bpf: avoid holding freeze_mutex during mmap operation

[ Upstream commit bc27c52eea189e8f7492d40739b7746d67b65beb ]

We use map->freeze_mutex to prevent races between map_freeze() and
memory mapping BPF map contents with writable permissions. The way we
naively do this means we'll hold freeze_mutex for entire duration of all
the mm and VMA manipulations, which is completely unnecessary. This can
potentially also lead to deadlocks, as reported by syzbot in [0].

So, instead, hold freeze_mutex only during writeability checks, bump
(proactively) "write active" count for the map, unlock the mutex and
proceed with mmap logic. And only if something went wrong during mmap
logic, then undo that "write active" counter increment.

  [0] https://lore.kernel.org/bpf/678dcbc9.050a0220.303755.0066.GAE@google.com/

Fixes: fc9702273e2e ("bpf: Add mmap() support for BPF_MAP_TYPE_ARRAY")
Reported-by: syzbot+4dc041c686b7c816a71e@syzkaller.appspotmail.com
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20250129012246.1515826-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index fa43f26ce0dac355244d9e38df419462851ce1da..3200372ea28ce976361183a33c53f55b1d547807 100644 (file)
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -936,7 +936,7 @@ static const struct vm_operations_struct bpf_map_default_vmops = {
 static int bpf_map_mmap(struct file *filp, struct vm_area_struct *vma)
 {
        struct bpf_map *map = filp->private_data;
-       int err;
+       int err = 0;
 
        if (!map->ops->map_mmap || !IS_ERR_OR_NULL(map->record))
                return -ENOTSUPP;
@@ -960,7 +960,12 @@ static int bpf_map_mmap(struct file *filp, struct vm_area_struct *vma)
                        err = -EACCES;
                        goto out;
                }
+               bpf_map_write_active_inc(map);
        }
+out:
+       mutex_unlock(&map->freeze_mutex);
+       if (err)
+               return err;
 
        /* set default open/close callbacks */
        vma->vm_ops = &bpf_map_default_vmops;
@@ -977,13 +982,11 @@ static int bpf_map_mmap(struct file *filp, struct vm_area_struct *vma)
                vm_flags_clear(vma, VM_MAYWRITE);
 
        err = map->ops->map_mmap(map, vma);
-       if (err)
-               goto out;
+       if (err) {
+               if (vma->vm_flags & VM_WRITE)
+                       bpf_map_write_active_dec(map);
+       }
 
-       if (vma->vm_flags & VM_WRITE)
-               bpf_map_write_active_inc(map);
-out:
-       mutex_unlock(&map->freeze_mutex);
        return err;
 }