Patch "bpf: avoid holding freeze_mutex during mmap operation" has been added to the 6.13-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    bpf: avoid holding freeze_mutex during mmap operation

to the 6.13-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     bpf-avoid-holding-freeze_mutex-during-mmap-operation.patch
and it can be found in the queue-6.13 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit a9a1958779d005cd3834293128ae824fd6730334
Author: Andrii Nakryiko <andrii@xxxxxxxxxx>
Date:   Tue Jan 28 17:22:46 2025 -0800

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

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 60417b79639e5..f086fd8f263f1 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -1031,7 +1031,7 @@ 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;
@@ -1055,7 +1055,12 @@ 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;
@@ -1072,13 +1077,11 @@ 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;
 }