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"