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>

diff --git a/arch/mips/net/bpf_jit.c b/arch/mips/net/bpf_jit.c
index 1bcd599..9476e7f 100644 (file)
--- a/arch/mips/net/bpf_jit.c
+++ b/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 {                                                   \
@@ -309,8 +311,11 @@ 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));
-       emit_instr(ctx, sll, dst, src, sa);
+       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);
 }
 
 static inline void emit_srlv(unsigned int dst, unsigned int src,
@@ -323,8 +328,11 @@ 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));
-       emit_instr(ctx, srl, dst, src, sa);
+       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);
 }
 
 static inline void emit_slt(unsigned int dst, unsigned int src1,