[PATCH v3 1/2] x86_64,signal: Fix SS handling for signals delivered to 64-bit programs

[Andy Lutomirski <luto@kernel.org>]

From: 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.

While we're at it, we need a second change to make this useful.  If
the signal we're delivering is caused by a bad SS value, saving that
value isn't enough.  We need to remove that bad value from the regs
before we try to deliver the signal.  Oddly, x32 already got this
right.

I suspect that 64-bit programs that try to run 16-bit code and use
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               | 22 +++++++++++++---------
 3 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/sigcontext.h b/arch/x86/include/asm/sigcontext.h
index 9dfce4e0417d..f910cdcb71fd 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 d8b9f9081e86..076b11fd6fa1 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 ed37a768d0fc..9511eb7f17b0 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);
@@ -457,9 +451,19 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig,
 
 	regs->sp = (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. */
+	/*
+	 * Set up the CS and SS registers to run signal handlers in
+	 * 64-bit mode, even if the handler happens to be interrupting
+	 * 32-bit or 16-bit code.
+	 *
+	 * SS is subtle.  In 64-bit mode, we don't need any particular
+	 * SS descriptor, but we do need SS to be valid.  It's possible
+	 * that the old SS is entirely bogus -- this can happen if the
+	 * signal we're trying to deliver is #GP or #SS caused by a bad
+	 * SS value.
+	 */
 	regs->cs = __USER_CS;
+	regs->ss = __USER_DS;
 
 	return 0;
 }
-- 
2.3.0