  10 files changed, 133 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > > index e7bfe1bde49e..f63268341739 100644
> > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > @@ -1538,6 +1538,17 @@
> > >                       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.
> > > +                     - 'no':  do not seal system mappings.
> > > +                     - 'yes': seal system mappings.
> > > +                     This overrides CONFIG_SEAL_SYSTEM_MAPPINGS=(y/n)
> > > +                     If not specified or invalid, default is the value set by
> > > +                     CONFIG_SEAL_SYSTEM_MAPPINGS.
> > > +                     This option has no effect if CONFIG_64BIT=n
> > > +
> > >       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/Documentation/userspace-api/mseal.rst b/Documentation/userspace-api/mseal.rst
> > > index 41102f74c5e2..bec122318a59 100644
> > > --- a/Documentation/userspace-api/mseal.rst
> > > +++ b/Documentation/userspace-api/mseal.rst
> > > @@ -130,6 +130,10 @@ Use cases
> > > 
> > >  - Chrome browser: protect some security sensitive data structures.
> > > 
> > > +- seal system mappings:
> > > +  kernel config CONFIG_SEAL_SYSTEM_MAPPINGS seals system mappings such
> > > +  as vdso, vvar, sigpage, uprobes and vsyscall.
> > > +
> > >  When not to use mseal
> > >  =====================
> > >  Applications can apply sealing to any virtual memory region from userspace,
> > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > index 63de71544d95..fc5da8f74342 100644
> > > --- a/arch/arm64/Kconfig
> > > +++ b/arch/arm64/Kconfig
> > > @@ -44,6 +44,7 @@ config ARM64
> > >       select ARCH_HAS_SETUP_DMA_OPS
> > >       select ARCH_HAS_SET_DIRECT_MAP
> > >       select ARCH_HAS_SET_MEMORY
> > > +     select ARCH_HAS_SEAL_SYSTEM_MAPPINGS
> > >       select ARCH_STACKWALK
> > >       select ARCH_HAS_STRICT_KERNEL_RWX
> > >       select ARCH_HAS_STRICT_MODULE_RWX
> > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > > index 1ea18662942c..5f6bac99974c 100644
> > > --- a/arch/x86/Kconfig
> > > +++ b/arch/x86/Kconfig
> > > @@ -26,6 +26,7 @@ config X86_64
> > >       depends on 64BIT
> > >       # Options that are inherently 64-bit kernel only:
> > >       select ARCH_HAS_GIGANTIC_PAGE
> > > +     select ARCH_HAS_SEAL_SYSTEM_MAPPINGS
> > >       select ARCH_SUPPORTS_INT128 if CC_HAS_INT128
> > >       select ARCH_SUPPORTS_PER_VMA_LOCK
> > >       select ARCH_SUPPORTS_HUGE_PFNMAP if TRANSPARENT_HUGEPAGE
> > > diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
> > > index 2fb7d53cf333..30e0958915ca 100644
> > > --- a/arch/x86/entry/vsyscall/vsyscall_64.c
> > > +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
> > > @@ -366,8 +366,12 @@ 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);
> > > +     }
> > > 
> > >       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/init/Kconfig b/init/Kconfig
> > > index 1aa95a5dfff8..614719259aa0 100644
> > > --- a/init/Kconfig
> > > +++ b/init/Kconfig
> > > @@ -1860,6 +1860,31 @@ config ARCH_HAS_MEMBARRIER_CALLBACKS
> > >  config ARCH_HAS_MEMBARRIER_SYNC_CORE
> > >       bool
> > > 
> > > +config ARCH_HAS_SEAL_SYSTEM_MAPPINGS
> > > +     bool
> > > +     help
> > > +       Control SEAL_SYSTEM_MAPPINGS access based on architecture.
> > > +
> > > +       A 64-bit kernel is required for the memory sealing feature.
> > > +       No specific hardware features from the CPU are needed.
> > > +
> > > +       To enable this feature, the architecture needs to be tested to
> > > +       confirm that it doesn't unmap/remap system mappings during the
> > > +       the life time of the process. After the architecture enables this,
> > > +       a distribution can set CONFIG_SEAL_SYSTEM_MAPPING to manage access
> > > +       to the feature.
> > > +
> > > +       The CONFIG_SEAL_SYSTEM_MAPPINGS already checks the CHECKPOINT_RESTORE
> > > +       feature, which is known to remap/unmap vdso.  Thus, the presence of
> > > +       CHECKPOINT_RESTORE is not considered a factor in enabling
> > > +       ARCH_HAS_SEAL_SYSTEM_MAPPINGS for a architecture.
> > > +
> > > +       For complete list of system mappings, please see
> > > +       CONFIG_SEAL_SYSTEM_MAPPINGS.
> > > +
> > > +       For complete descriptions of memory sealing, please see
> > > +       Documentation/userspace-api/mseal.rst
> > > +
> > >  config HAVE_PERF_EVENTS
> > >       bool
> > >       help
> > > 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.
> > > +      */
> > > +     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..80126d6231bb 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 override 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..5bbb8d989d79 100644
> > > --- a/security/Kconfig
> > > +++ b/security/Kconfig
> > > @@ -51,6 +51,30 @@ config PROC_MEM_NO_FORCE
> > > 
> > >  endchoice
> > > 
> > > +config SEAL_SYSTEM_MAPPINGS
> > > +     bool "seal system mappings"
> > > +     default n
> > > +     depends on 64BIT
> > > +     depends on ARCH_HAS_SEAL_SYSTEM_MAPPINGS
> > > +     depends on !CHECKPOINT_RESTORE
> > > +     help
> > > +       Seal system mappings such as vdso, vvar, sigpage, vsyscall, uprobes.
> > > +
> > > +       A 64-bit kernel is required for the memory sealing feature.
> > > +       No specific hardware features from the CPU are needed.
> > > +
> > > +       Depends on the ARCH_HAS_SEAL_SYSTEM_MAPPINGS.
> > > +
> > > +       CHECKPOINT_RESTORE might relocate vdso mapping during restore,
> > > +       and remap/unmap will fail when the mapping is sealed, therefore
> > > +       !CHECKPOINT_RESTORE is added as dependency.
> > > +
> > > +       Kernel command line exec.seal_system_mappings=(no/yes) overrides
> > > +       this.
> > > +
> > > +       For complete descriptions of memory sealing, please see
> > > +       Documentation/userspace-api/mseal.rst
> > > +
> > >  config SECURITY
> > >       bool "Enable different security models"
> > >       depends on SYSFS
> > 
> > 
> 

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