netdevsim: fib: fix use-after-free of FIB data via debugfs

Writing to the netdevsim debugfs file
"netdevsim/netdevsimN/fib/nexthop_bucket_activity" enters
nsim_nexthop_bucket_activity_write(), which looks up a nexthop in
data->nexthop_ht under rtnl_lock(). If a network namespace teardown,
devlink reload or device deletion runs concurrently, nsim_fib_destroy()
frees that rhashtable (and the surrounding nsim_fib_data) while the
write is still in flight, leading to a slab-use-after-free:

  BUG: KASAN: slab-use-after-free in nsim_nexthop_bucket_activity_write+0xb9e/0xdf0
  Read of size 4 at addr ff1100001a379808 by task syz.0.11967/27894

  CPU: 0 UID: 0 PID: 27894 Comm: syz.0.11967 Not tainted 7.1.0-rc4-gf6f1bfc1980a #4
  Call Trace:
   nsim_nexthop_bucket_activity_write+0xb9e/0xdf0
   full_proxy_write+0x135/0x1a0
   vfs_write+0x2e2/0x1040
   ksys_write+0x146/0x270
   __x64_sys_write+0x76/0xb0
   do_syscall_64+0xb9/0x5b0
   entry_SYSCALL_64_after_hwframe+0x74/0x7c

  Allocated by task 15957:
   rhashtable_init_noprof+0x3ec/0x860
   nsim_fib_create+0x371/0xca0
   nsim_drv_probe+0xd60/0x15c0
   ...
   new_device_store+0x425/0x7f0

  Freed by task 24:
   rhashtable_free_and_destroy+0x10d/0x620
   nsim_fib_destroy+0xc9/0x1c0
   nsim_dev_reload_destroy+0x1e7/0x530
   nsim_dev_reload_down+0x6b/0xd0
   devlink_reload+0x1b5/0x770
   devlink_pernet_pre_exit+0x25d/0x3a0
   ops_undo_list+0x1b7/0xb90
   cleanup_net+0x47f/0x8a0

  The buggy address belongs to the object at ff1100001a379800
   which belongs to the cache kmalloc-1k of size 1024

The freed 1k object is the bucket table of data->nexthop_ht. Shortly
after, the dangling table is dereferenced again and the machine also
takes a GPF in __rht_bucket_nested() from the same call site.

The root cause is a lifetime mismatch: the debugfs files reference
nsim_fib_data (the writer dereferences data->nexthop_ht), but the
interface is not bracketed around the lifetime of that data.
nsim_fib_destroy() freed both rhashtables and only removed the debugfs
directory afterwards, and nsim_fib_create() created the debugfs files
before the rhashtables were initialized and, on the error path, freed
them before removing the files. debugfs keeps the file itself alive
across a ->write() via debugfs_file_get()/debugfs_file_put()
(fs/debugfs/file.c), but it does not keep data->nexthop_ht alive, so the
in-flight writer dereferenced freed memory. rtnl_lock() in the writer
does not help, because the teardown path does not take rtnl around
rhashtable_free_and_destroy().

Fix it by bracketing the debugfs interface around the data it exposes,
keeping nsim_fib_create() and nsim_fib_destroy() symmetric:

 - In nsim_fib_destroy(), tear down the debugfs files before the data
   structures they reference. debugfs_remove_recursive() drops the
   initial active-user reference and then waits for every in-flight
   ->write() to drop its reference before returning, and rejects new
   opens (__debugfs_file_removed(), fs/debugfs/inode.c). Once it returns,
   no debugfs accessor can reach the FIB data, so the rhashtables and
   nsim_fib_data can be destroyed safely. This also covers the bool knobs
   in the same directory, which store pointers into the same
   nsim_fib_data, and the final kfree(data).

 - In nsim_fib_create(), create the debugfs files after the rhashtables
   and notifiers are set up. This closes the same race on the
   error-unwind path, where a concurrent writer could otherwise observe a
   half-constructed instance or a table that the unwind has already
   freed. (With only the destroy-side change, a writer racing the create
   window instead dereferences an uninitialized data->nexthop_ht.)

This is reproducible by racing, in a loop, writes to
/sys/kernel/debug/netdevsim/netdevsimN/fib/nexthop_bucket_activity
against a teardown of the same netdevsim instance -- a devlink reload
("devlink dev reload netdevsim/netdevsimN"), destroying the network
namespace it lives in, or "echo N > /sys/bus/netdevsim/del_device". It
was found with syzkaller; a syzkaller reproducer is available. A
standalone C reproducer does not trigger it reliably because the race
needs the netns-teardown/reload path.

Cc: <stable+noautosel@kernel.org> # netdevsim is a test harness, it's never loaded on production systems
Signed-off-by: Zijing Yin <yzjaurora@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260529135718.1804031-1-yzjaurora@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions