The current implementation of mmap() is set up such that a struct file object is obtained for the input fd in ksys_mmap_pgoff() via fget(), and its reference count decremented at the end of the function via. fput(). If a merge can be achieved, we are fine to simply decrement the refcount on the file. Otherwise, in __mmap_new_file_vma(), we increment the reference count on the file via get_file() such that the fput() in ksys_mmap_pgoff() does not free the now-referenced file object. The introduction of the f_op->mmap_prepare hook changes things, as it becomes possible for a driver to replace the file object right at the beginning of the mmap operation. The current implementation is buggy if this happens because it unconditionally calls get_file() on the mapping's file whether or not it was replaced (and thus whether or not its reference count will be decremented at the end of ksys_mmap_pgoff()). This results in a memory leak, and was exposed in commit ab04945f91bc ("mm: update mem char driver to use mmap_prepare"). This patch solves the problem by explicitly tracking whether we actually need to call get_file() on the file or not, and only doing so if required. Signed-off-by: Lorenzo Stoakes Fixes: ab04945f91bc ("mm: update mem char driver to use mmap_prepare") Reported-by: syzbot+bf5de69ebb4bdf86f59f@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/6964a92b.050a0220.eaf7.008a.GAE@google.com/ --- mm/vma.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/mm/vma.c b/mm/vma.c index 5fdf38325b0e..3dbe414eff89 100644 --- a/mm/vma.c +++ b/mm/vma.c @@ -37,6 +37,8 @@ struct mmap_state { bool check_ksm_early :1; /* If we map new, hold the file rmap lock on mapping. */ bool hold_file_rmap_lock :1; + /* If .mmap_prepare changed the file, we don't need to pin. */ + bool file_doesnt_need_get :1; }; #define MMAP_STATE(name, mm_, vmi_, addr_, len_, pgoff_, vm_flags_, file_) \ @@ -2450,7 +2452,9 @@ static int __mmap_new_file_vma(struct mmap_state *map, struct vma_iterator *vmi = map->vmi; int error; - vma->vm_file = get_file(map->file); + vma->vm_file = map->file; + if (!map->file_doesnt_need_get) + get_file(map->file); if (!map->file->f_op->mmap) return 0; @@ -2638,7 +2642,10 @@ static int call_mmap_prepare(struct mmap_state *map, /* Update fields permitted to be changed. */ map->pgoff = desc->pgoff; - map->file = desc->vm_file; + if (desc->vm_file != map->file) { + map->file_doesnt_need_get = true; + map->file = desc->vm_file; + } map->vm_flags = desc->vm_flags; map->page_prot = desc->page_prot; /* User-defined fields. */ -- 2.52.0