From: Sohil Mehta Date: Mon, 9 Mar 2026 18:10:28 +0000 (-0700) Subject: x86/vsyscall: Disable LASS if vsyscall mode is set to EMULATE X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=b36d1f53d90c869d5f02fe0d8603f825013e746e;p=thirdparty%2Flinux.git x86/vsyscall: Disable LASS if vsyscall mode is set to EMULATE 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 include the necessary 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 Signed-off-by: Dave Hansen Reviewed-by: Rick Edgecombe Reviewed-by: Dave Hansen Reviewed-by: H. Peter Anvin (Intel) Tested-by: Maciej Wieczor-Retman Link: https://patch.msgid.link/20260309181029.398498-5-sohil.mehta@intel.com --- diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index cb850e5290c2b..64df2c52b2e51 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -8376,7 +8376,9 @@ Kernel parameters 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 e740f3b422788..ea36de9fa8641 100644 --- a/arch/x86/entry/vsyscall/vsyscall_64.c +++ b/arch/x86/entry/vsyscall/vsyscall_64.c @@ -62,6 +62,11 @@ static int __init vsyscall_setup(char *str) else return -EINVAL; + if (cpu_feature_enabled(X86_FEATURE_LASS) && vsyscall_mode == EMULATE) { + setup_clear_cpu_cap(X86_FEATURE_LASS); + pr_warn_once("x86/cpu: Disabling LASS due to vsyscall=emulate\n"); + } + return 0; }