From fa81511bb0bbb2b1aace3695ce869da9762624ff Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Wed, 14 May 2014 16:33:54 -0700 Subject: [PATCH] x86-64, modify_ldt: Make support for 16-bit segments a runtime option Checkin: b3b42ac2cbae x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels disabled 16-bit segments on 64-bit kernels due to an information leak. However, it does seem that people are genuinely using Wine to run old 16-bit Windows programs on Linux. A proper fix for this ("espfix64") is coming in the upcoming merge window, but as a temporary fix, create a sysctl to allow the administrator to re-enable support for 16-bit segments. It adds a "/proc/sys/abi/ldt16" sysctl that defaults to zero (off). If you hit this issue and care about your old Windows program more than you care about a kernel stack address information leak, you can do echo 1 > /proc/sys/abi/ldt16 as root (add it to your startup scripts), and you should be ok. The sysctl table is only added if you have COMPAT support enabled on x86-64, but I assume anybody who runs old windows binaries very much does that ;) Signed-off-by: H. Peter Anvin Link: http://lkml.kernel.org/r/CA%2B55aFw9BPoD10U1LfHbOMpHWZkvJTkMcfCs9s3urPr1YyWBxw@mail.gmail.com Cc: --- arch/x86/kernel/ldt.c | 4 +++- arch/x86/vdso/vdso32-setup.c | 8 ++++++++ 2 files changed, 11 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c index af1d14a..dcbbaa1 100644 --- a/arch/x86/kernel/ldt.c +++ b/arch/x86/kernel/ldt.c @@ -20,6 +20,8 @@ #include #include +int sysctl_ldt16 = 0; + #ifdef CONFIG_SMP static void flush_ldt(void *current_mm) { @@ -234,7 +236,7 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode) * IRET leaking the high bits of the kernel stack address. */ #ifdef CONFIG_X86_64 - if (!ldt_info.seg_32bit) { + if (!ldt_info.seg_32bit && !sysctl_ldt16) { error = -EINVAL; goto out_unlock; } diff --git a/arch/x86/vdso/vdso32-setup.c b/arch/x86/vdso/vdso32-setup.c index 0034898..e1f220e 100644 --- a/arch/x86/vdso/vdso32-setup.c +++ b/arch/x86/vdso/vdso32-setup.c @@ -39,6 +39,7 @@ #ifdef CONFIG_X86_64 #define vdso_enabled sysctl_vsyscall32 #define arch_setup_additional_pages syscall32_setup_pages +extern int sysctl_ldt16; #endif /* @@ -249,6 +250,13 @@ static struct ctl_table abi_table2[] = { .mode = 0644, .proc_handler = proc_dointvec }, + { + .procname = "ldt16", + .data = &sysctl_ldt16, + .maxlen = sizeof(int), + .mode = 0644, + .proc_handler = proc_dointvec + }, {} }; -- 2.7.4