Commit 8caa016b authored Nov 20, 2019 by Andy Lutomirski Committed by Peter Zijlstra Nov 21, 2019

selftests/x86/mov_ss_trap: Fix the SYSENTER test



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: 59c2a722 ("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

parent 89542907

tools/testing/selftests/x86/mov_ss_trap.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -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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