1 files changed, 17 insertions, 0 deletions

diff --git a/arch/x86_64/kernel/signal.c b/arch/x86_64/kernel/signal.c
index d642fbf3da29..5876df116c92 100644
--- a/arch/x86_64/kernel/signal.c
+++ b/arch/x86_64/kernel/signal.c
@@ -110,6 +110,15 @@ restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc, unsigned
         COPY(r14);
         COPY(r15);
 
+        /* Kernel saves and restores only the CS segment register on signals,
+         * which is the bare minimum needed to allow mixed 32/64-bit code.
+         * App's signal handler can save/restore other segments if needed. */
+        {
+                unsigned cs;
+                err |= __get_user(cs, &sc->cs);
+                regs->cs = cs | 3;      /* Force into user mode */
+        }
+
         {
                 unsigned int tmpflags;
                 err |= __get_user(tmpflags, &sc->eflags);
@@ -187,6 +196,7 @@ setup_sigcontext(struct sigcontext __user *sc, struct pt_regs *regs, unsigned lo
 {
         int err = 0;
 
+        err |= __put_user(regs->cs, &sc->cs);
         err |= __put_user(0, &sc->gs);
         err |= __put_user(0, &sc->fs);
 
@@ -318,7 +328,14 @@ static int setup_rt_frame(int sig, struct k_sigaction *ka, siginfo_t *info,
 
         regs->rsp = (unsigned long)frame;
 
+        /* Set up the CS register to run signal handlers in 64-bit mode,
+           even if the handler happens to be interrupting 32-bit code. */
+        regs->cs = __USER_CS;
+
+        /* This, by contrast, has nothing to do with segment registers -
+           see include/asm-x86_64/uaccess.h for details. */
         set_fs(USER_DS);
+
         regs->eflags &= ~TF_MASK;
         if (test_thread_flag(TIF_SINGLESTEP))
                 ptrace_notify(SIGTRAP);