[PATCH] uml: skas0 - separate kernel address space on stock hosts

UML has had two modes of operation - an insecure, slow mode (tt mode) in
which the kernel is mapped into every process address space which requires
no host kernel modifications, and a secure, faster mode (skas mode) in
which the UML kernel is in a separate host address space, which requires a
patch to the host kernel.

This patch implements something very close to skas mode for hosts which
don't support skas - I'm calling this skas0.  It provides the security of
the skas host patch, and some of the performance gains.

The two main things that are provided by the skas patch, /proc/mm and
PTRACE_FAULTINFO, are implemented in a way that require no host patch.

For the remote address space changing stuff (mmap, munmap, and mprotect),
we set aside two pages in the process above its stack, one of which
contains a little bit of code which can call mmap et al.

To update the address space, the system call information (system call
number and arguments) are written to the stub page above the code.  The
%esp is set to the beginning of the data, the %eip is set the the start of
the stub, and it repeatedly pops the information into its registers and
makes the system call until it sees a system call number of zero.  This is
to amortize the cost of the context switch across multiple address space
updates.

When the updates are done, it SIGSTOPs itself, and the kernel process
continues what it was doing.

For a PTRACE_FAULTINFO replacement, we set up a SIGSEGV handler in the
child, and let it handle segfaults rather than nullifying them.  The
handler is in the same page as the mmap stub.  The second page is used as
the stack.  The handler reads cr2 and err from the sigcontext, sticks them
at the base of the stack in a faultinfo struct, and SIGSTOPs itself.  The
kernel then reads the faultinfo and handles the fault.

A complication on x86_64 is that this involves resetting the registers to
the segfault values when the process is inside the kill system call.  This
breaks on x86_64 because %rcx will contain %rip because you tell SYSRET
where to return to by putting the value in %rcx.  So, this corrupts $rcx on
return from the segfault.  To work around this, I added an
arch_finish_segv, which on x86 does nothing, but which on x86_64 ptraces
the child back through the sigreturn.  This causes %rcx to be restored by
sigreturn and avoids the corruption.  Ultimately, I think I will replace
this with the trick of having it send itself a blocked signal which will be
unblocked by the sigreturn.  This will allow it to be stopped just after
the sigreturn, and PTRACE_SYSCALLed without all the back-and-forth of
PTRACE_SYSCALLing it through sigreturn.

This runs on a stock host, so theoretically (and hopefully), tt mode isn't
needed any more.  We need to make sure that this is better in every way
than tt mode, though.  I'm concerned about the speed of address space
updates and page fault handling, since they involve extra round-trips to
the child.  We can amortize the round-trip cost for large address space
updates by writing all of the operations to the data page and having the
child execute them all at the same time.  This will help fork and exec, but
not page faults, since they involve only one page.

I can't think of any way to help page faults, except to add something like
PTRACE_FAULTINFO to the host.  There is PTRACE_SIGINFO, but UML doesn't use
siginfo for SIGSEGV (or anything else) because there isn't enough
information in the siginfo struct to handle page faults (the faulting
operation type is missing).  Adding that would make PTRACE_SIGINFO a usable
equivalent to PTRACE_FAULTINFO.

As for the code itself:

- The system call stub is in arch/um/kernel/sys-$(SUBARCH)/stub.S.  It is
  put in its own section of the binary along with stub_segv_handler in
  arch/um/kernel/skas/process.c.  This is manipulated with run_syscall_stub
  in arch/um/kernel/skas/mem_user.c.  syscall_stub will execute any system
  call at all, but it's only used for mmap, munmap, and mprotect.

- The x86_64 stub calls sigreturn by hand rather than allowing the normal
  sigreturn to happen, because the normal sigreturn is a SA_RESTORER in
  UML's address space provided by libc.  Needless to say, this is not
  available in the child's address space.  Also, it does a couple of odd
  pops before that which restore the stack to the state it was in at the
  time the signal handler was called.

- There is a new field in the arch mmu_context, which is now a union.
  This is the pid to be manipulated rather than the /proc/mm file
  descriptor.  Code which deals with this now checks proc_mm to see whether
  it should use the usual skas code or the new code.

- userspace_tramp is now used to create a new host process for every UML
  process, rather than one per UML processor.  It checks proc_mm and
  ptrace_faultinfo to decide whether to map in the pages above its stack.

- start_userspace now makes CLONE_VM conditional on proc_mm since we need
  separate address spaces now.

- switch_mm_skas now just sets userspace_pid[0] to the new pid rather
  than PTRACE_SWITCH_MM.  There is an addition to userspace which updates
  its idea of the pid being manipulated each time around the loop.  This is
  important on exec, when the pid will change underneath userspace().

- The stub page has a pte, but it can't be mapped in using tlb_flush
  because it is part of tlb_flush.  This is why it's required for it to be
  mapped in by userspace_tramp.

Other random things:

- The stub section in uml.lds.S is page aligned.  This page is written
  out to the backing vm file in setup_physmem because it is mapped from
  there into user processes.

- There's some confusion with TASK_SIZE now that there are a couple of
  extra pages that the process can't use.  TASK_SIZE is considered by the
  elf code to be the usable process memory, which is reasonable, so it is
  decreased by two pages.  This confuses the definition of
  USER_PGDS_IN_LAST_PML4, making it too small because of the rounding down
  of the uneven division.  So we round it to the nearest PGDIR_SIZE rather
  than the lower one.

- I added a missing PT_SYSCALL_ARG6_OFFSET macro.

- um_mmu.h was made into a userspace-usable file.

- proc_mm and ptrace_faultinfo are globals which say whether the host
  supports these features.

- There is a bad interaction between the mm.nr_ptes check at the end of
  exit_mmap, stack randomization, and skas0.  exit_mmap will stop freeing
  pages at the PGDIR_SIZE boundary after the last vma.  If the stack isn't
  on the last page table page, the last pte page won't be freed, as it
  should be since the stub ptes are there, and exit_mmap will BUG because
  there is an unfreed page.  To get around this, TASK_SIZE is set to the
  next lowest PGDIR_SIZE boundary and mm->nr_ptes is decremented after the
  calls to init_stub_pte.  This ensures that we know the process stack (and
  all other process mappings) will be below the top page table page, and
  thus we know that mm->nr_ptes will be one too many, and can be
  decremented.

Things that need fixing:

- We may need better assurrences that the stub code is PIC.

- The stub pte is set up in init_new_context_skas.

- alloc_pgdir is probably the right place.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

7 files changed, 78 insertions, 18 deletions

diff --git a/arch/um/include/mem.h b/arch/um/include/mem.h
index 10c46c38949a..99d3ad4a03e5 100644
--- a/arch/um/include/mem.h
+++ b/arch/um/include/mem.h
@@ -13,6 +13,7 @@ extern int physmem_subst_mapping(void *virt, int fd, __u64 offset, int w);
 extern int is_remapped(void *virt);
 extern int physmem_remove_mapping(void *virt);
 extern void physmem_forget_descriptor(int fd);
+extern unsigned long to_phys(void *virt);
 
 #endif
 
diff --git a/arch/um/include/registers.h b/arch/um/include/registers.h
index 8744abb5224f..0a35e6d0baa0 100644
--- a/arch/um/include/registers.h
+++ b/arch/um/include/registers.h
@@ -14,6 +14,7 @@ extern int restore_fp_registers(int pid, unsigned long *fp_regs);
 extern void save_registers(int pid, union uml_pt_regs *regs);
 extern void restore_registers(int pid, union uml_pt_regs *regs);
 extern void init_registers(int pid);
+extern void get_safe_registers(unsigned long * regs);
 
 #endif
 
diff --git a/arch/um/include/sysdep-i386/ptrace_user.h b/arch/um/include/sysdep-i386/ptrace_user.h
index eca8066e7a43..899aa4b2a78d 100644
--- a/arch/um/include/sysdep-i386/ptrace_user.h
+++ b/arch/um/include/sysdep-i386/ptrace_user.h
@@ -20,11 +20,24 @@
 #define PT_SYSCALL_ARG3_OFFSET PT_OFFSET(EDX)
 #define PT_SYSCALL_ARG4_OFFSET PT_OFFSET(ESI)
 #define PT_SYSCALL_ARG5_OFFSET PT_OFFSET(EDI)
+#define PT_SYSCALL_ARG6_OFFSET PT_OFFSET(EBP)
 
 #define PT_SYSCALL_RET_OFFSET PT_OFFSET(EAX)
 
+#define REGS_SYSCALL_NR EAX /* This is used before a system call */
+#define REGS_SYSCALL_ARG1 EBX
+#define REGS_SYSCALL_ARG2 ECX
+#define REGS_SYSCALL_ARG3 EDX
+#define REGS_SYSCALL_ARG4 ESI
+#define REGS_SYSCALL_ARG5 EDI
+#define REGS_SYSCALL_ARG6 EBP
+
+#define REGS_IP_INDEX EIP
+#define REGS_SP_INDEX UESP
+
 #define PT_IP_OFFSET PT_OFFSET(EIP)
 #define PT_IP(regs) ((regs)[EIP])
+#define PT_SP_OFFSET PT_OFFSET(UESP)
 #define PT_SP(regs) ((regs)[UESP])
 
 #ifndef FRAME_SIZE
diff --git a/arch/um/include/sysdep-i386/stub.h b/arch/um/include/sysdep-i386/stub.h
new file mode 100644
index 000000000000..fed9ff1cea52
--- /dev/null
+++ b/arch/um/include/sysdep-i386/stub.h
@@ -0,0 +1,18 @@
+/*
+ * Copyright (C) 2004 Jeff Dike (jdike@addtoit.com)
+ * Licensed under the GPL
+ */
+
+#ifndef __SYSDEP_STUB_H
+#define __SYSDEP_STUB_H
+
+#include <asm/ptrace.h>
+#include <asm/unistd.h>
+
+extern void stub_segv_handler(int sig);
+
+#define STUB_SYSCALL_RET EAX
+#define STUB_MMAP_NR __NR_mmap2
+#define MMAP_OFFSET(o) ((o) >> PAGE_SHIFT)
+
+#endif
diff --git a/arch/um/include/sysdep-x86_64/ptrace_user.h b/arch/um/include/sysdep-x86_64/ptrace_user.h
index 31729973fb14..128faf027364 100644
--- a/arch/um/include/sysdep-x86_64/ptrace_user.h
+++ b/arch/um/include/sysdep-x86_64/ptrace_user.h
@@ -55,6 +55,20 @@
 #define PTRACE_OLDSETOPTIONS 21
 #endif
 
+/* These are before the system call, so the the system call number is RAX
+ * rather than ORIG_RAX, and arg4 is R10 rather than RCX
+ */
+#define REGS_SYSCALL_NR PT_INDEX(RAX)
+#define REGS_SYSCALL_ARG1 PT_INDEX(RDI)
+#define REGS_SYSCALL_ARG2 PT_INDEX(RSI)
+#define REGS_SYSCALL_ARG3 PT_INDEX(RDX)
+#define REGS_SYSCALL_ARG4 PT_INDEX(R10)
+#define REGS_SYSCALL_ARG5 PT_INDEX(R8)
+#define REGS_SYSCALL_ARG6 PT_INDEX(R9)
+
+#define REGS_IP_INDEX PT_INDEX(RIP)
+#define REGS_SP_INDEX PT_INDEX(RSP)
+
 #endif
 
 /*
diff --git a/arch/um/include/sysdep-x86_64/stub.h b/arch/um/include/sysdep-x86_64/stub.h
new file mode 100644
index 000000000000..6b5447ad590d
--- /dev/null
+++ b/arch/um/include/sysdep-x86_64/stub.h
@@ -0,0 +1,19 @@
+/*
+ * Copyright (C) 2004 Jeff Dike (jdike@addtoit.com)
+ * Licensed under the GPL
+ */
+
+#ifndef __SYSDEP_STUB_H
+#define __SYSDEP_STUB_H
+
+#include <asm/ptrace.h>
+#include <asm/unistd.h>
+#include <sysdep/ptrace_user.h>
+
+extern void stub_segv_handler(int sig);
+
+#define STUB_SYSCALL_RET PT_INDEX(RAX)
+#define STUB_MMAP_NR __NR_mmap
+#define MMAP_OFFSET(o) (o)
+
+#endif
diff --git a/arch/um/include/tlb.h b/arch/um/include/tlb.h
index da1097285b8c..c6f9628f39bf 100644
--- a/arch/um/include/tlb.h
+++ b/arch/um/include/tlb.h
@@ -37,31 +37,25 @@ struct host_vm_op {
 extern void mprotect_kernel_vm(int w);
 extern void force_flush_all(void);
 extern void fix_range_common(struct mm_struct *mm, unsigned long start_addr,
-                             unsigned long end_addr, int force, int data,
-                             void (*do_ops)(int, struct host_vm_op *, int));
+                             unsigned long end_addr, int force,
+                             void (*do_ops)(union mm_context *,
+                                            struct host_vm_op *, int));
 extern int flush_tlb_kernel_range_common(unsigned long start,
                                          unsigned long end);
 
 extern int add_mmap(unsigned long virt, unsigned long phys, unsigned long len,
                     int r, int w, int x, struct host_vm_op *ops, int index,
-                    int last_filled, int data,
-                    void (*do_ops)(int, struct host_vm_op *, int));
+                    int last_filled, union mm_context *mmu,
+                    void (*do_ops)(union mm_context *, struct host_vm_op *,
+                                   int));
 extern int add_munmap(unsigned long addr, unsigned long len,
                       struct host_vm_op *ops, int index, int last_filled,
-                      int data, void (*do_ops)(int, struct host_vm_op *, int));
+                      union mm_context *mmu,
+                      void (*do_ops)(union mm_context *, struct host_vm_op *,
+                                     int));
 extern int add_mprotect(unsigned long addr, unsigned long len, int r, int w,
                         int x, struct host_vm_op *ops, int index,
-                        int last_filled, int data,
-                        void (*do_ops)(int, struct host_vm_op *, int));
+                        int last_filled, union mm_context *mmu,
+                        void (*do_ops)(union mm_context *, struct host_vm_op *,
+                                       int));
 #endif
-
-/*
- * Overrides for Emacs so that we follow Linus's tabbing style.
- * Emacs will notice this stuff at the end of the file and automatically
- * adjust the settings for this buffer only.  This must remain at the end
- * of the file.
- * ---------------------------------------------------------------------------
- * Local variables:
- * c-file-style: "linux"
- * End:
- */