Re: [PATCH v2 3/5] fs/resctrl: Fix use-after-free during unmount

[Babu Moger <babu.moger@amd.com>]

Hi Tony,


On 5/15/26 14:39, Tony Luck wrote:
> Sashiko reported[1] this issue:
> 
>    During unmount or failure teardown, resctrl_fs_teardown() calls
>    mon_put_kn_priv() (which frees all mon_data structures) followed
>    by rdtgroup_destroy_root() (which destroys kernfs nodes). However, the
>    RDT_DELETED flag is never set for rdtgroup_default.
> 
>    If a concurrent reader (e.g., rdtgroup_mondata_show()) invokes
>    rdtgroup_kn_lock_live(), it drops kernfs active protection and blocks on
>    rdtgroup_mutex. resctrl_fs_teardown() (holding the mutex) proceeds to free
>    the private data and destroy the nodes without waiting for the reader.
> 
>    When the mutex is released, the reader wakes up, observes that RDT_DELETED is
>    not set for the default group, and dereferences the already-freed of->kn->priv
>    pointer.
> 
> Set RDT_DELETED for the default group (if there are any tasks waiting).

It only mentions about the one fix.  It is better to add a line about 
fixing  "Avoid races from pending operations from a previous mount".

> 
> Fixes: 60cf5e101fd4 ("x86/intel_rdt: Add mkdir to resctrl file system")
> Signed-off-by: Tony Luck <tony.luck@intel.com>
> Link: https://sashiko.dev/#/patchset/20260508182143.14592-1-tony.luck%40intel.com?part=2 [1]
> ---
>   fs/resctrl/rdtgroup.c | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
> 
> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
> index 506b40dc9430..97d1a3648b9e 100644
> --- a/fs/resctrl/rdtgroup.c
> +++ b/fs/resctrl/rdtgroup.c
> @@ -593,6 +593,13 @@ static ssize_t rdtgroup_cpus_write(struct kernfs_open_file *of,
>    */
>   static void rdtgroup_remove(struct rdtgroup *rdtgrp)
>   {
> +	/*
> +	 * Groups created with mkdir() have an extra hold, that doesn't
> +	 * apply to the default group. It is stacically allocated, so

stacically -> statically

Thanks
Babu

> +	 * does not need to be freed.
> +	 */
> +	if (rdtgrp == &rdtgroup_default)
> +		return;
>   	kernfs_put(rdtgrp->kn);
>   	kfree(rdtgrp);
>   }
> @@ -2965,6 +2972,7 @@ static void resctrl_fs_teardown(void)
>   	mon_put_kn_priv();
>   	rdt_pseudo_lock_release();
>   	rdtgroup_default.mode = RDT_MODE_SHAREABLE;
> +	rdtgroup_default.flags = RDT_DELETED;
>   	closid_exit();
>   	schemata_list_destroy();
>   	rdtgroup_destroy_root();
> @@ -2990,6 +2998,12 @@ static int rdt_get_tree(struct fs_context *fc)
>   		goto out;
>   	}
>   
> +	/* Avoid races from pending operations from a previous mount */
> +	if (atomic_read(&rdtgroup_default.waitcount) != 0) {
> +		ret = -EBUSY;
> +		goto out;
> +	}
> +
>   	ret = setup_rmid_lru_list();
>   	if (ret)
>   		goto out;
> @@ -4265,6 +4279,7 @@ static int rdtgroup_setup_root(struct rdt_fs_context *ctx)
>   
>   	ctx->kfc.root = rdt_root;
>   	rdtgroup_default.kn = kernfs_root_to_node(rdt_root);
> +	rdtgroup_default.flags = 0;
>   
>   	return 0;
>   }