Re: [PATCH module v2] module: Fix uninitialized count read of unloaded taint tracking record

[Petr Pavlu <petr.pavlu@suse.com>]

On 10/10/26 10:03 AM, Binbin Deng wrote:
> try_add_tainted_module() allocates the tracking record without zeroing
> it and initializes the fields in this order:
> 
> 	mod_taint = kmalloc_obj(*mod_taint);
> 	strscpy(mod_taint->name, mod->name, MODULE_NAME_LEN);
> 	mod_taint->taints = mod->taints;
> 	list_add_rcu(&mod_taint->list, &unloaded_tainted_modules);
> 	mod_taint->count = 1;
> 
> The record is published by list_add_rcu() before the count field is
> written.  The readers, unloaded_tainted_modules_seq_show() (debugfs,
> under rcu_read_lock()) and print_unloaded_tainted_modules() (panic
> path, under guard(rcu)), traverse the list under RCU and read
> mod_taint->count.  module_mutex, which the writer holds, does not
> exclude RCU readers.
> 
> A reader that observes the new node between the list_add_rcu() and
> the count store reads an uninitialized u64 left over from previous
> slab use.  This is a data race and leaks kernel heap residue through
> the debugfs file and the panic message.
> 
> The race window is two adjacent instructions; it is reachable when a
> tainted module is unloaded (rmmod) concurrently with a read of
> /sys/kernel/debug/module/unloaded_tainted, or with the module list
> printout of an oops on another CPU.
> 
> Fix this by initializing count before the record is published.
> 
> Found during a static-analysis-assisted audit: candidate sites were
> surfaced by a Coccinelle rule, and the reader/writer paths were
> confirmed by LLM-assisted (GLM-5.3) code inspection.  The issue was
> not observed at runtime; it is identified by code inspection only.
> 
> Fixes: 99bd9956551b ("module: Introduce module unload taint tracking")
> Signed-off-by: Binbin Deng <18983559317@163.com>

Looks ok, I've queued the patch on modules-next for v7.4-rc1.

-- 
Thanks,
Petr