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Message-ID: <5d1ce4a1-aa46-49e5-9890-3551b9223551@lucifer.local>
Date: Mon, 11 Aug 2025 15:12:44 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
Linus Torvalds <torvalds@...uxfoundation.org>,
Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
Arnaldo Carvalho de Melo <acme@...hat.com>,
Kees Cook <kees@...nel.org>
Subject: Re: [patch V2 RESEND 6/6] perf/core: Convert mmap() refcounts to
refcount_t
On Mon, Aug 11, 2025 at 02:36:39PM +0200, Thomas Gleixner wrote:
> The recently fixed reference count leaks could have been detected by using
> refcount_t and refcount_t would have mitigated the potential overflow at
> least.
>
> Now that the code is properly structured, convert the mmap() related
> mmap_count variants over to refcount_t.
>
> No functional change intended.
>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
OK, this LGTM on basis that the _inc() -> _set() change mentioned is all good,
so:
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> ---
> include/linux/perf_event.h | 2 +-
> kernel/events/core.c | 40 ++++++++++++++++++++--------------------
> kernel/events/internal.h | 4 ++--
> kernel/events/ring_buffer.c | 2 +-
> 4 files changed, 24 insertions(+), 24 deletions(-)
>
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -859,7 +859,7 @@ struct perf_event {
>
> /* mmap bits */
> struct mutex mmap_mutex;
> - atomic_t mmap_count;
> + refcount_t mmap_count;
>
> struct perf_buffer *rb;
> struct list_head rb_entry;
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3968,7 +3968,7 @@ static noinline int visit_groups_merge(s
> */
> static inline bool event_update_userpage(struct perf_event *event)
> {
> - if (likely(!atomic_read(&event->mmap_count)))
> + if (likely(!refcount_read(&event->mmap_count)))
> return false;
>
> perf_event_update_time(event);
> @@ -6704,11 +6704,11 @@ static void perf_mmap_open(struct vm_are
> struct perf_event *event = vma->vm_file->private_data;
> mapped_f mapped = get_mapped(event, event_mapped);
>
> - atomic_inc(&event->mmap_count);
> - atomic_inc(&event->rb->mmap_count);
> + refcount_inc(&event->mmap_count);
> + refcount_inc(&event->rb->mmap_count);
>
> if (vma->vm_pgoff)
> - atomic_inc(&event->rb->aux_mmap_count);
> + refcount_inc(&event->rb->aux_mmap_count);
>
> if (mapped)
> mapped(event, vma->vm_mm);
> @@ -6743,7 +6743,7 @@ static void perf_mmap_close(struct vm_ar
> * to avoid complications.
> */
> if (rb_has_aux(rb) && vma->vm_pgoff == rb->aux_pgoff &&
> - atomic_dec_and_mutex_lock(&rb->aux_mmap_count, &rb->aux_mutex)) {
> + refcount_dec_and_mutex_lock(&rb->aux_mmap_count, &rb->aux_mutex)) {
> /*
> * Stop all AUX events that are writing to this buffer,
> * so that we can free its AUX pages and corresponding PMU
> @@ -6763,10 +6763,10 @@ static void perf_mmap_close(struct vm_ar
> mutex_unlock(&rb->aux_mutex);
> }
>
> - if (atomic_dec_and_test(&rb->mmap_count))
> + if (refcount_dec_and_test(&rb->mmap_count))
> detach_rest = true;
>
> - if (!atomic_dec_and_mutex_lock(&event->mmap_count, &event->mmap_mutex))
> + if (!refcount_dec_and_mutex_lock(&event->mmap_count, &event->mmap_mutex))
> goto out_put;
>
> ring_buffer_attach(event, NULL);
> @@ -6991,17 +6991,17 @@ static int perf_mmap_rb(struct vm_area_s
> if (data_page_nr(rb) != nr_pages)
> return -EINVAL;
>
> - if (atomic_inc_not_zero(&event->rb->mmap_count)) {
> + if (refcount_inc_not_zero(&event->rb->mmap_count)) {
> /*
> * Success -- managed to mmap() the same buffer
> * multiple times.
> */
> - atomic_inc(&event->mmap_count);
> + refcount_inc(&event->mmap_count);
> return 0;
> }
> /*
> * Raced against perf_mmap_close()'s
> - * atomic_dec_and_mutex_lock() remove the event and
> + * refcount_dec_and_mutex_lock() remove the event and
> * continue as if !event->rb
> */
> ring_buffer_attach(event, NULL);
> @@ -7019,7 +7019,7 @@ static int perf_mmap_rb(struct vm_area_s
> if (!rb)
> return -ENOMEM;
>
> - atomic_set(&rb->mmap_count, 1);
> + refcount_set(&rb->mmap_count, 1);
> rb->mmap_user = get_current_user();
> rb->mmap_locked = extra;
>
> @@ -7030,7 +7030,7 @@ static int perf_mmap_rb(struct vm_area_s
> perf_event_update_userpage(event);
>
> perf_mmap_account(vma, user_extra, extra);
> - atomic_set(&event->mmap_count, 1);
> + refcount_set(&event->mmap_count, 1);
This is the really pertinent change - with this now a _set (asked about on 5/6
just to confirm this is all good) - the refcount doesn't think there's a UAF and
we're all good.
> return 0;
> }
>
> @@ -7071,17 +7071,17 @@ static int perf_mmap_aux(struct vm_area_
> return -EINVAL;
>
> /* If this succeeds, subsequent failures have to undo it */
> - if (!atomic_inc_not_zero(&rb->mmap_count))
> + if (!refcount_inc_not_zero(&rb->mmap_count))
> return -EINVAL;
>
> /* If mapped, attach to it */
> if (rb_has_aux(rb)) {
> - atomic_inc(&rb->aux_mmap_count);
> + refcount_inc(&rb->aux_mmap_count);
> return 0;
> }
>
> if (!perf_mmap_calc_limits(vma, &user_extra, &extra)) {
> - atomic_dec(&rb->mmap_count);
> + refcount_dec(&rb->mmap_count);
> return -EPERM;
> }
>
> @@ -7091,14 +7091,14 @@ static int perf_mmap_aux(struct vm_area_
> ret = rb_alloc_aux(rb, event, vma->vm_pgoff, nr_pages,
> event->attr.aux_watermark, rb_flags);
> if (ret) {
> - atomic_dec(&rb->mmap_count);
> + refcount_dec(&rb->mmap_count);
> return ret;
> }
>
> - atomic_set(&rb->aux_mmap_count, 1);
> + refcount_set(&rb->aux_mmap_count, 1);
> rb->aux_mmap_locked = extra;
> perf_mmap_account(vma, user_extra, extra);
> - atomic_inc(&event->mmap_count);
> + refcount_inc(&event->mmap_count);
> return 0;
> }
>
> @@ -13247,7 +13247,7 @@ perf_event_set_output(struct perf_event
> mutex_lock_double(&event->mmap_mutex, &output_event->mmap_mutex);
> set:
> /* Can't redirect output if we've got an active mmap() */
> - if (atomic_read(&event->mmap_count))
> + if (refcount_read(&event->mmap_count))
> goto unlock;
>
> if (output_event) {
> @@ -13260,7 +13260,7 @@ perf_event_set_output(struct perf_event
> goto unlock;
>
> /* did we race against perf_mmap_close() */
> - if (!atomic_read(&rb->mmap_count)) {
> + if (!refcount_read(&rb->mmap_count)) {
> ring_buffer_put(rb);
> goto unlock;
> }
> --- a/kernel/events/internal.h
> +++ b/kernel/events/internal.h
> @@ -35,7 +35,7 @@ struct perf_buffer {
> spinlock_t event_lock;
> struct list_head event_list;
>
> - atomic_t mmap_count;
> + refcount_t mmap_count;
> unsigned long mmap_locked;
> struct user_struct *mmap_user;
>
> @@ -47,7 +47,7 @@ struct perf_buffer {
> unsigned long aux_pgoff;
> int aux_nr_pages;
> int aux_overwrite;
> - atomic_t aux_mmap_count;
> + refcount_t aux_mmap_count;
> unsigned long aux_mmap_locked;
> void (*free_aux)(void *);
> refcount_t aux_refcount;
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -400,7 +400,7 @@ void *perf_aux_output_begin(struct perf_
> * the same order, see perf_mmap_close. Otherwise we end up freeing
> * aux pages in this path, which is a bug, because in_atomic().
> */
> - if (!atomic_read(&rb->aux_mmap_count))
> + if (!refcount_read(&rb->aux_mmap_count))
> goto err;
>
> if (!refcount_inc_not_zero(&rb->aux_refcount))
>
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