From: Alexei Starovoitov Date: Mon, 13 Apr 2026 19:42:45 +0000 (-0700) Subject: bpf: Fix use-after-free in arena_vm_close on fork X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=4fddde2a732de60bb97e3307d4eb69ac5f1d2b74;p=thirdparty%2Fkernel%2Flinux.git bpf: Fix use-after-free in arena_vm_close on fork arena_vm_open() only bumps vml->mmap_count but never registers the child VMA in arena->vma_list. The vml->vma always points at the parent VMA, so after parent munmap the pointer dangles. If the child then calls bpf_arena_free_pages(), zap_pages() reads the stale vml->vma triggering use-after-free. Fix this by preventing the arena VMA from being inherited across fork with VM_DONTCOPY, and preventing VMA splits via the may_split callback. Also reject mremap with a .mremap callback returning -EINVAL. A same-size mremap(MREMAP_FIXED) on the full arena VMA reaches copy_vma() through the following path: check_prep_vma() - returns 0 early: new_len == old_len skips VM_DONTEXPAND check prep_move_vma() - vm_start == old_addr and vm_end == old_addr + old_len so may_split is never called move_vma() copy_vma_and_data() copy_vma() vm_area_dup() - copies vm_private_data (vml pointer) vm_ops->open() - bumps vml->mmap_count vm_ops->mremap() - returns -EINVAL, rollback unmaps new VMA The refcount ensures the rollback's arena_vm_close does not free the vml shared with the original VMA. Reported-by: Weiming Shi Reported-by: Xiang Mei Fixes: 317460317a02 ("bpf: Introduce bpf_arena.") Reviewed-by: Emil Tsalapatis Link: https://lore.kernel.org/r/20260413194245.21449-1-alexei.starovoitov@gmail.com Signed-off-by: Alexei Starovoitov --- diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c index f355cf1c1a169..9c68c9b0b24ad 100644 --- a/kernel/bpf/arena.c +++ b/kernel/bpf/arena.c @@ -341,6 +341,16 @@ static void arena_vm_open(struct vm_area_struct *vma) refcount_inc(&vml->mmap_count); } +static int arena_vm_may_split(struct vm_area_struct *vma, unsigned long addr) +{ + return -EINVAL; +} + +static int arena_vm_mremap(struct vm_area_struct *vma) +{ + return -EINVAL; +} + static void arena_vm_close(struct vm_area_struct *vma) { struct bpf_map *map = vma->vm_file->private_data; @@ -417,6 +427,8 @@ out_unlock_sigsegv: static const struct vm_operations_struct arena_vm_ops = { .open = arena_vm_open, + .may_split = arena_vm_may_split, + .mremap = arena_vm_mremap, .close = arena_vm_close, .fault = arena_vm_fault, }; @@ -486,10 +498,11 @@ static int arena_map_mmap(struct bpf_map *map, struct vm_area_struct *vma) arena->user_vm_end = vma->vm_end; /* * bpf_map_mmap() checks that it's being mmaped as VM_SHARED and - * clears VM_MAYEXEC. Set VM_DONTEXPAND as well to avoid - * potential change of user_vm_start. + * clears VM_MAYEXEC. Set VM_DONTEXPAND to avoid potential change + * of user_vm_start. Set VM_DONTCOPY to prevent arena VMA from + * being copied into the child process on fork. */ - vm_flags_set(vma, VM_DONTEXPAND); + vm_flags_set(vma, VM_DONTEXPAND | VM_DONTCOPY); vma->vm_ops = &arena_vm_ops; return 0; }