bpf: Prevent mmap()'ing read-only maps as writable
As discussed in [0], it's dangerous to allow mapping BPF map, that's meant to
be frozen and is read-only on BPF program side, because that allows user-space
to actually store a writable view to the page even after it is frozen. This is
exacerbated by BPF verifier making a strong assumption that contents of such
frozen map will remain unchanged. To prevent this, disallow mapping
BPF_F_RDONLY_PROG mmap()'able BPF maps as writable, ever.
[0] https://lore.kernel.org/bpf/CAEf4BzYGWYhXdp6BJ7_=9OQPJxQpgug080MMjdSB72i9R+5c6g@mail.gmail.com/
Fixes: fc9702273e2e ("bpf: Add mmap() support for BPF_MAP_TYPE_ARRAY")
Suggested-by: Jann Horn <jannh@google.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/bpf/20200519053824.1089415-1-andriin@fb.com
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 2843bbb..4e6dee1 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -623,9 +623,20 @@ static int bpf_map_mmap(struct file *filp, struct vm_area_struct *vma)
mutex_lock(&map->freeze_mutex);
- if ((vma->vm_flags & VM_WRITE) && map->frozen) {
- err = -EPERM;
- goto out;
+ if (vma->vm_flags & VM_WRITE) {
+ if (map->frozen) {
+ err = -EPERM;
+ goto out;
+ }
+ /* map is meant to be read-only, so do not allow mapping as
+ * writable, because it's possible to leak a writable page
+ * reference and allows user-space to still modify it after
+ * freezing, while verifier will assume contents do not change
+ */
+ if (map->map_flags & BPF_F_RDONLY_PROG) {
+ err = -EACCES;
+ goto out;
+ }
}
/* set default open/close callbacks */