Re: [RFC] syscalls, x86: Add __NR_kcmp syscall

From: Cyrill Gorcunov
Date: Tue Jan 17 2012 - 16:15:25 EST

On Tue, Jan 17, 2012 at 10:47:37AM -0800, H. Peter Anvin wrote:
> On 01/17/2012 06:44 AM, Cyrill Gorcunov wrote:
> > On Tue, Jan 17, 2012 at 04:38:14PM +0200, Alexey Dobriyan wrote:
> >> On 1/17/12, Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
> >>> +#define KCMP_EQ 0
> >>> +#define KCMP_LT 1
> >>> +#define KCMP_GT 2
> >>
> >> LT and GT are meaningless.
> >>
> >
> > I found symbolic names better than open-coded values. But sure,
> > if this is problem it could be dropped.
> >
> > Or you mean that in general anything but 'equal' is useless?
> >
>
> Why on Earth would user space need to know which order in memory certain
> kernel objects are?
>
> Keep in mind that this is *exactly* the kind of information which makes
> rootkits easier.
>

Hmm, indeed this might help narrow down the target address I fear. So
after some conversation with Pavel I think we can try to live with just
one result -- is objects are at same location in kernel memory or not.
The updated version is below. Please review if you get a chance. Thanks
a lot for comments!

Cyrill
---
From: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
Subject: [RFC] syscalls, x86: Add __NR_kcmp syscall v2

While doing the checkpoint-restore in the userspace one need to determine
whether various kernel objects (like mm_struct-s of file_struct-s) are shared
between tasks and restore this state.

The 2nd step can be solved by using appropriate CLONE_ flags and the unshare
syscall, while there's currently no ways for solving the 1st one.

One of the ways for checking whether two tasks share e.g. mm_struct is to
provide some mm_struct ID of a task to its proc file, but showing such
info considered to be not that good for security reasons.

Thus after some debates we end up in conclusion that using that named
'comparision' syscall might be the best candidate. So here is it --
__NR_kcmp.

It takes up to 5 agruments - the pids of the two tasks (which
characteristics should be compared), the comparision type and
(in case of comparision of files) two file descriptors.

At moment only x86 is supported.

v2: The result of syscall is restricted to only two variants:
either objects match and lay at same memory address (0),
either not and (1) is returned.

Signed-off-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
CC: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
CC: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
CC: Andrey Vagin <avagin@xxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxx>
CC: H. Peter Anvin <hpa@xxxxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Glauber Costa <glommer@xxxxxxxxxxxxx>
CC: Andi Kleen <andi@xxxxxxxxxxxxxx>
CC: Tejun Heo <tj@xxxxxxxxxx>
CC: Matt Helsley <matthltc@xxxxxxxxxx>
CC: Pekka Enberg <penberg@xxxxxxxxxx>
CC: Eric Dumazet <eric.dumazet@xxxxxxxxx>
CC: Vasiliy Kulikov <segoon@xxxxxxxxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: Alexey Dobriyan <adobriyan@xxxxxxxxx>
CC: Valdis.Kletnieks@xxxxxx
---
arch/x86/include/asm/kcmp.h | 20 ++++++
arch/x86/include/asm/syscalls.h | 4 +
arch/x86/include/asm/unistd_32.h | 1
arch/x86/include/asm/unistd_64.h | 2
arch/x86/kernel/Makefile | 1
arch/x86/kernel/kcmp.c | 121 +++++++++++++++++++++++++++++++++++++
arch/x86/kernel/syscall_table_32.S | 1
7 files changed, 150 insertions(+)

Index: linux-2.6.git/arch/x86/include/asm/kcmp.h
===================================================================
--- /dev/null
+++ linux-2.6.git/arch/x86/include/asm/kcmp.h
@@ -0,0 +1,20 @@
+#ifndef _LINUX_KCMP_H
+#define _LINUX_KCMP_H
+
+/* Comparision type */
+enum {
+ KCMP_FILE,
+ KCMP_VM,
+ KCMP_FILES,
+ KCMP_FS,
+ KCMP_SIGHAND,
+ KCMP_IO,
+ KCMP_SYSVSEM,
+
+ KCMP_TYPES,
+};
+
+#define KCMP_EQ 0
+#define KCMP_NE 1
+
+#endif /* _LINUX_KCMP_H */
Index: linux-2.6.git/arch/x86/include/asm/syscalls.h
===================================================================
--- linux-2.6.git.orig/arch/x86/include/asm/syscalls.h
+++ linux-2.6.git/arch/x86/include/asm/syscalls.h
@@ -42,6 +42,10 @@ long sys_sigaltstack(const stack_t __use
asmlinkage int sys_set_thread_area(struct user_desc __user *);
asmlinkage int sys_get_thread_area(struct user_desc __user *);

+/* kenrel/kcmp.c */
+asmlinkage long sys_kcmp(pid_t pid1, pid_t pid2, int type,
+ unsigned long idx1, unsigned long idx2);
+
/* X86_32 only */
#ifdef CONFIG_X86_32

Index: linux-2.6.git/arch/x86/include/asm/unistd_32.h
===================================================================
--- linux-2.6.git.orig/arch/x86/include/asm/unistd_32.h
+++ linux-2.6.git/arch/x86/include/asm/unistd_32.h
@@ -354,6 +354,7 @@
#define __NR_setns 346
#define __NR_process_vm_readv 347
#define __NR_process_vm_writev 348
+#define __NR_kcmp 349

#ifdef __KERNEL__

Index: linux-2.6.git/arch/x86/include/asm/unistd_64.h
===================================================================
--- linux-2.6.git.orig/arch/x86/include/asm/unistd_64.h
+++ linux-2.6.git/arch/x86/include/asm/unistd_64.h
@@ -686,6 +686,8 @@ __SYSCALL(__NR_getcpu, sys_getcpu)
__SYSCALL(__NR_process_vm_readv, sys_process_vm_readv)
#define __NR_process_vm_writev 311
__SYSCALL(__NR_process_vm_writev, sys_process_vm_writev)
+#define __NR_kcmp 312
+__SYSCALL(__NR_kcmp, sys_kcmp)

#ifndef __NO_STUBS
#define __ARCH_WANT_OLD_READDIR
Index: linux-2.6.git/arch/x86/kernel/Makefile
===================================================================
--- linux-2.6.git.orig/arch/x86/kernel/Makefile
+++ linux-2.6.git/arch/x86/kernel/Makefile
@@ -33,6 +33,7 @@ obj-y += alternative.o i8253.o pci-nom
obj-y += tsc.o io_delay.o rtc.o
obj-y += pci-iommu_table.o
obj-y += resource.o
+obj-y += kcmp.o

obj-y += trampoline.o trampoline_$(BITS).o
obj-y += process.o
Index: linux-2.6.git/arch/x86/kernel/kcmp.c
===================================================================
--- /dev/null
+++ linux-2.6.git/arch/x86/kernel/kcmp.c
@@ -0,0 +1,121 @@
+#include <linux/kernel.h>
+#include <linux/syscalls.h>
+#include <linux/fdtable.h>
+#include <linux/err.h>
+
+#include <asm/unistd.h>
+#include <asm/kcmp.h>
+
+#define KCMP_PTR(ptr1, ptr2) \
+({ \
+ long ___r = (long)ptr1 - (long)ptr2; \
+ (___r == 0 ? KCMP_EQ : KCMP_NE); \
+})
+
+#define KCMP_TASK_PTR(task1, task2, member) \
+ KCMP_PTR((task1)->member, (task2)->member)
+
+/* A caller must be sure the task is presented in memory */
+static struct file *
+get_file_raw_ptr(struct task_struct *task, unsigned int idx)
+{
+ struct fdtable *fdt;
+ struct file *file;
+
+ spin_lock(&task->files->file_lock);
+ fdt = files_fdtable(task->files);
+ if (idx < fdt->max_fds)
+ file = fdt->fd[idx];
+ else
+ file = NULL;
+ spin_unlock(&task->files->file_lock);
+
+ return file;
+}
+
+SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
+ unsigned long, idx1, unsigned long, idx2)
+{
+ struct task_struct *task1;
+ struct task_struct *task2;
+ int ret = 0;
+
+ rcu_read_lock();
+
+ task1 = find_task_by_vpid(pid1);
+ if (!task1) {
+ rcu_read_unlock();
+ return -ESRCH;
+ }
+
+ task2 = find_task_by_vpid(pid2);
+ if (!task2) {
+ put_task_struct(task1);
+ rcu_read_unlock();
+ return -ESRCH;
+ }
+
+ get_task_struct(task1);
+ get_task_struct(task2);
+
+ rcu_read_unlock();
+
+ if (!ptrace_may_access(task1, PTRACE_MODE_READ) ||
+ !ptrace_may_access(task2, PTRACE_MODE_READ)) {
+ ret = -EACCES;
+ goto err;
+ }
+
+ /*
+ * Note for all cases but the KCMP_FILE we
+ * don't take any locks and do a plain pointer
+ * comparision in a sake of speed.
+ */
+
+ switch (type) {
+ case KCMP_FILE: {
+ struct file *filp1, *filp2;
+
+ filp1 = get_file_raw_ptr(task1, idx1);
+ filp2 = get_file_raw_ptr(task2, idx2);
+
+ if (filp1 && filp2)
+ ret = KCMP_PTR(filp1, filp2);
+ else
+ ret = -ENOENT;
+ break;
+ }
+ case KCMP_VM:
+ ret = KCMP_TASK_PTR(task1, task2, mm);
+ break;
+ case KCMP_FILES:
+ ret = KCMP_TASK_PTR(task1, task2, files);
+ break;
+ case KCMP_FS:
+ ret = KCMP_TASK_PTR(task1, task2, fs);
+ break;
+ case KCMP_SIGHAND:
+ ret = KCMP_TASK_PTR(task1, task2, sighand);
+ break;
+ case KCMP_IO:
+ ret = KCMP_TASK_PTR(task1, task2, io_context);
+ break;
+ case KCMP_SYSVSEM:
+#ifdef CONFIG_SYSVIPC
+ ret = KCMP_TASK_PTR(task1, task2, sysvsem.undo_list);
+#else
+ ret = -ENOENT;
+ goto err;
+#endif
+ break;
+ default:
+ ret = -EINVAL;
+ goto err;
+ }
+
+err:
+ put_task_struct(task1);
+ put_task_struct(task2);
+
+ return ret;
+}
Index: linux-2.6.git/arch/x86/kernel/syscall_table_32.S
===================================================================
--- linux-2.6.git.orig/arch/x86/kernel/syscall_table_32.S
+++ linux-2.6.git/arch/x86/kernel/syscall_table_32.S
@@ -348,3 +348,4 @@ ENTRY(sys_call_table)
.long sys_setns
.long sys_process_vm_readv
.long sys_process_vm_writev
+ .long sys_kcmp
--
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