s390/bpf: Fix backward jumps

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.

Cc: stable@vger.kernel.org # 4.0+
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index d3766dd67e23266b6670d90eedc9b02c26304e7b..fee782acc2ee51f6a3aae4b28152ec76981c8350 100644 (file)
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -250,7 +250,7 @@ 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);                                       \
 })