[PATCH bpf] bpf: fix BPF_ALU32 | BPF_ARSH on BE arches

[Jiong Wang <jiong.wang@netronome.com>]

Yauheni reported the following code do not work correctly on BE arches:

       ALU_ARSH_X:
               DST = (u64) (u32) ((*(s32 *) &DST) >> SRC);
               CONT;
       ALU_ARSH_K:
               DST = (u64) (u32) ((*(s32 *) &DST) >> IMM);
               CONT;

and are causing failure of test_verifier test 'arsh32 on imm 2' on BE
arches.

The code is taking address and interpreting memory directly, so is not
endianness neutral. We should instead perform standard C type casting on
the variable. A u64 to s32 conversion will drop the high 32-bit and reserve
the low 32-bit as signed integer, this is all we want.

Fixes: 2dc6b100f928 ("bpf: interpreter support BPF_ALU | BPF_ARSH")
Reported-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
---
 kernel/bpf/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 080e2bb..f2148db 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1364,10 +1364,10 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
 		insn++;
 		CONT;
 	ALU_ARSH_X:
-		DST = (u64) (u32) ((*(s32 *) &DST) >> SRC);
+		DST = (u64) (u32) (((s32) DST) >> SRC);
 		CONT;
 	ALU_ARSH_K:
-		DST = (u64) (u32) ((*(s32 *) &DST) >> IMM);
+		DST = (u64) (u32) (((s32) DST) >> IMM);
 		CONT;
 	ALU64_ARSH_X:
 		(*(s64 *) &DST) >>= SRC;
-- 
2.7.4