[PATCH 5.10 46/54] bpf: Fix verifier jsgt branch analysis on max bound

Greg Kroah-Hartman
2021-02-11T15:38:46Z

From: Daniel Borkmann <daniel@iogearbox.net>

commit ee114dd64c0071500345439fc79dd5e0f9d106ed upstream.

Fix incorrect is_branch{32,64}_taken() analysis for the jsgt case. The return
code for both will tell the caller whether a given conditional jump is taken
or not, e.g. 1 means branch will be taken [for the involved registers] and the
goto target will be executed, 0 means branch will not be taken and instead we
fall-through to the next insn, and last but not least a -1 denotes that it is
not known at verification time whether a branch will be taken or not. Now while
the jsgt has the branch-taken case correct with reg->s32_min_value > sval, the
branch-not-taken case is off-by-one when testing for reg->s32_max_value < sval
since the branch will also be taken for reg->s32_max_value == sval. The jgt
branch analysis, for example, gets this right.

Fixes: 3f50f132d840 ("bpf: Verifier, do explicit ALU32 bounds tracking")
Fixes: 4f7b3e82589e ("bpf: improve verifier branch analysis")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/bpf/verifier.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6822,7 +6822,7 @@ static int is_branch32_taken(struct bpf_
case BPF_JSGT:
if (reg->s32_min_value > sval)
return 1;
- else if (reg->s32_max_value < sval)
+ else if (reg->s32_max_value <= sval)
return 0;
break;
case BPF_JLT:
@@ -6895,7 +6895,7 @@ static int is_branch64_taken(struct bpf_
case BPF_JSGT:
if (reg->smin_value > sval)
return 1;
- else if (reg->smax_value < sval)
+ else if (reg->smax_value <= sval)
return 0;
break;