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bpf: Prevent mmap()'ing read-only maps as writable

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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: fc9702273e ("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
This commit is contained in:
Andrii Nakryiko
2020-05-18 22:38:24 -07:00
committed by Alexei Starovoitov
parent 0550cfe8c2
commit dfeb376dd4
3 changed files with 34 additions and 4 deletions
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8
tools/testing/selftests/bpf/progs/test_mmap.c
View File

@@ -7,6 +7,14 @@
char _license[] SEC("license") = "GPL";
struct {
__uint(type, BPF_MAP_TYPE_ARRAY);
__uint(max_entries, 4096);
__uint(map_flags, BPF_F_MMAPABLE | BPF_F_RDONLY_PROG);
__type(key, __u32);
__type(value, char);
} rdonly_map SEC(".maps");
struct {
__uint(type, BPF_MAP_TYPE_ARRAY);
__uint(max_entries, 512 * 4); /* at least 4 pages of data */