netdev - [PATCH bpf 1/2] bpf: prevent re-mmap()'ing BPF map as writable for initially r/o mapping

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Date:   Thu, 9 Apr 2020 17:04:24 -0700
From:   Andrii Nakryiko <andriin@...com>
To:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>, <ast@...com>,
        <daniel@...earbox.net>
CC:     <andrii.nakryiko@...il.com>, <kernel-team@...com>,
        Andrii Nakryiko <andriin@...com>, Jann Horn <jannh@...gle.com>
Subject: [PATCH bpf 1/2] bpf: prevent re-mmap()'ing BPF map as writable for initially r/o mapping

VM_MAYWRITE flag during initial memory mapping determines if already mmap()'ed
pages can be later remapped as writable ones through mprotect() call. To
prevent user application to rewrite contents of memory-mapped as read-only and
subsequently frozen BPF map, remove VM_MAYWRITE flag completely on initially
read-only mapping.

Alternatively, we could treat any memory-mapping on unfrozen map as writable
and bump writecnt instead. But there is little legitimate reason to map
BPF map as read-only and then re-mmap() it as writable through mprotect(),
instead of just mmap()'ing it as read/write from the very beginning.

Fixes: fc9702273e2e ("bpf: Add mmap() support for BPF_MAP_TYPE_ARRAY")
Reported-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Andrii Nakryiko <andriin@...com>
---
 kernel/bpf/syscall.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 64783da34202..f7f6db50a085 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -635,6 +635,10 @@ static int bpf_map_mmap(struct file *filp, struct vm_area_struct *vma)
 	/* set default open/close callbacks */
 	vma->vm_ops = &bpf_map_default_vmops;
 	vma->vm_private_data = map;
+	vma->vm_flags &= ~VM_MAYEXEC;
+	if (!(vma->vm_flags & VM_WRITE))
+		/* disallow re-mapping with PROT_WRITE */
+		vma->vm_flags &= ~VM_MAYWRITE;

 	err = map->ops->map_mmap(map, vma);
 	if (err)
-- 
2.24.1