x86/signal/64: Fix SS if needed when delivering a 64-bit signal

Signals are always delivered to 64-bit tasks with CS set to a long
mode segment.  In long mode, SS doesn't matter as long as it's a
present writable segment.

If SS starts out invalid (this can happen if the signal was caused
by an IRET fault or was delivered on the way out of set_thread_area
or modify_ldt), then IRET to the signal handler can fail, eventually
killing the task.

The straightforward fix would be to simply reset SS when delivering
a signal.  That breaks DOSEMU, though: 64-bit builds of DOSEMU rely
on SS being set to the faulting SS when signals are delivered.

As a compromise, this patch leaves SS alone so long as it's valid.

The net effect should be that the behavior of successfully delivered
signals is unchanged.  Some signals that would previously have
failed to be delivered will now be delivered successfully.

This has no effect for x32 or 32-bit tasks: their signal handlers
were already called with SS == __USER_DS.

(On Xen, there's a slight hole: if a task sets SS to a writable
 *kernel* data segment, then we will fail to identify it as invalid
 and we'll still kill the task.  If anyone cares, this could be fixed
 with a new paravirt hook.)

Signed-off-by: Andy Lutomirski <[email protected]>
Acked-by: Borislav Petkov <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stas Sergeev <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/163c6e1eacde41388f3ff4d2fe6769be651d7b6e.1455664054.git.luto@kernel.org
Signed-off-by: Ingo Molnar <[email protected]>

0 files changed, 0 insertions, 0 deletions