s390/signal: switch to using vdso for sigreturn and syscall restart

with generic entry, there's a bug when it comes to restarting of signals.
The failing sequence is:

a) a signal is coming in, and no handler is registered, so the lower
   part of arch_do_signal_or_restart() in arch/s390/kernel/signal.c
   sets PIF_SYSCALL_RESTART.

b) a second signal gets pending while the kernel is still in the exit
   loop, and for that one, a handler exists.

c) The first part of arch_do_signal_or_restart() is called. That part
   calls handle_signal(), which sets up stack + registers for handling
   the signal.

d) __do_syscall() in arch/s390/kernel/syscall.c checks for
   PIF_SYSCALL_RESTART right before leaving to userspace. If it is set,
   it restart's the syscall. However, the registers are already setup
   for handling a signal from c). The syscall is now restarted with the
   wrong arguments.

Change the code to:

- use vdso for syscall_restart() instead of PIF_SYSCALL_RESTART because
  we cannot rewind and go back to userspace on s390 because the system call
  number might be encoded in the svc instruction.
- for all other syscalls we rewind the PSW and return to userspace.

Cc: <stable@kernel.org> # v5.12+ d57778feb987: s390/vdso: always enable vdso
Cc: <stable@kernel.org> # v5.12+ 686341f2548b: s390/vdso64: add sigreturn,rt_sigreturn and restart_syscall
Cc: <stable@kernel.org> # v5.12+ 43e1f76b0b69: s390/vdso: rename VDSO64_LBASE to VDSO_LBASE
Cc: <stable@kernel.org> # v5.12+ 779df2248739: s390/vdso: add minimal compat vdso
Cc: <stable@kernel.org> # v5.12+
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

1 files changed, 3 insertions, 10 deletions

diff --git a/arch/s390/kernel/compat_signal.c b/arch/s390/kernel/compat_signal.c
index 1d0e17ec93eb..cca142fbb516 100644
--- a/arch/s390/kernel/compat_signal.c
+++ b/arch/s390/kernel/compat_signal.c
@@ -28,6 +28,7 @@
 #include <linux/uaccess.h>
 #include <asm/lowcore.h>
 #include <asm/switch_to.h>
+#include <asm/vdso.h>
 #include "compat_linux.h"
 #include "compat_ptrace.h"
 #include "entry.h"
@@ -118,7 +119,6 @@ static int restore_sigregs32(struct pt_regs *regs,_sigregs32 __user *sregs)
 	fpregs_load((_s390_fp_regs *) &user_sregs.fpregs, &current->thread.fpu);
 
 	clear_pt_regs_flag(regs, PIF_SYSCALL); /* No longer in a system call */
-	clear_pt_regs_flag(regs, PIF_SYSCALL_RESTART);
 	return 0;
 }
 
@@ -304,11 +304,7 @@ static int setup_frame32(struct ksignal *ksig, sigset_t *set,
 		restorer = (unsigned long __force)
 			ksig->ka.sa.sa_restorer | PSW32_ADDR_AMODE;
 	} else {
-		/* Signal frames without vectors registers are short ! */
-		__u16 __user *svc = (void __user *) frame + frame_size - 2;
-		if (__put_user(S390_SYSCALL_OPCODE | __NR_sigreturn, svc))
-			return -EFAULT;
-		restorer = (unsigned long __force) svc | PSW32_ADDR_AMODE;
+		restorer = VDSO32_SYMBOL(current, sigreturn);
         }
 
 	/* Set up registers for signal handler */
@@ -371,10 +367,7 @@ static int setup_rt_frame32(struct ksignal *ksig, sigset_t *set,
 		restorer = (unsigned long __force)
 			ksig->ka.sa.sa_restorer | PSW32_ADDR_AMODE;
 	} else {
-		__u16 __user *svc = &frame->svc_insn;
-		if (__put_user(S390_SYSCALL_OPCODE | __NR_rt_sigreturn, svc))
-			return -EFAULT;
-		restorer = (unsigned long __force) svc | PSW32_ADDR_AMODE;
+		restorer = VDSO32_SYMBOL(current, rt_sigreturn);
 	}
 
 	/* Create siginfo on the signal stack */