diff mbox series

[v3,09/12] x86/vsyscall: Disable LASS if vsyscall mode is set to EMULATE

Message ID 20230609183632.48706-10-alexander.shishkin@linux.intel.com
State New
Headers
Series Enable Linear Address Space Separation support |

Commit Message

Alexander Shishkin June 9, 2023, 6:36 p.m. UTC 
  From: Sohil Mehta <sohil.mehta@intel.com>

The EMULATE mode of vsyscall maps the vsyscall page into user address
space which can be read directly by the user application. This mode has
been deprecated recently and can only be enabled from a special command
line parameter vsyscall=emulate. See commit bf00745e7791 ("x86/vsyscall:
Remove CONFIG_LEGACY_VSYSCALL_EMULATE")

Fixing the LASS violations during the EMULATE mode would need complex
instruction decoding since the resulting #GP fault does not include any
useful error information and the vsyscall address is not readily
available in the RIP.

At this point, no one is expected to be using the insecure and
deprecated EMULATE mode. The rare usages that need support probably
don't care much about security anyway. Disable LASS when EMULATE mode is
requested during command line parsing to avoid breaking user software.
LASS will be supported if vsyscall mode is set to XONLY or NONE.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
---
 arch/x86/entry/vsyscall/vsyscall_64.c | 8 ++++++++
 1 file changed, 8 insertions(+)
diff mbox series

Patch

diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
index 76e1344997d2..edd58eda8f50 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -36,6 +36,7 @@ 
 #include <asm/vsyscall.h>
 #include <asm/unistd.h>
 #include <asm/fixmap.h>
+#include <asm/tlbflush.h>
 #include <asm/traps.h>
 #include <asm/paravirt.h>
 
@@ -63,6 +64,13 @@  static int __init vsyscall_setup(char *str)
 		else
 			return -EINVAL;
 
+		if (cpu_feature_enabled(X86_FEATURE_LASS) &&
+		    vsyscall_mode == EMULATE) {
+			cr4_clear_bits(X86_CR4_LASS);
+			setup_clear_cpu_cap(X86_FEATURE_LASS);
+			pr_info_once("x86/cpu: Disabling LASS support due to vsyscall=emulate\n");
+		}
+
 		return 0;
 	}