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net: Ensure net namespace isolation of sysctls 

This adds an ensure_safe_net_sysctl() check during register_net_sysctl()
to validate that sysctl table entries for a non-init_net netns are
sufficiently isolated. To be netns-safe, an entry must adhere to at
least (and usually exactly) one of these rules:

1. It is marked read-only inside the netns.
2. Its data pointer does not point to kernel/module global data.

An entry which fails both of these checks is indicative of a bug,
whereby a child netns can affect global net sysctl values.

If such an entry is found, this code will issue a warning to the kernel
log, and force the entry to be read-only to prevent a leak.

To test, simply create a new netns:

    $ sudo ip netns add dummy

As it sits now, this patch will WARN for two sysctls which will be
addressed in a subsequent patch:
- /proc/sys/net/netfilter/nf_conntrack_max
- /proc/sys/net/netfilter/nf_conntrack_expect_max

Signed-off-by: Jonathon Reinhart <Jonathon.Reinhart@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Jonathon Reinhart 2021-04-12 00:24:52 -04:00 committed by David S. Miller
parent a115d24a63
commit 31c4d2f160
1 changed files with 48 additions and 0 deletions
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48
net/sysctl_net.c
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@ -115,9 +115,57 @@ out1:
goto out;
}
/* Verify that sysctls for non-init netns are safe by either:
* 1) being read-only, or
* 2) having a data pointer which points outside of the global kernel/module
* data segment, and rather into the heap where a per-net object was
* allocated.
*/
static void ensure_safe_net_sysctl(struct net *net, const char *path,
struct ctl_table *table)
{
struct ctl_table *ent;
pr_debug("Registering net sysctl (net %p): %s\n", net, path);
for (ent = table; ent->procname; ent++) {
unsigned long addr;
const char *where;
pr_debug(" procname=%s mode=%o proc_handler=%ps data=%p\n",
ent->procname, ent->mode, ent->proc_handler, ent->data);
/* If it's not writable inside the netns, then it can't hurt. */
if ((ent->mode & 0222) == 0) {
pr_debug(" Not writable by anyone\n");
continue;
}
/* Where does data point? */
addr = (unsigned long)ent->data;
if (is_module_address(addr))
where = "module";
else if (core_kernel_data(addr))
where = "kernel";
else
continue;
/* If it is writable and points to kernel/module global
* data, then it's probably a netns leak.
*/
WARN(1, "sysctl %s/%s: data points to %s global data: %ps\n",
path, ent->procname, where, ent->data);
/* Make it "safe" by dropping writable perms */
ent->mode &= ~0222;
}
}
struct ctl_table_header *register_net_sysctl(struct net *net,
const char *path, struct ctl_table *table)
{
if (!net_eq(net, &init_net))
ensure_safe_net_sysctl(net, path, table);
return __register_sysctl_table(&net->sysctls, path, table);
}
EXPORT_SYMBOL_GPL(register_net_sysctl);