s390/bpf: Fix backward jumps

commit b035b60ded132592055c0f9bd1cc280259c7de4b upstream.

Currently all backward jumps crash for JITed s390x eBPF programs
with an illegal instruction program check and kernel panic. Because
for negative values the opcode of the jump instruction is overriden
by the negative branch offset an illegal instruction is generated
by the JIT:

 000003ff802da378: c01100000002   lgfi    %r1,2
 000003ff802da37e: fffffff52065   unknown <-- illegal instruction
 000003ff802da384: b904002e       lgr     %r2,%r14

So fix this and mask the offset in order not to damage the opcode.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index 55423d8be580..9afb9d602f84 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -227,7 +227,7 @@ static inline void reg_set_seen(struct bpf_jit *jit, u32 b1)
 ({                                                              \
         /* Branch instruction needs 6 bytes */                  \
         int rel = (addrs[i + off + 1] - (addrs[i + 1] - 6)) / 2;\
-        _EMIT6(op1 | reg(b1, b2) << 16 | rel, op2 | mask);      \
+        _EMIT6(op1 | reg(b1, b2) << 16 | (rel & 0xffff), op2 | mask);   \
         REG_SET_SEEN(b1);                                       \
         REG_SET_SEEN(b2);                                       \
 })