From: Andrei Vagin <avagin@gmail.com>
To: Jann Horn <jannh@google.com>
Cc: the arch/x86 maintainers <x86@kernel.org>,
Andy Lutomirski <luto@kernel.org>,
linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
linux-um@lists.infradead.org, criu@openvz.org, avagin@google.com,
Andrew Morton <akpm@linux-foundation.org>,
Anton Ivanov <anton.ivanov@cambridgegreys.com>,
Christian Brauner <christian.brauner@ubuntu.com>,
Dmitry Safonov <0x7f454c46@gmail.com>,
Ingo Molnar <mingo@redhat.com>, Jeff Dike <jdike@addtoit.com>,
Mike Rapoport <rppt@linux.ibm.com>,
Michael Kerrisk <mtk.manpages@gmail.com>,
Oleg Nesterov <oleg@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Richard Weinberger <richard@nod.at>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 2/4] arch/x86: implement the process_vm_exec syscall
Date: Fri, 2 Jul 2021 15:48:53 -0700 [thread overview]
Message-ID: <YN+X1QKWQOKekK4E@gmail.com> (raw)
In-Reply-To: <CAG48ez37ZUNvWy1eOvrW13kFRM-_ZW175x99Nyjq43w4Qz1qJQ@mail.gmail.com>
On Fri, Jul 02, 2021 at 10:56:38PM +0200, Jann Horn wrote:
> On Wed, Apr 14, 2021 at 7:59 AM Andrei Vagin <avagin@gmail.com> wrote:
> > This change introduces the new system call:
> > process_vm_exec(pid_t pid, struct sigcontext *uctx, unsigned long flags,
> > siginfo_t * uinfo, sigset_t *sigmask, size_t sizemask)
> >
> > process_vm_exec allows to execute the current process in an address
> > space of another process.
> >
> > process_vm_exec swaps the current address space with an address space of
> > a specified process, sets a state from sigcontex and resumes the process.
> > When a process receives a signal or calls a system call,
> > process_vm_exec saves the process state back to sigcontext, restores the
> > origin address space, restores the origin process state, and returns to
> > userspace.
> >
> > If it was interrupted by a signal and the signal is in the user_mask,
> > the signal is dequeued and information about it is saved in uinfo.
> > If process_vm_exec is interrupted by a system call, a synthetic siginfo
> > for the SIGSYS signal is generated.
> >
> > The behavior of this system call is similar to PTRACE_SYSEMU but
> > everything is happing in the context of one process, so
> > process_vm_exec shows a better performance.
> >
> > PTRACE_SYSEMU is primarily used to implement sandboxes (application
> > kernels) like User-mode Linux or gVisor. These type of sandboxes
> > intercepts applications system calls and acts as the guest kernel.
> > A simple benchmark, where a "tracee" process executes systems calls in a
> > loop and a "tracer" process traps syscalls and handles them just
> > incrementing the tracee instruction pointer to skip the syscall
> > instruction shows that process_vm_exec works more than 5 times faster
> > than PTRACE_SYSEMU.
> [...]
> > +long swap_vm_exec_context(struct sigcontext __user *uctx)
> > +{
> > + struct sigcontext ctx = {};
> > + sigset_t set = {};
> > +
> > +
> > + if (copy_from_user(&ctx, uctx, CONTEXT_COPY_SIZE))
> > + return -EFAULT;
> > + /* A floating point state is managed from user-space. */
> > + if (ctx.fpstate != 0)
> > + return -EINVAL;
> > + if (!user_access_begin(uctx, sizeof(*uctx)))
> > + return -EFAULT;
> > + unsafe_put_sigcontext(uctx, NULL, current_pt_regs(), (&set), Efault);
> > + user_access_end();
> > +
> > + if (__restore_sigcontext(current_pt_regs(), &ctx, 0))
> > + goto badframe;
> > +
> > + return 0;
> > +Efault:
> > + user_access_end();
> > +badframe:
> > + signal_fault(current_pt_regs(), uctx, "swap_vm_exec_context");
> > + return -EFAULT;
> > +}
>
> Comparing the pieces of context that restore_sigcontext() restores
> with what a normal task switch does (see __switch_to() and callees), I
> noticed: On CPUs with FSGSBASE support, I think sandboxed code could
> overwrite FSBASE/GSBASE using the WRFSBASE/WRGSBASE instructions,
> causing the supervisor to access attacker-controlled addresses when it
> tries to access a thread-local variable like "errno"? Signal handling
> saves the segment registers, but not the FS/GS base addresses.
>
>
> jannh@laptop:~/test$ cat signal_gsbase.c
> // compile with -mfsgsbase
> #include <stdio.h>
> #include <signal.h>
> #include <immintrin.h>
>
> void signal_handler(int sig, siginfo_t *info, void *ucontext_) {
> puts("signal handler");
> _writegsbase_u64(0x12345678);
> }
>
> int main(void) {
> struct sigaction new_act = {
> .sa_sigaction = signal_handler,
> .sa_flags = SA_SIGINFO
> };
> sigaction(SIGUSR1, &new_act, NULL);
>
> printf("original gsbase is 0x%lx\n", _readgsbase_u64());
> raise(SIGUSR1);
> printf("post-signal gsbase is 0x%lx\n", _readgsbase_u64());
> }
> jannh@laptop:~/test$ gcc -o signal_gsbase signal_gsbase.c -mfsgsbase
> jannh@laptop:~/test$ ./signal_gsbase
> original gsbase is 0x0
> signal handler
> post-signal gsbase is 0x12345678
> jannh@laptop:~/test$
>
>
> So to make this usable for a sandboxing usecase, you'd also have to
> save and restore FSBASE/GSBASE, just like __switch_to().
You are right. I've found this too when I implemented the gviosr user-space
part.
Here is the tree whether this problem has been fixed:
https://github.com/avagin/linux-task-diag/commits/wip/gvisor-5.10
next prev parent reply other threads:[~2021-07-02 22:52 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-14 5:52 [PATCH 0/4 POC] Allow executing code and syscalls in another address space Andrei Vagin
2021-04-14 5:52 ` [PATCH 1/4] signal: add a helper to restore a process state from sigcontex Andrei Vagin
2021-04-14 5:52 ` [PATCH 2/4] arch/x86: implement the process_vm_exec syscall Andrei Vagin
2021-04-14 17:09 ` Oleg Nesterov
2021-04-23 6:59 ` Andrei Vagin
2021-06-28 16:13 ` Jann Horn
2021-06-28 16:30 ` Andy Lutomirski
2021-06-28 17:14 ` Jann Horn
2021-06-28 18:18 ` Eric W. Biederman
2021-06-29 1:01 ` Andrei Vagin
2021-07-02 6:22 ` Andrei Vagin
2021-07-02 11:51 ` Jann Horn
2021-07-02 20:40 ` Andy Lutomirski
2021-07-02 8:51 ` Peter Zijlstra
2021-07-02 22:21 ` Andrei Vagin
2021-07-02 20:56 ` Jann Horn
2021-07-02 22:48 ` Andrei Vagin [this message]
2021-04-14 5:52 ` [PATCH 3/4] arch/x86: allow to execute syscalls via process_vm_exec Andrei Vagin
2021-04-14 5:52 ` [PATCH 4/4] selftests: add tests for process_vm_exec Andrei Vagin
2021-04-14 6:46 ` [PATCH 0/4 POC] Allow executing code and syscalls in another address space Jann Horn
2021-04-14 22:10 ` Andrei Vagin
2021-07-02 6:57 ` Andrei Vagin
2021-07-02 15:12 ` Jann Horn
2021-07-18 0:38 ` Andrei Vagin
2021-04-14 7:22 ` Anton Ivanov
2021-04-14 7:34 ` Johannes Berg
2021-04-14 9:24 ` Benjamin Berg
2021-04-14 10:27 ` Florian Weimer
2021-04-14 11:24 ` Jann Horn
2021-04-14 12:20 ` Florian Weimer
2021-04-14 13:58 ` Jann Horn
2021-04-16 19:29 ` Kirill Smelkov
2021-04-17 16:28 ` sbaugh
2021-07-02 22:44 ` Andy Lutomirski
2021-07-18 1:34 ` Andrei Vagin
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