Merge branch 'x86-entry-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 vsyscall updates from Thomas Gleixner:
 "Further hardening of the legacy vsyscall by providing support for
  execute only mode and switching the default to it.

  This prevents a certain class of attacks which rely on the vsyscall
  page being accessible at a fixed address in the canonical kernel
  address space"

* 'x86-entry-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  selftests/x86: Add a test for process_vm_readv() on the vsyscall page
  x86/vsyscall: Add __ro_after_init to global variables
  x86/vsyscall: Change the default vsyscall mode to xonly
  selftests/x86/vsyscall: Verify that vsyscall=none blocks execution
  x86/vsyscall: Document odd SIGSEGV error code for vsyscalls
  x86/vsyscall: Show something useful on a read fault
  x86/vsyscall: Add a new vsyscall=xonly mode
  Documentation/admin: Remove the vsyscall=native documentation

1 files changed, 25 insertions, 10 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 51a98d6eae8e..fbabf59692ff 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2288,7 +2288,7 @@ config COMPAT_VDSO
 choice
 	prompt "vsyscall table for legacy applications"
 	depends on X86_64
-	default LEGACY_VSYSCALL_EMULATE
+	default LEGACY_VSYSCALL_XONLY
 	help
 	  Legacy user code that does not know how to find the vDSO expects
 	  to be able to issue three syscalls by calling fixed addresses in
@@ -2296,23 +2296,38 @@ choice
 	  it can be used to assist security vulnerability exploitation.
 
 	  This setting can be changed at boot time via the kernel command
-	  line parameter vsyscall=[emulate|none].
+	  line parameter vsyscall=[emulate|xonly|none].
 
 	  On a system with recent enough glibc (2.14 or newer) and no
 	  static binaries, you can say None without a performance penalty
 	  to improve security.
 
-	  If unsure, select "Emulate".
+	  If unsure, select "Emulate execution only".
 
 	config LEGACY_VSYSCALL_EMULATE
-		bool "Emulate"
+		bool "Full emulation"
 		help
-		  The kernel traps and emulates calls into the fixed
-		  vsyscall address mapping. This makes the mapping
-		  non-executable, but it still contains known contents,
-		  which could be used in certain rare security vulnerability
-		  exploits. This configuration is recommended when userspace
-		  still uses the vsyscall area.
+		  The kernel traps and emulates calls into the fixed vsyscall
+		  address mapping. This makes the mapping non-executable, but
+		  it still contains readable known contents, which could be
+		  used in certain rare security vulnerability exploits. This
+		  configuration is recommended when using legacy userspace
+		  that still uses vsyscalls along with legacy binary
+		  instrumentation tools that require code to be readable.
+
+		  An example of this type of legacy userspace is running
+		  Pin on an old binary that still uses vsyscalls.
+
+	config LEGACY_VSYSCALL_XONLY
+		bool "Emulate execution only"
+		help
+		  The kernel traps and emulates calls into the fixed vsyscall
+		  address mapping and does not allow reads.  This
+		  configuration is recommended when userspace might use the
+		  legacy vsyscall area but support for legacy binary
+		  instrumentation of legacy code is not needed.  It mitigates
+		  certain uses of the vsyscall area as an ASLR-bypassing
+		  buffer.
 
 	config LEGACY_VSYSCALL_NONE
 		bool "None"