MIPS: BPF: Prevent kernel fall over for >=32bit shifts

Remove BUG_ON() if the shift immediate is >=32 to avoid kernel crashes
due to malicious user input. If the shift immediate is >= 32,
we simply load the destination register with 0 since only
32-bit instructions are used by JIT so this will do the
correct thing even on MIPS64.
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Daniel Borkmann <dborkman@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: netdev@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7179/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/net/bpf_jit.c

@@ -151,6 +151,8 @@ static inline int optimize_div(u32 *k)
 	return 0;
 }
 
+static inline void emit_jit_reg_move(ptr dst, ptr src, struct jit_ctx *ctx);
+
 /* Simply emit the instruction if the JIT memory space has been allocated */
 #define emit_instr(ctx, func, ...)			\
 do {							\
@@ -309,7 +311,10 @@ static inline void emit_sll(unsigned int dst, unsigned int src,
 			    unsigned int sa, struct jit_ctx *ctx)
 {
 	/* sa is 5-bits long */
-	BUG_ON(sa >= BIT(5));
+	if (sa >= BIT(5))
+		/* Shifting >= 32 results in zero */
+		emit_jit_reg_move(dst, r_zero, ctx);
+	else
 		emit_instr(ctx, sll, dst, src, sa);
 }
 
@@ -323,7 +328,10 @@ static inline void emit_srl(unsigned int dst, unsigned int src,
 			    unsigned int sa, struct jit_ctx *ctx)
 {
 	/* sa is 5-bits long */
-	BUG_ON(sa >= BIT(5));
+	if (sa >= BIT(5))
+		/* Shifting >= 32 results in zero */
+		emit_jit_reg_move(dst, r_zero, ctx);
+	else
 		emit_instr(ctx, srl, dst, src, sa);
 }