CVE-2026-90400md: recheck spare changes before starting sync

Official description

In the Linux kernel, the following vulnerability has been resolved: md: recheck spare changes before starting sync remove_spares() and remove_and_add_spares() modify the array's rdev configuration. These operations are only safe after the array has been suspended. md_start_sync() checks whether spare configuration changes are needed before taking reconfig_mutex. However, the rdev state can change before the mutex is acquired, so the initial check can become stale. In that case, md_choose_sync_action() may remove or replace rdevs while normal I/O is still accessing them. The race can occur as follows: raid10d Worker Normal IO ____________ _______________________ ______________________ raid10_write_request() wait_blocked_dev() set Blocked set Faulty Skip Faulty rdev rrdev->nr_pending++ .repl_bio = bio removeable_rdev = false . array not suspended . lock mddev goto err_handle lock mddev (wait) . update sb . clear Blocked . . unlock mddev . lock mddev (acquires) remove_spares() removeable_rdev = true raid10_remove_disk() rdev = replacement replacement = NULL rdev_dec_pending(NULL) unlock mddev (NULL)->nr_pending-- In this case, rdev_dec_pending() is called with a NULL pointer, resulting in a NULL pointer dereference when attempting to decrement nr_pending. Fix this by suspending the array when spare configuration changes are needed, including for non-read-write arrays, and checking again after taking reconfig_mutex. If the array was not already suspended and a change is now needed, release the mutex, suspend the array, and reacquire the mutex before continuing.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/c3777d16bc3335c0ac4bdad0551c80d38c5d94cc
• https://git.kernel.org/stable/c/e5ac7ab78467b064f1da8b0f3042a63595fafcfd
• https://git.kernel.org/stable/c/81b39df5d701976cf20e52f33106c1fc1603b4cb
• https://git.kernel.org/stable/c/c7d34d17ea43ebc86b45d439ebb435e11ca44bca