[PATCHv8 10/17] x86/vsyscall: Disable LASS if vsyscall mode is set to EMULATE

Kirill A. Shutemov
2025-07-01T09:59:11Z

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>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
Documentation/admin-guide/kernel-parameters.txt | 4 +++-
arch/x86/entry/vsyscall/vsyscall_64.c | 7 +++++++
2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f1f2c0874da9..796c987372df 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -7926,7 +7926,9 @@

emulate Vsyscalls turn into traps and are emulated
reasonably safely. The vsyscall page is
- readable.
+ readable. This disables the Linear
+ Address Space Separation (LASS) security
+ feature and makes the system less secure.

xonly [default] Vsyscalls turn into traps and are
emulated reasonably safely. The vsyscall
diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
index be77385b311e..d37df40bfb26 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -63,6 +63,13 @@ 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_warn_once("x86/cpu: Disabling LASS support due to vsyscall=emulate\n");
+ }
+
return 0;
}

--
2.47.2