Re: [RFC][PATCH 1/4] checkpoint-restart: general infrastructure

From: Oren Laadan
Date: Fri Aug 08 2008 - 17:02:17 EST



Dave Hansen wrote:
On Fri, 2008-08-08 at 11:46 +0200, Arnd Bergmann wrote:
On Friday 08 August 2008, Dave Hansen wrote:
+ hh->magic = 0x00a2d200;
+ hh->major = (LINUX_VERSION_CODE >> 16) & 0xff;
+ hh->minor = (LINUX_VERSION_CODE >> 8) & 0xff;
+ hh->patch = (LINUX_VERSION_CODE) & 0xff;
...
+}
Do you rely on the kernel version in order to determine the format
of the binary data, or is it just informational?

If you think the format can change in incompatible ways, you
probably need something more specific than the version number
these days, because there are just so many different trees with
the same numbers.

Yeah, this is very true. My guess is that we'll need something like
what we do with modversions.

Exactly. The header should eventually contain sufficient information
to describe the kernel version, configuration, compiler, cpu (arch and
capabilities), and checkpoint code version.

How would you suggest to identify the origin tree with an identifier
(or a text field) in the header ?


+/* debugging */
+#if 0
+#define CR_PRINTK(str, args...) \
+ printk(KERN_ERR "cr@%s#%d: " str, __func__, __LINE__, ##args)
+#else
+#define CR_PRINTK(...) do {} while (0)
+#endif
+
Please use the existing pr_debug and dev_debug here, instead of creating
yet another version.

Sure thing. Will do.

+struct cr_hdr_tail {
+ __u32 magic;
+ __u32 cksum[2];
+};
This structure has an odd multiple of 32-bit members, which means
that if you put it into a larger structure that also contains
64-bit members, the larger structure may get different alignment
on x86-32 and x86-64, which you might want to avoid.
I can't tell if this is an actual problem here.

Can't we just declare all these things __packed__ and stop worrying
about aligning them all manually?

+
+/*
+ * During restart the code reads in data from the chekcpoint image into a
+ * temporary buffer (ctx->hbuf). Because operations can be nested, one
+ * should call cr_hbuf_get() to reserve space in the buffer, and then
+ * cr_hbuf_put() when it no longer needs that space
+ */
+
+#include <linux/version.h>
+#include <linux/sched.h>
+#include <linux/file.h>
+
+#include "ckpt.h"
+#include "ckpt_hdr.h"
+
+/**
+ * cr_hbuf_get - reserve space on the hbuf
+ * @ctx: checkpoint context
+ * @n: number of bytes to reserve
+ */
+void *cr_hbuf_get(struct cr_ctx *ctx, int n)
+{
+ void *ptr;
+
+ BUG_ON(ctx->hpos + n > CR_HBUF_TOTAL);
+ ptr = (void *) (((char *) ctx->hbuf) + ctx->hpos);
+ ctx->hpos += n;
+ return ptr;
+}
Can (ctx->hpos + n > CR_HBUF_TOTAL) be controlled by the input
data? If so, this is a denial-of-service attack.

Ugh, this is crappy code anyway. It needs to return an error and have
someone else handle it.

Actually not quite. 'n' is _not_ controlled by the input data, and at
the same time ctx->hpos should always carry enough room by design. If
that is not the case, then it's a logical bug, not DoS attack.

To avoid repetitive malloc/free, ctx->hbuf is a buffer to host headers
as they are read; since headers can be read in a nested manner, ctx->hpos
points to the next free position in that buffer. So 'n' is the size of
the header that we are about to read - decided at compile time, not the
user input. The BUG_ON() statement asserts that by design we have enough
buffer (like you'd check that you didn't run out of kernel stack...)

If it is preferred, we can change this to write a kernel message and
return a special error telling that a logical error has occurred.


+int cr_kwrite(struct cr_ctx *ctx, void *buf, int count)
+{
+ mm_segment_t oldfs;
+ int ret;
+
+ oldfs = get_fs();
+ set_fs(KERNEL_DS);
+ ret = cr_uwrite(ctx, buf, count);
+ set_fs(oldfs);
+
+ return ret;
+}
get_fs()/set_fs() always feels a bit ouch, and this way you have
to use __force to avoid the warnings about __user pointer casts
in sparse.
I wonder if you can use splice_read/splice_write to get around
this problem.

I have to wonder if this is just a symptom of us trying to do this the
wrong way. We're trying to talk the kernel into writing internal gunk
into a FD. You're right, it is like a splice where one end of the pipe
is in the kernel.

Any thoughts on a better way to do this?

+ struct cr_ctx *ctx;
+ struct file *file;
+ int fput_needed;
+ int ret;
+
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
+
Why do you need CAP_SYS_ADMIN for this? Can't regular users
be allowed to checkpoint/restart their own tasks?

Yes, eventually. I think one good point is that we should probably
remove this now so that we *have* to think about security implications
as we add each individual patch. For instance, what kind of checking do
we do when we restore an mlock()'d VMA?

I'll pull this check out so it causes pain. (the good kind)

Hmmm... even if not strictly now, we *will* need admin privileges for
the CR operations, for the following reasons:

checkpoint: we save the entire state of a set of processes to a file - so
we must have privileges to do so, at least within (or with respect to) the
said container. Even if we are the user who owns the container, we'll need
root access within that container.

restart: we restore the entire set of a set of processes, which may require
some privileged operations (again, at least within or with respect to the
said container). Otherwise any user could inject any restart data into the
kernel and create any set of processes with arbitrary permissions.

In a sense, we need something similar to granting ptrace access.


--- linux-2.6.git/Makefile~handle_a_single_task_with_private_memory_maps 2008-08-05 09:04:27.000000000 -0700
+++ linux-2.6.git-dave/Makefile 2008-08-05 09:07:53.000000000 -0700
@@ -611,7 +611,7 @@ export mod_strip_cmd
ifeq ($(KBUILD_EXTMOD),)
-core-y += kernel/ mm/ fs/ ipc/ security/ crypto/ block/
+core-y += kernel/ mm/ fs/ ipc/ security/ crypto/ block/ ckpt/
The name 'ckpt' is a bit unobvious, how about naming it 'checkpoint' instead?

Fine with me. Renamed in new patches, hopefully.

I'll send new patches out later today.

-- Dave

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