[PATCH v10 11/15] x86/vsyscall: Disable LASS if vsyscall mode is set to EMULATE

Sohil Mehta
2025-10-07T06:54:05Z

The EMULATE mode of vsyscall maps the vsyscall page with a high kernel
address directly into user address space. Reading the vsyscall page in
EMULATE mode would cause LASS to trigger a #GP.

Fixing the LASS violation in EMULATE mode would require complex
instruction decoding because the resulting #GP does not include any
useful error information, and the vsyscall address is not readily
available in the RIP.

The EMULATE mode has been deprecated since 2022 and can only be enabled
using the command line parameter vsyscall=emulate. See commit
bf00745e7791 ("x86/vsyscall: Remove CONFIG_LEGACY_VSYSCALL_EMULATE") for
details. At this point, no one is expected to be using this insecure
mode. The rare usages that need it obviously do not care about security.

Disable LASS when EMULATE mode is requested to avoid breaking legacy
user software. Also, update the vsyscall documentation to reflect this.
LASS will only be supported if vsyscall mode is set to XONLY (default)
or NONE.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
---
v10:
- No significant change. Minor changes to code formatting.

Eventually, we want to get rid of the EMULATE mode altogether. Linus and
AndyL seem to be okay with such a change. However, those changes are
beyond the scope of this series.
---
Documentation/admin-guide/kernel-parameters.txt | 4 +++-
arch/x86/entry/vsyscall/vsyscall_64.c | 6 ++++++
2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 3edc5ce0e2a3..29a2ee9e1001 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -8079,7 +8079,9 @@

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 ff319d7e778c..57498609b1f0 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -63,6 +63,12 @@ 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 due to vsyscall=emulate\n");
+ }
+
return 0;
}

--
2.43.0