[PATCH v9 6/7] mseal sysmap: update mseal.rst

[jeffxu]

From: Jeff Xu <jeffxu@xxxxxxxxxxxx>

Update memory sealing documentation to include details about system
mappings.

Signed-off-by: Jeff Xu <jeffxu@xxxxxxxxxxxx>
Reviewed-by: Kees Cook <kees@xxxxxxxxxx>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
---
 Documentation/userspace-api/mseal.rst | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/Documentation/userspace-api/mseal.rst b/Documentation/userspace-api/mseal.rst
index 41102f74c5e2..56aee46a9307 100644
--- a/Documentation/userspace-api/mseal.rst
+++ b/Documentation/userspace-api/mseal.rst
@@ -130,6 +130,26 @@ Use cases
 
 - Chrome browser: protect some security sensitive data structures.
 
+- System mappings:
+  The system mappings are created by the kernel and includes vdso, vvar,
+  vvar_vclock, vectors (arm compat-mode), sigpage (arm compat-mode), uprobes.
+
+  Those system mappings are readonly only or execute only, memory sealing can
+  protect them from ever changing to writable or unmmap/remapped as different
+  attributes. This is useful to mitigate memory corruption issues where a
+  corrupted pointer is passed to a memory management system.
+
+  If supported by an architecture (CONFIG_ARCH_SUPPORTS_MSEAL_SYSTEM_MAPPINGS),
+  the CONFIG_MSEAL_SYSTEM_MAPPINGS seals all system mappings of this
+  architecture.
+
+  The following architectures currently support this feature: x86-64 and arm64.
+
+  WARNING: This feature breaks programs which rely on relocating
+  or unmapping system mappings. Known broken software at the time
+  of writing includes CHECKPOINT_RESTORE, UML, gVisor, rr. Therefore
+  this config can't be enabled universally.
+
 When not to use mseal
 =====================
 Applications can apply sealing to any virtual memory region from userspace,
-- 
2.48.1.711.g2feabab25a-goog