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Subject[PATCH 4.19 274/306] selftests/x86/mov_ss_trap: Fix the SYSENTER test
From: Andy Lutomirski <>

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 <>
Signed-off-by: Peter Zijlstra (Intel) <>
Signed-off-by: Greg Kroah-Hartman <>

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"

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