Re: [PATCH v2] mm/damon/sysfs: fix use-after-free in state_show()

[SeongJae Park <sj@kernel.org>]

+ Andrew

On Fri,  5 Sep 2025 13:10:46 +0300 Stanislav Fort <stanislav.fort@aisle.com> wrote:

> state_show() reads kdamond->damon_ctx without holding damon_sysfs_lock.
> This allows a use-after-free race:
> 
> CPU 0                         CPU 1
> -----                         -----
> state_show()                  damon_sysfs_turn_damon_on()
> ctx = kdamond->damon_ctx;     mutex_lock(&damon_sysfs_lock);
>                               damon_destroy_ctx(kdamond->damon_ctx);
>                               kdamond->damon_ctx = NULL;
>                               mutex_unlock(&damon_sysfs_lock);
> damon_is_running(ctx);        /* ctx is freed */
> mutex_lock(&ctx->kdamond_lock); /* UAF */
> 
> (The race can also occur with damon_sysfs_kdamonds_rm_dirs() and
> damon_sysfs_kdamond_release(), which free or replace the context under
> damon_sysfs_lock.)
> 
> Fix by taking damon_sysfs_lock before dereferencing the context,
> mirroring the locking used in pid_show().
> 
> The bug has existed since state_show() first accessed kdamond->damon_ctx.
> 
> Fixes: a61ea561c871 ("mm/damon/sysfs: link DAMON for virtual address spaces monitoring")
> Reported-by: Stanislav Fort <disclosure@aisle.com>
> Closes: N/A # non-publicly reported
> Signed-off-by: Stanislav Fort <disclosure@aisle.com>

Reviewed-by: SeongJae Park <sj@kernel.org>


Thanks,
SJ

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