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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Andy Lutomirski <luto@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>, stable@kernel.org
Subject: [PATCH 4.14 183/211] selftests/x86/mov_ss_trap: Fix the SYSENTER test
Date:   Wed, 27 Nov 2019 21:31:56 +0100
Message-Id: <20191127203111.057361704@linuxfoundation.org>
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From: Andy Lutomirski <luto@kernel.org>

commit 8caa016bfc129f2c925d52da43022171d1d1de91 upstream.

For reasons that I haven't quite fully diagnosed, running
mov_ss_trap_32 on a 32-bit kernel results in an infinite loop in
userspace.  This appears to be because the hacky SYSENTER test
doesn't segfault as desired; instead it corrupts the program state
such that it infinite loops.

Fix it by explicitly clearing EBP before doing SYSENTER.  This will
give a more reliable segfault.

Fixes: 59c2a7226fc5 ("x86/selftests: Add mov_to_ss test")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 tools/testing/selftests/x86/mov_ss_trap.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/tools/testing/selftests/x86/mov_ss_trap.c
+++ b/tools/testing/selftests/x86/mov_ss_trap.c
@@ -257,7 +257,8 @@ int main()
 			err(1, "sigaltstack");
 		sethandler(SIGSEGV, handle_and_longjmp, SA_RESETHAND | SA_ONSTACK);
 		nr = SYS_getpid;
-		asm volatile ("mov %[ss], %%ss; SYSENTER" : "+a" (nr)
+		/* Clear EBP first to make sure we segfault cleanly. */
+		asm volatile ("xorl %%ebp, %%ebp; mov %[ss], %%ss; SYSENTER" : "+a" (nr)
 			      : [ss] "m" (ss) : "flags", "rcx"
 #ifdef __x86_64__
 				, "r11"