x86, fpu: shift drop_init_fpu() from save_xstate_sig() to handle_signal()

save_xstate_sig()->drop_init_fpu() doesn't look right. setup_rt_frame()
can fail after that, in this case the next setup_rt_frame() triggered
by SIGSEGV won't save fpu simply because the old state was lost. This
obviously mean that fpu won't be restored after sys_rt_sigreturn() from
SIGSEGV handler.

Shift drop_init_fpu() into !failed branch in handle_signal().

Test-case (needs -O2):

	#include <stdio.h>
	#include <signal.h>
	#include <unistd.h>
	#include <sys/syscall.h>
	#include <sys/mman.h>
	#include <pthread.h>
	#include <assert.h>

	volatile double D;

	void test(double d)
	{
		int pid = getpid();

		for (D = d; D == d; ) {
			/* sys_tkill(pid, SIGHUP); asm to avoid save/reload
			 * fp regs around "C" call */
			asm ("" : : "a"(200), "D"(pid), "S"(1));
			asm ("syscall" : : : "ax");
		}

		printf("ERR!!\n");
	}

	void sigh(int sig)
	{
	}

	char altstack[4096 * 10] __attribute__((aligned(4096)));

	void *tfunc(void *arg)
	{
		for (;;) {
			mprotect(altstack, sizeof(altstack), PROT_READ);
			mprotect(altstack, sizeof(altstack), PROT_READ|PROT_WRITE);
		}
	}

	int main(void)
	{
		stack_t st = {
			.ss_sp = altstack,
			.ss_size = sizeof(altstack),
			.ss_flags = SS_ONSTACK,
		};

		struct sigaction sa = {
			.sa_handler = sigh,
		};

		pthread_t pt;

		sigaction(SIGSEGV, &sa, NULL);
		sigaltstack(&st, NULL);
		sa.sa_flags = SA_ONSTACK;
		sigaction(SIGHUP, &sa, NULL);

		pthread_create(&pt, NULL, tfunc, NULL);

		test(123.456);
		return 0;
	}

Reported-by: Bean Anderson <bean@azulsystems.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Link: http://lkml.kernel.org/r/20140902175713.GA21646@redhat.com
Cc: <stable@kernel.org> # v3.7+
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
This commit is contained in:
Oleg Nesterov 2014-09-02 19:57:13 +02:00 committed by H. Peter Anvin
parent f3761db164
commit 66463db4fc
2 changed files with 5 additions and 2 deletions

View File

@ -675,6 +675,11 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
* handler too.
*/
regs->flags &= ~(X86_EFLAGS_DF|X86_EFLAGS_RF|X86_EFLAGS_TF);
/*
* Ensure the signal handler starts with the new fpu state.
*/
if (used_math())
drop_init_fpu(current);
}
signal_setup_done(failed, ksig, test_thread_flag(TIF_SINGLESTEP));
}

View File

@ -271,8 +271,6 @@ int save_xstate_sig(void __user *buf, void __user *buf_fx, int size)
if (use_fxsr() && save_xstate_epilog(buf_fx, ia32_fxstate))
return -1;
drop_init_fpu(tsk); /* trigger finit */
return 0;
}