[PATCH 1/1] get_nr_restart_syscall() should return __NR_ia32_restart_syscall if __USER32_CS

Oleg Nesterov
2017-03-28T14:55:37Z

get_nr_restart_syscall() checks TS_I386_REGS_POKED but this bit is only
set if debugger is 32-bit. If a 64-bit debugger restores the registers
of a 32-bit debugee outside of syscall exit path get_nr_restart_syscall()
wrongly returns __NR_restart_syscall.

Test-case:

$ cvs -d :pserver:anoncvs:anoncvs@sourceware.org:/cvs/systemtap co ptrace-tests
$ gcc -o erestartsys-trap-debuggee ptrace-tests/tests/erestartsys-trap-debuggee.c --m32
$ gcc -o erestartsys-trap-debugger ptrace-tests/tests/erestartsys-trap-debugger.c -lutil
$ ./erestartsys-trap-debugger
Unexpected: retval 1, errno 22
erestartsys-trap-debugger: ptrace-tests/tests/erestartsys-trap-debugger.c:421

As Jan explains this is what "(gdb) call func()" actually does:

* Tracee calls sleep(2).
* Debugger interrupts the tracee by CTRL-C after 1 sec.
* Save regs by PTRACE_GETREGS.
* Use PTRACE_SETREGS changing %rip to some 'func' and setting %orig_rax=-1
* PTRACE_CONT
* func() uses int3.
* Debugger catches SIGTRAP.
* Restore original regs by PTRACE_SETREGS.
* PTRACE_CONT

Change get_nr_restart_syscall() to take __USER32_CS into account, to me
this looks a bit better than TIF_IA32 check but either way this logic
can't be always right as the comment explains.

Alternatively we could change putreg() to set TS_I386_REGS_POKED just like
putreg32() does if "child" is 32-bit, but this won't fix all the problems
too and I think it would be beter to kill TS_I386_REGS_POKED after this
change.

Reported-by: Jan Kratochvil <jan.kratochvil@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: stable@vger.kernel.org
---
arch/x86/kernel/signal.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 763af1d..1b05448 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -785,7 +785,8 @@ static inline unsigned long get_nr_restart_syscall(const struct pt_regs *regs)
* than the tracee.
*/
#ifdef CONFIG_IA32_EMULATION
- if (current->thread.status & (TS_COMPAT|TS_I386_REGS_POKED))
+ if ((current->thread.status & (TS_COMPAT|TS_I386_REGS_POKED)) ||
+ regs->cs == __USER32_CS)
return __NR_ia32_restart_syscall;
#endif
#ifdef CONFIG_X86_X32_ABI
--
2.5.0