selftests/x86/sigreturn/32: Invalidate DS and ES when abusing the kernel

If the kernel accidentally uses DS or ES while the user values are
loaded, it will work fine for sane userspace.  In the interest of
simulating maximally insane userspace, make sigreturn_32 zero out DS
and ES for the nasty parts so that inadvertent use of these segments
will crash.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@kernel.org

tools/testing/selftests/x86/sigreturn.c

@@ -451,6 +451,19 @@ static void sigusr1(int sig, siginfo_t *info, void *ctx_void)
 	ctx->uc_mcontext.gregs[REG_SP] = (unsigned long)0x8badf00d5aadc0deULL;
 	ctx->uc_mcontext.gregs[REG_CX] = 0;
 
+#ifdef __i386__
+	/*
+	 * Make sure the kernel doesn't inadvertently use DS or ES-relative
+	 * accesses in a region where user DS or ES is loaded.
+	 *
+	 * Skip this for 64-bit builds because long mode doesn't care about
+	 * DS and ES and skipping it increases test coverage a little bit,
+	 * since 64-bit kernels can still run the 32-bit build.
+	 */
+	ctx->uc_mcontext.gregs[REG_DS] = 0;
+	ctx->uc_mcontext.gregs[REG_ES] = 0;
+#endif
+
 	memcpy(&requested_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t));
 	requested_regs[REG_CX] = *ssptr(ctx);	/* The asm code does this. */