binfmt_misc: correctly account pre-opened interpreters

An 'F' entry, and every interpreter a 'B' entry binds, holds a file open
from registration until the entry goes away, pinning the file, its inode,
the mount it came from and that mount's superblock. Nothing bounds how
many of those a user namespace can hold. An entry binds at most
BINFMT_MISC_INTERP_MAX interpreters, but nothing caps the entries.

Charge each binding to the user namespace and uid that makes it against a
new UCOUNT_BINFMT_MISC_INTERPRETERS. Going over budget causes -ENOSPC.

A per-instance cap would suck. Instances are keyed on the user
namespace. So any constant is multiplied by the number of namespaces the
caller creates. Creating those is virtually free. A ucount charges the
namespace and every one of its ancestors. And a namespace can raise only
its own limit. So nesting buys nothing.

The knob is /proc/sys/user/max_binfmt_misc_interpreters. Leave it at the
max_threads/2 default fork_init() gives a new type. No existing
configuration comes close to that.

binfmt_misc is tristate, which makes it the first ucount user that can be
built as a module. Export inc_ucount() and dec_ucount(); without them
CONFIG_BINFMT_MISC=m fails to link. Export them to binfmt_misc alone:
charging a ucount type is not something a module has any business doing
in general, and the list is trivial to extend if a second user shows up.
init_user_ns and init_binfmt_misc are already exported for the same
module.

Link: https://patch.msgid.link/20260803-work-binfmt_misc-interplimit-v1-1-4a2435500bd9@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/binfmt_misc.h b/include/linux/binfmt_misc.h
index 072e4b3dd78d..8045b10dd3e5 100644
--- a/include/linux/binfmt_misc.h
+++ b/include/linux/binfmt_misc.h
@@ -7,6 +7,7 @@
 struct bpf_prog;
 struct file;
 struct linux_binprm;
+struct ucounts;
 struct user_namespace;
 
 #define BINFMT_MISC_OPS_NAME_MAX 16
@@ -21,6 +22,7 @@ struct user_namespace;
  * struct binfmt_misc_interp - an interpreter an entry was registered with
  * @list: link in the entry's list, in registration order
  * @file: the file, opened at registration and never resolved again
+ * @ucounts: the UCOUNT_BINFMT_MISC_INTERPRETERS charge the binding took
  * @path: the path it was registered under, used as the name the interpreter
  *        runs under; stored after @name in the same allocation
  * @name: the name the load program selects it by; empty for the fixed
@@ -33,6 +35,7 @@ struct user_namespace;
 struct binfmt_misc_interp {
 	struct list_head	list;
 	struct file		*file;
+	struct ucounts		*ucounts;
 	const char		*path;
 	char			name[];
 };