On Tue, Mar 28, 2017 at 9:27 AM, Oleg Nesterov <[email protected]> wrote: > On 03/28, Andy Lutomirski wrote: >> >> On Tue, Mar 28, 2017 at 7:54 AM, Oleg Nesterov <[email protected]> wrote: >> > 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. >> >> I had sent a patch that introduced a new syscall nr, but it's not >> quite safe because it could break seccomp-using programs. > > Ah, indeed...
This is, in theory, solvable. It would be ugly and would pollute seccomp a bit. > >> But your >> patch here is also screwy. > > Yes, yes, it doesn't try to solve all possible problems, I even mentioned > this in the changelog. > >> How about we store the syscall arch to be restored in task_struct >> along with restart_block? > > Yes, perhaps we will have to finally do this. Not really nice too. > >> the way there without heuristics as nasty as yours. > > I agree it will be better, but I refuse to treat them as mine checks ;) :) > >> P.S. __USER32_CS is the wrong check even if we used your approach. >> user_64bit_regs() is much better. > > Yes, thanks. If only I understood what cs == pv_info.extra_user_64bit_cs > actually means... > It means that, if Linux is a Xen PV guest, the GDT contains a bunch of entries supplied by Xen and outside of Linux's control, and one of those entries is a 64-bit DPL=3 code segment. On the one hand, it's annoying. On the other hand, it serves a real purpose performance-wise. --Andy
