x86_64: Ignore compat mode SYSCALL when IA32_EMULATION is not defined

Previously a program could switch to a compat mode segment and then
execute SYSCALL and it would jump to an uninitialized MSR and crash
the kernel.

Instead supply a dummy target for this case.

Pointed out by Jan Beulich

Cc: jbeulich@novell.com
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 10 insertions, 0 deletions

diff --git a/arch/x86_64/kernel/entry.S b/arch/x86_64/kernel/entry.S
index fa984b53e7e6..a67f87bf4015 100644
--- a/arch/x86_64/kernel/entry.S
+++ b/arch/x86_64/kernel/entry.S
@@ -1163,3 +1163,10 @@ ENTRY(call_softirq)
         ret
         CFI_ENDPROC
 ENDPROC(call_softirq)
+
+KPROBE_ENTRY(ignore_sysret)
+        CFI_STARTPROC
+        mov $-ENOSYS,%eax
+        sysret
+        CFI_ENDPROC
+ENDPROC(ignore_sysret)
diff --git a/arch/x86_64/kernel/setup64.c b/arch/x86_64/kernel/setup64.c
index 64379a80d763..1200aaac403e 100644
--- a/arch/x86_64/kernel/setup64.c
+++ b/arch/x86_64/kernel/setup64.c
@@ -150,6 +150,8 @@ void pda_init(int cpu)
 char boot_exception_stacks[(N_EXCEPTION_STACKS - 1) * EXCEPTION_STKSZ + DEBUG_STKSZ]
 __attribute__((section(".bss.page_aligned")));
 
+extern asmlinkage void ignore_sysret(void);
+
 /* May not be marked __init: used by software suspend */
 void syscall_init(void)
 {
@@ -160,6 +162,7 @@ void syscall_init(void)
          */ 
         wrmsrl(MSR_STAR,  ((u64)__USER32_CS)<<48  | ((u64)__KERNEL_CS)<<32); 
         wrmsrl(MSR_LSTAR, system_call); 
+        wrmsrl(MSR_CSTAR, ignore_sysret);
 
 #ifdef CONFIG_IA32_EMULATION            
         syscall32_cpu_init ();