[Xen-devel] [PATCH RFC v2 09/12] x86: enhance syscall stub to work in per-domain mapping

[Juergen Gross <jgross@xxxxxxxx>]

Use indirect jump via register in case the target address isn't
reachable via a 32 bit relative jump.

Add macros for stub size and use those instead of returning the size
when writing the stub trampoline in order to support easy switching
between different sized stubs.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 xen/arch/x86/x86_64/traps.c  | 47 +++++++++++++++++++++++++-------------------
 xen/include/asm-x86/system.h |  7 +++++++
 2 files changed, 34 insertions(+), 20 deletions(-)

diff --git a/xen/arch/x86/x86_64/traps.c b/xen/arch/x86/x86_64/traps.c
index 3652f5ff21..b4836f623c 100644
--- a/xen/arch/x86/x86_64/traps.c
+++ b/xen/arch/x86/x86_64/traps.c
@@ -260,10 +260,11 @@ void do_double_fault(struct cpu_user_regs *regs)
     panic("DOUBLE FAULT -- system shutdown");
 }
 
-static unsigned int write_stub_trampoline(
-    unsigned char *stub, unsigned long stub_va,
-    unsigned long stack_bottom, unsigned long target_va)
+void write_stub_trampoline(unsigned char *stub, unsigned long stub_va,
+                           unsigned long stack_bottom, unsigned long target_va)
 {
+    long target_diff;
+
     /* movabsq %rax, stack_bottom - 8 */
     stub[0] = 0x48;
     stub[1] = 0xa3;
@@ -282,24 +283,32 @@ static unsigned int write_stub_trampoline(
     /* pushq %rax */
     stub[23] = 0x50;
 
-    /* jmp target_va */
-    stub[24] = 0xe9;
-    *(int32_t *)&stub[25] = target_va - (stub_va + 29);
-
-    /* Round up to a multiple of 16 bytes. */
-    return 32;
+    target_diff = target_va - (stub_va + 29);
+    if ( target_diff >> 31 == target_diff >> 63 )
+    {
+        /* jmp target_va */
+        stub[24] = 0xe9;
+        *(int32_t *)&stub[25] = target_diff;
+    }
+    else
+    {
+        /* movabs target_va, %rax */
+        stub[24] = 0x48;
+        stub[25] = 0xb8;
+        *(uint64_t *)&stub[26] = target_va;
+        /* jmpq *%rax */
+        stub[34] = 0xff;
+        stub[35] = 0xe0;
+    }
 }
 
 DEFINE_PER_CPU(struct stubs, stubs);
-void lstar_enter(void);
-void cstar_enter(void);
 
 void subarch_percpu_traps_init(void)
 {
     unsigned long stack_bottom = get_stack_bottom();
     unsigned long stub_va = this_cpu(stubs.addr);
     unsigned char *stub_page;
-    unsigned int offset;
 
     /* IST_MAX IST pages + 1 syscall page + 1 guard page + primary stack. */
     BUILD_BUG_ON((IST_MAX + 2) * PAGE_SIZE + PRIMARY_STACK_SIZE > STACK_SIZE);
@@ -312,10 +321,9 @@ void subarch_percpu_traps_init(void)
      * start of the stubs.
      */
     wrmsrl(MSR_LSTAR, stub_va);
-    offset = write_stub_trampoline(stub_page + (stub_va & ~PAGE_MASK),
-                                   stub_va, stack_bottom,
-                                   (unsigned long)lstar_enter);
-    stub_va += offset;
+    write_stub_trampoline(stub_page + (stub_va & ~PAGE_MASK), stub_va,
+                          stack_bottom, (unsigned long)lstar_enter);
+    stub_va += STUB_TRAMPOLINE_SIZE_PERCPU;
 
     if ( boot_cpu_data.x86_vendor == X86_VENDOR_INTEL ||
          boot_cpu_data.x86_vendor == X86_VENDOR_CENTAUR )
@@ -328,12 +336,11 @@ void subarch_percpu_traps_init(void)
 
     /* Trampoline for SYSCALL entry from compatibility mode. */
     wrmsrl(MSR_CSTAR, stub_va);
-    offset += write_stub_trampoline(stub_page + (stub_va & ~PAGE_MASK),
-                                    stub_va, stack_bottom,
-                                    (unsigned long)cstar_enter);
+    write_stub_trampoline(stub_page + (stub_va & ~PAGE_MASK), stub_va,
+                          stack_bottom, (unsigned long)cstar_enter);
 
     /* Don't consume more than half of the stub space here. */
-    ASSERT(offset <= STUB_BUF_SIZE / 2);
+    ASSERT(2 * STUB_TRAMPOLINE_SIZE_PERCPU <= STUB_BUF_SIZE / 2);
 
     unmap_domain_page(stub_page);
 
diff --git a/xen/include/asm-x86/system.h b/xen/include/asm-x86/system.h
index 2cf50d1d49..c5baf7c991 100644
--- a/xen/include/asm-x86/system.h
+++ b/xen/include/asm-x86/system.h
@@ -231,6 +231,13 @@ static inline int local_irq_is_enabled(void)
 void trap_init(void);
 void init_idt_traps(void);
 void tss_init(struct tss_struct *tss, unsigned long stack_bottom);
+void write_stub_trampoline(unsigned char *stub, unsigned long stub_va,
+                           unsigned long stack_bottom,
+                           unsigned long target_va);
+#define STUB_TRAMPOLINE_SIZE_PERCPU   32
+#define STUB_TRAMPOLINE_SIZE_PERVCPU  64
+void lstar_enter(void);
+void cstar_enter(void);
 void load_system_tables(void);
 void percpu_traps_init(void);
 void subarch_percpu_traps_init(void);
-- 
2.13.6


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