[PATCH 10/10] x86/entry/32: Add and check a stack canary for the SYSENTER stack

[Andy Lutomirski]

Signed-off-by: Andy Lutomirski <l...@kernel.org>
---
 arch/x86/include/asm/processor.h | 3 ++-
 arch/x86/kernel/process.c        | 3 +++
 arch/x86/kernel/traps.c          | 8 ++++++++
 3 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 7cd01b71b5bd..50a6dc871cc0 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -299,8 +299,9 @@ struct tss_struct {
 
 #ifdef CONFIG_X86_32
        /*
-        * Space for the temporary SYSENTER stack:
+        * Space for the temporary SYSENTER stack.
         */
+       unsigned long           SYSENTER_stack_canary;
        unsigned long           SYSENTER_stack[64];
 #endif
 
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 9f7c21c22477..ee9a9792caeb 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -57,6 +57,9 @@ __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, 
cpu_tss) = {
          */
        .io_bitmap              = { [0 ... IO_BITMAP_LONGS] = ~0 },
 #endif
+#ifdef CONFIG_X86_32
+       .SYSENTER_stack_canary  = STACK_END_MAGIC,
+#endif
 };
 EXPORT_PER_CPU_SYMBOL(cpu_tss);
 
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 80928ea78373..590110119e6a 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -713,6 +713,14 @@ dotraplinkage void do_debug(struct pt_regs *regs, long 
error_code)
        debug_stack_usage_dec();
 
 exit:
+#if defined(CONFIG_X86_32)
+       /*
+        * This is the most likely code path that involves non-trivial use
+        * of the SYSENTER stack.  Check that we haven't overrun it.
+        */
+       WARN(this_cpu_read(cpu_tss.SYSENTER_stack_canary) != STACK_END_MAGIC,
+            "Overran or corrupted SYSENTER stack\n");
+#endif
        ist_exit(regs);
 }
 NOKPROBE_SYMBOL(do_debug);
-- 
2.5.0