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Joel Granados says:
====================
net: sysctl: Const Qualify sysctl ctl_table arrays
What?
=====
We do two things:
1. Reject netns-unsafe: Replace warning and file permission change with
an error (reject registration) when an "unsafe" net sysctl
registration is detected.
2. Const qualify: Const qualify network templated ctl_table arrays and
unconditional kmemdup'ed ctl_table arrays.
Why?
====
The main motivation for this is to continue with the const qualification
of the ctl_table arrays [1]. The permission change inside
ensure_safe_net_sysctl disallows cons qualifiaction as it basically
modifies the entries before running the sysctl registration.
ent->mode &= ~0222;
On reject netns-unsafe?
=======================
* I believe that there is currently now way that the permission change
gets executed [2]
* I found one case where the warning message was posted to lore
(vsock_sysctl_register) [3], but it made its to mainline as part of
the second case in [2].
* We should error anyway because writing to the global sysctl value
through a child netns is indicative of a bug [4].
On Const qualification?
=======================
We can separate the places where network registers sysctl tables into
three groups:
1. Static global: The unchanged global static arrays are passed along to
sysctl register.
2. Always kmemdup: The global static arrays are always kmemdup'ed before
passing them along to sysctl register.
3. Dynamic global: The global static array is changed in place before
passing it along to sysctl register.
This series handles case 1 and 2. It leaves 3 for a later point as
const qualifying those global ctl_tables is more involved.
I would be very thankful if you point me to anything that I have missed
in my analysis that shows that this cannot/shouldn't be done.
[1]
https://git.kernel.org/pub/scm/linux/kernel/git/sysctl/sysctl.git/commit/?h=constfy-sysctl-6.14-rc1&id=1751f872cc97f992ed5c4c72c55588db1f0021e1
[2]
I have identified 4 contexts relevant to the ensure_safe_net_sysctl call
inside the network sysctl registration.
1. When the (struct net) == &init_net (like in iw_cm_init): In this case
ensure_safe_net_sysctl is not executed and permission modification
never happens.
2. When the ctl_table data (->data) gets "manually" assigned to
something other init_net (like in vsock_sysctl_register): In this
case ensure_safe_net_sysctl *is* executed but the data that is passed
is neither a module address (!is_module_address) nor a kernel core
address (!is_kernel_core_data); so the permission modification never
happens.
3. When the permissions are explicitly changed on a kmemdup'ed ctl_table
array (like in sysctl_core_net_init): in this case
ensure_safe_net_sysctl *is* executed but the permission modification
never happens as the mode is not writable.
4. When ctl have custom proc_handlers (like in nf_lwtunnel_net_init): In
this case ->data is NULL so it is not a module address
(!is_module_address) nor a kernel core address
(!is_kernel_core_data), so permission modification never happens.
It seems like there is no way of executing the permission change in
ensure_safe_net_sysctl. Please correct me if this is inaccurate and help
me find the case that I missed.
[3]
https://lore.kernel.org/all/20260302194926.90378-1-graf@amazon.com/
[4]
The ensure_safe_net_sysctl function was introduced in Commit:
31c4d2f160 ("net: Ensure net namespace
isolation of sysctls") which states that it is trying to prevent a
leak (indicative of a bug).
[5]
https://patchwork.kernel.org/project/netdevbpf/patch/20260713-jag-net_const_qualify-v3-1-7289fe9eaea6@kernel.org/
====================
Link: https://patch.msgid.link/20260810-jag-net_const_qualify-v4-0-77e888237c69@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
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