[PATCH AUTOSEL 6.13 02/32] bpf: unify VM_WRITE vs VM_MAYWRITE use in BPF map mmaping logic

Sasha Levin
2025-02-24T11:16:43Z

From: Andrii Nakryiko <andrii@kernel.org>

[ Upstream commit 98671a0fd1f14e4a518ee06b19037c20014900eb ]

For all BPF maps we ensure that VM_MAYWRITE is cleared when
memory-mapping BPF map contents as initially read-only VMA. This is
because in some cases BPF verifier relies on the underlying data to not
be modified afterwards by user space, so once something is mapped
read-only, it shouldn't be re-mmap'ed as read-write.

As such, it's not necessary to check VM_MAYWRITE in bpf_map_mmap() and
map->ops->map_mmap() callbacks: VM_WRITE should be consistently set for
read-write mappings, and if VM_WRITE is not set, there is no way for
user space to upgrade read-only mapping to read-write one.

This patch cleans up this VM_WRITE vs VM_MAYWRITE handling within
bpf_map_mmap(), which is an entry point for any BPF map mmap()-ing
logic. We also drop unnecessary sanitization of VM_MAYWRITE in BPF
ringbuf's map_mmap() callback implementation, as it is already performed
by common code in bpf_map_mmap().

Note, though, that in bpf_map_mmap_{open,close}() callbacks we can't
drop VM_MAYWRITE use, because it's possible (and is outside of
subsystem's control) to have initially read-write memory mapping, which
is subsequently dropped to read-only by user space through mprotect().
In such case, from BPF verifier POV it's read-write data throughout the
lifetime of BPF map, and is counted as "active writer".

But its VMAs will start out as VM_WRITE|VM_MAYWRITE, then mprotect() can
change it to just VM_MAYWRITE (and no VM_WRITE), so when its finally
munmap()'ed and bpf_map_mmap_close() is called, vm_flags will be just
VM_MAYWRITE, but we still need to decrement active writer count with
bpf_map_write_active_dec() as it's still considered to be a read-write
mapping by the rest of BPF subsystem.

Similar reasoning applies to bpf_map_mmap_open(), which is called
whenever mmap(), munmap(), and/or mprotect() forces mm subsystem to
split original VMA into multiple discontiguous VMAs.

Memory-mapping handling is a bit tricky, yes.

Cc: Jann Horn <jannh@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20250129012246.1515826-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/bpf/ringbuf.c | 4 ----
kernel/bpf/syscall.c | 10 ++++++++--
2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/ringbuf.c b/kernel/bpf/ringbuf.c
index e1cfe890e0be6..1499d8caa9a35 100644
--- a/kernel/bpf/ringbuf.c
+++ b/kernel/bpf/ringbuf.c
@@ -268,8 +268,6 @@ static int ringbuf_map_mmap_kern(struct bpf_map *map, struct vm_area_struct *vma
/* allow writable mapping for the consumer_pos only */
if (vma->vm_pgoff != 0 || vma->vm_end - vma->vm_start != PAGE_SIZE)
return -EPERM;
- } else {
- vm_flags_clear(vma, VM_MAYWRITE);
}
/* remap_vmalloc_range() checks size and offset constraints */
return remap_vmalloc_range(vma, rb_map->rb,
@@ -289,8 +287,6 @@ static int ringbuf_map_mmap_user(struct bpf_map *map, struct vm_area_struct *vma
* position, and the ring buffer data itself.
*/
return -EPERM;
- } else {
- vm_flags_clear(vma, VM_MAYWRITE);
}
/* remap_vmalloc_range() checks size and offset constraints */
return remap_vmalloc_range(vma, rb_map->rb, vma->vm_pgoff + RINGBUF_PGOFF);
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 5684e8ce132d5..60417b79639e5 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -1061,15 +1061,21 @@ static int bpf_map_mmap(struct file *filp, struct vm_area_struct *vma)
vma->vm_ops = &bpf_map_default_vmops;
vma->vm_private_data = map;
vm_flags_clear(vma, VM_MAYEXEC);
+ /* If mapping is read-only, then disallow potentially re-mapping with
+ * PROT_WRITE by dropping VM_MAYWRITE flag. This VM_MAYWRITE clearing
+ * means that as far as BPF map's memory-mapped VMAs are concerned,
+ * VM_WRITE and VM_MAYWRITE and equivalent, if one of them is set,
+ * both should be set, so we can forget about VM_MAYWRITE and always
+ * check just VM_WRITE
+ */
if (!(vma->vm_flags & VM_WRITE))
- /* disallow re-mapping with PROT_WRITE */
vm_flags_clear(vma, VM_MAYWRITE);

err = map->ops->map_mmap(map, vma);
if (err)
goto out;

- if (vma->vm_flags & VM_MAYWRITE)
+ if (vma->vm_flags & VM_WRITE)
bpf_map_write_active_inc(map);
out:
mutex_unlock(&map->freeze_mutex);
--
2.39.5