From 77de62ad3de3967818c3dbe656b7336ebee461d2 Mon Sep 17 00:00:00 2001 From: Haocheng Yu Date: Tue, 3 Feb 2026 00:20:56 +0800 Subject: [PATCH] perf/core: Fix refcount bug and potential UAF in perf_mmap Syzkaller reported a refcount_t: addition on 0; use-after-free warning in perf_mmap. The issue is caused by a race condition between a failing mmap() setup and a concurrent mmap() on a dependent event (e.g., using output redirection). In perf_mmap(), the ring_buffer (rb) is allocated and assigned to event->rb with the mmap_mutex held. The mutex is then released to perform map_range(). If map_range() fails, perf_mmap_close() is called to clean up. However, since the mutex was dropped, another thread attaching to this event (via inherited events or output redirection) can acquire the mutex, observe the valid event->rb pointer, and attempt to increment its reference count. If the cleanup path has already dropped the reference count to zero, this results in a use-after-free or refcount saturation warning. Fix this by extending the scope of mmap_mutex to cover the map_range() call. This ensures that the ring buffer initialization and mapping (or cleanup on failure) happens atomically effectively, preventing other threads from accessing a half-initialized or dying ring buffer. Closes: https://lore.kernel.org/oe-kbuild-all/202602020208.m7KIjdzW-lkp@intel.com/ Reported-by: kernel test robot Signed-off-by: Haocheng Yu Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20260202162057.7237-1-yuhaocheng035@gmail.com --- kernel/events/core.c | 42 +++++++++++++++++++++--------------------- 1 file changed, 21 insertions(+), 21 deletions(-) diff --git a/kernel/events/core.c b/kernel/events/core.c index 4f86d22f281a..22a0f405585b 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -7465,29 +7465,29 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma) ret = perf_mmap_aux(vma, event, nr_pages); if (ret) return ret; + + /* + * Since pinned accounting is per vm we cannot allow fork() to copy our + * vma. + */ + vm_flags_set(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_DONTDUMP); + vma->vm_ops = &perf_mmap_vmops; + + mapped = get_mapped(event, event_mapped); + if (mapped) + mapped(event, vma->vm_mm); + + /* + * Try to map it into the page table. On fail, invoke + * perf_mmap_close() to undo the above, as the callsite expects + * full cleanup in this case and therefore does not invoke + * vmops::close(). + */ + ret = map_range(event->rb, vma); + if (ret) + perf_mmap_close(vma); } - /* - * Since pinned accounting is per vm we cannot allow fork() to copy our - * vma. - */ - vm_flags_set(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_DONTDUMP); - vma->vm_ops = &perf_mmap_vmops; - - mapped = get_mapped(event, event_mapped); - if (mapped) - mapped(event, vma->vm_mm); - - /* - * Try to map it into the page table. On fail, invoke - * perf_mmap_close() to undo the above, as the callsite expects - * full cleanup in this case and therefore does not invoke - * vmops::close(). - */ - ret = map_range(event->rb, vma); - if (ret) - perf_mmap_close(vma); - return ret; }