Indan Zupancic wrote: > On Tue, January 17, 2012 18:45, Andrew Lutomirski wrote: > > On Tue, Jan 17, 2012 at 9:05 AM, Oleg Nesterov <oleg@xxxxxxxxxx> 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? PTRACE_PEEKTEXT won't securely tell you if it's int 0x80 if there's another thread modifying the code, or changing the mappings, or it's executing from a file or shared memory that someone's writing to. > 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? It's a surprise to me too. And like you I'm using ptrace, to trace what a process touches, not restrict it, but it's subject to the same problem. This looks like it needs a kernel patch. -- Jamie -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
