From: Bodo Stroesser

The implementation of sys_sigreturn() and sys_rt_sigreturn() in UML
must be changed.
This is necessary, since the return value of sys_*_sigreturn()
is the value of eax in the thread, that was interrupted by the
signal handler. If accidentaly eax contains -ERESTART_*, orig_eax
*must* be -1 to avoid syscall restart processing in kern_do_signal().
If orig_eax is >=0, eip might be lowered by 2, the process will fail.
In UML PT_REGS_SYSCALL_NR() or UPT_SYSCALL_NR() have to be used
instead of orig_eax.

While writing and testing an exploit for this, I saw that for most
interrupts, the syscall number is undefined. So even on a return from
interrupt a wrong syscall restart handling could happen.

And also: UML resumes a process with ptrace(PTRACE_SYSCALL/SYSEMU/SINGLESTEP
when a syscall in UML in SKAS mode has been processed. But since there
is a valid syscall number in the host's orig_eax, the host could do
a wrong syscall restarting if the syscall in UML was a sigreturn() returning
-ERESTART* To avoid this, in SKAS -1 should be written to regs.orig_eax
before restore_registers().
Signed-off-by: Bodo Stroesser <bstroesser@fujitsu-siemens.com>
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade_spam@yahoo.it>
Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>