bpf: prevent out-of-bounds speculation

commit b2157399cc9898260d6031c5bfe45fe137c1fbe7 upstream.

Under speculation, CPUs may mis-predict branches in bounds checks. Thus,
memory accesses under a bounds check may be speculated even if the
bounds check fails, providing a primitive for building a side channel.

To avoid leaking kernel data round up array-based maps and mask the index
after bounds check, so speculated load with out of bounds index will load
either valid value from the array or zero from the padded area.

Unconditionally mask index for all array types even when max_entries
are not rounded to power of 2 for root user.
When map is created by unpriv user generate a sequence of bpf insns
that includes AND operation to make sure that JITed code includes
the same 'index & index_mask' operation.

If prog_array map is created by unpriv user replace
  bpf_tail_call(ctx, map, index);
with
  if (index >= max_entries) {
    index &= map->index_mask;
    bpf_tail_call(ctx, map, index);
  }
(along with roundup to power 2) to prevent out-of-bounds speculation.
There is secondary redundant 'if (index >= max_entries)' in the interpreter
and in all JITs, but they can be optimized later if necessary.

Other array-like maps (cpumap, devmap, sockmap, perf_event_array, cgroup_array)
cannot be used by unpriv, so no changes there.

That fixes bpf side of "Variant 1: bounds check bypass (CVE-2017-5753)" on
all architectures with and without JIT.

v2->v3:
Daniel noticed that attack potentially can be crafted via syscall commands
without loading the program, so add masking to those paths as well.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jiri Slaby <jslaby@suse.cz>
[ Backported to 4.9 - gregkh ]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 6 insertions, 1 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 97498be2ca2e..75ffd3b2149e 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -43,6 +43,7 @@ struct bpf_map {
 	u32 max_entries;
 	u32 map_flags;
 	u32 pages;
+	bool unpriv_array;
 	struct user_struct *user;
 	const struct bpf_map_ops *ops;
 	struct work_struct work;
@@ -189,6 +190,7 @@ struct bpf_prog_aux {
 struct bpf_array {
 	struct bpf_map map;
 	u32 elem_size;
+	u32 index_mask;
 	/* 'ownership' of prog_array is claimed by the first program that
 	 * is going to use this map or by the first program which FD is stored
 	 * in the map to make sure that all callers and callees have the same
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 4c4e9358c146..070fc49e39e2 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -67,7 +67,10 @@ struct bpf_verifier_state_list {
 };
 
 struct bpf_insn_aux_data {
-	enum bpf_reg_type ptr_type;	/* pointer type for load/store insns */
+	union {
+		enum bpf_reg_type ptr_type;     /* pointer type for load/store insns */
+		struct bpf_map *map_ptr;        /* pointer for call insn into lookup_elem */
+	};
 	bool seen; /* this insn was processed by the verifier */
 };