x86/PV: use 64-bit subtract to adjust guest RIP upon missing SYSCALL callbacks

When discussing the shrunk down version of the commit in question it
was said (in reply to my conditional choosing of the width):

"However, the 32bit case isn't actually interesting here.  A
 guest can't execute a SYSCALL instruction on/across the 4G->0 boundary
 because the M2P is mapped NX up to the 4G boundary, so we can never
 reach this point with %eip < 2.

 Therefore, the 64bit-only form is the appropriate one to use, which
 solves any question of cleverness, or potential decode stalls it
 causes."

Fixes: ca6fcf4321b3 ("x86/pv: Inject #UD for missing SYSCALL callbacks")
Signed-off-by: Jan Beulich <JBeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index b5688e2c34e0ef1f1cc450025bdf2813d2bf0b68..6422687fbf0ad39d856b22563b531de7011cea0f 100644 (file)
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -42,7 +42,7 @@ ENTRY(switch_to_kernel)
 UNLIKELY_START(z, syscall_no_callback) /* TB_eip == 0 => #UD */
         mov   VCPU_trap_ctxt(%rbx), %rdi
         movl  $X86_EXC_UD, UREGS_entry_vector(%rsp)
-        subl  $2, UREGS_rip(%rsp)
+        subq  $2, UREGS_rip(%rsp)
         mov   X86_EXC_UD * TRAPINFO_sizeof + TRAPINFO_eip(%rdi), %rax
         testb $4, X86_EXC_UD * TRAPINFO_sizeof + TRAPINFO_flags(%rdi)
         setnz %cl