[PATCH 1/2] x86_64,signal: Save and restore SS in signal frames

[Andy Lutomirski <luto@amacapital.net>]

The comment in the signal code says that apps can save/restore other
segments on their own.  It's true that apps can *save* SS on their
own, but there's no way for apps to restore it: SYSCALL effectively
resets SS to __USER_DS, so any value that user code tries to load
into SS gets lost on entry to sigreturn.

This recycles two padding bytes in the segment selector area for SS.

I suspect that 64-bit programs that try to run 16-bit code and uses
signals will have a lot of trouble without this.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 arch/x86/include/asm/sigcontext.h      | 2 +-
 arch/x86/include/uapi/asm/sigcontext.h | 2 +-
 arch/x86/kernel/signal.c               | 8 +-------
 3 files changed, 3 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/sigcontext.h b/arch/x86/include/asm/sigcontext.h
index 9dfce4e..f910cdc 100644
--- a/arch/x86/include/asm/sigcontext.h
+++ b/arch/x86/include/asm/sigcontext.h
@@ -59,7 +59,7 @@ struct sigcontext {
 	unsigned short cs;
 	unsigned short gs;
 	unsigned short fs;
-	unsigned short __pad0;
+	unsigned short ss;
 	unsigned long err;
 	unsigned long trapno;
 	unsigned long oldmask;
diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
index d8b9f90..076b11f 100644
--- a/arch/x86/include/uapi/asm/sigcontext.h
+++ b/arch/x86/include/uapi/asm/sigcontext.h
@@ -179,7 +179,7 @@ struct sigcontext {
 	__u16 cs;
 	__u16 gs;
 	__u16 fs;
-	__u16 __pad0;
+	__u16 ss;
 	__u64 err;
 	__u64 trapno;
 	__u64 oldmask;
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 2851d63..08d7a9e 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -94,15 +94,8 @@ int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc,
 		COPY(r15);
 #endif /* CONFIG_X86_64 */
 
-#ifdef CONFIG_X86_32
 		COPY_SEG_CPL3(cs);
 		COPY_SEG_CPL3(ss);
-#else /* !CONFIG_X86_32 */
-		/* 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. */
-		COPY_SEG_CPL3(cs);
-#endif /* CONFIG_X86_32 */
 
 		get_user_ex(tmpflags, &sc->flags);
 		regs->flags = (regs->flags & ~FIX_EFLAGS) | (tmpflags & FIX_EFLAGS);
@@ -164,6 +157,7 @@ int setup_sigcontext(struct sigcontext __user *sc, void __user *fpstate,
 		put_user_ex(regs->cs, &sc->cs);
 		put_user_ex(0, &sc->gs);
 		put_user_ex(0, &sc->fs);
+		put_user_ex(regs->ss, &sc->ss);
 #endif /* CONFIG_X86_32 */
 
 		put_user_ex(fpstate, &sc->fpstate);
-- 
1.9.3