[PATCH 4.1 46/56] s390/bpf: Fix backward jumps

[Greg Kroah-Hartman]

4.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Michael Holzheu <holzheu@xxxxxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/s390/net/bpf_jit_comp.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 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 b
 ({								\
 	/* 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);					\
 })


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