Messages in this thread |  | | | From | Andrew Lutomirski <> | | Date | Tue, 17 Jan 2012 17:01:41 -0800 | | Subject | Re: Compat 32-bit syscall entry from 64-bit task!? [was: Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF] |
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On Tue, Jan 17, 2012 at 4:56 PM, Indan Zupancic <indan@nul.nu> wrote: > On Tue, January 17, 2012 18:45, Andrew Lutomirski wrote: >> On Tue, Jan 17, 2012 at 9:05 AM, Oleg Nesterov <oleg@redhat.com> wrote: >>> On 01/17, Andrew Lutomirski wrote: >>>> >>>> (is_compat_task says whether the executable was marked as 32-bit. �The >>>> actual execution mode is determined by the cs register, which the user >>>> can control. >>> >>> Confused... Afaics, TIF_IA32 says that the binary is 32-bit (this comes >>> along with TS_COMPAT). >>> >>> TS_COMPAT says that, say, the task did "int 80" to enters the kernel. >>> 64-bit or not, we should treat is as 32-bit in this case. >> >> I think you're right, and checking which entry was used is better than >> checking the cs register (since 64-bit code can use int80). That's >> what I get for insufficiently careful reading of the assembly. (And >> for going from memory from when I wrote the vsyscall emulation code -- >> that code is entered from a page fault, so the entry point used is >> irrelevant.) > > Wait: If a tasks is set to 64 bit mode, but calls into the kernel via > int 0x80 it's changed to 32 bit mode for that system call and back to > 64 bit mode when the system call is finished!? > > Our ptrace jailer is checking cs to figure out if a task is a compat task > or not, if the kernel can change that behind our back it means our jailer > isn't secure for x86_64 with compat enabled. Or is cs changed before the > ptrace stuff and ptrace sees the "right" cs value? If not, we have to add > an expensive PTRACE_PEEKTEXT to check if it's an int 0x80 or not. Or is > there another way?
I don't know what your ptrace jailer does. But a task can switch itself between 32-bit and 64-bit execution at will, and there's nothing the kernel can do about it. (That isn't quite true -- in theory the kernel could fiddle with the GDT, but that would be expensive and wouldn't work on Xen.)
That being said, is_compat_task is apparently a good indication of whether the current *syscall* entry is a 64-bit syscall or a 32-bit syscall. Perhaps the function should be renamed to in_compat_syscall, because that's what it does.
> > I think this behaviour is so unexpected that it can only cause security > problems in the long run. Is anyone counting on this? Where is this > behaviour documented?
Nowhere, I think.
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