[PATCH 4.9 19/30] bpf: Fix buggy rsh min/max bounds tracking

Greg Kroah-Hartman
2021-01-29T12:12:01Z

From: Daniel Borkmann <daniel@iogearbox.net>

[ no upstream commit ]

Fix incorrect bounds tracking for RSH opcode. Commit f23cc643f9ba ("bpf: fix
range arithmetic for bpf map access") had a wrong assumption about min/max
bounds. The new dst_reg->min_value needs to be derived by right shifting the
max_val bounds, not min_val, and likewise new dst_reg->max_value needs to be
derived by right shifting the min_val bounds, not max_val. Later stable kernels
than 4.9 are not affected since bounds tracking was overall reworked and they
already track this similarly as in the fix.

Fixes: f23cc643f9ba ("bpf: fix range arithmetic for bpf map access")
Reported-by: Ryota Shiga (Flatt Security)
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Cc: Josef Bacik <jbacik@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/bpf/verifier.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1732,12 +1732,11 @@ static void adjust_reg_min_max_vals(stru
* unsigned shift, so make the appropriate casts.
*/
if (min_val < 0 || dst_reg->min_value < 0)
- dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
+ reset_reg_range_values(regs, insn->dst_reg);
else
- dst_reg->min_value =
- (u64)(dst_reg->min_value) >> min_val;
+ dst_reg->min_value = (u64)(dst_reg->min_value) >> max_val;
if (dst_reg->max_value != BPF_REGISTER_MAX_RANGE)
- dst_reg->max_value >>= max_val;
+ dst_reg->max_value >>= min_val;
break;
default:
reset_reg_range_values(regs, insn->dst_reg);