Subject: Re: [PATCH] arm64: bpf: fix buffer pointer

From: "Shi, Yang" <[email-masked]>
Date: Wed, 18 Nov 2015 13:07:18 -0800

On 11/18/2015 12:56 AM, Zi Shen Lim wrote:
> During code review, I noticed we were passing a bad buffer pointer
> to bpf_load_pointer helper function called by jitted code.
>
> Point to the buffer allocated by JIT, so we don't silently corrupt
> other parts of the stack.
>
> Signed-off-by: Zi Shen Lim <[email-masked]>
> ---
>   arch/arm64/net/bpf_jit_comp.c | 27 +++++++++++++--------------
>   1 file changed, 13 insertions(+), 14 deletions(-)
>
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index d6a53ef..7cf032b 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -139,6 +139,12 @@ static inline int epilogue_offset(const struct jit_ctx *ctx)
>   /* Stack must be multiples of 16B */
>   #define STACK_ALIGN(sz) (((sz) + 15) & ~15)
>
> +#define _STACK_SIZE \
> +	(MAX_BPF_STACK \
> +	 + 4 /* extra for skb_copy_bits buffer */)
> +
> +#define STACK_SIZE STACK_ALIGN(_STACK_SIZE)
> +
>   static void build_prologue(struct jit_ctx *ctx)
>   {
>   	const u8 r6 = bpf2a64[BPF_REG_6];
> @@ -150,10 +156,6 @@ static void build_prologue(struct jit_ctx *ctx)
>   	const u8 rx = bpf2a64[BPF_REG_X];
>   	const u8 tmp1 = bpf2a64[TMP_REG_1];
>   	const u8 tmp2 = bpf2a64[TMP_REG_2];
> -	int stack_size = MAX_BPF_STACK;
> -
> -	stack_size += 4; /* extra for skb_copy_bits buffer */
> -	stack_size = STACK_ALIGN(stack_size);
>
>   	/*
>   	 * BPF prog stack layout
> @@ -165,12 +167,13 @@ static void build_prologue(struct jit_ctx *ctx)
>   	 *                        | ... | callee saved registers
>   	 *                        +-----+
>   	 *                        |     | x25/x26
> -	 * BPF fp register => -80:+-----+
> +	 * BPF fp register => -80:+-----+ <= (BPF_FP)
>   	 *                        |     |
>   	 *                        | ... | BPF prog stack
>   	 *                        |     |
> -	 *                        |     |
> -	 * current A64_SP =>      +-----+
> +	 *                        +-----+ <= (BPF_FP - MAX_BPF_STACK)
> +	 *                        |RSVD | JIT scratchpad
> +	 * current A64_SP =>      +-----+ <= (BPF_FP - STACK_SIZE)
>   	 *                        |     |
>   	 *                        | ... | Function call stack
>   	 *                        |     |
> @@ -196,7 +199,7 @@ static void build_prologue(struct jit_ctx *ctx)
>   	emit(A64_MOV(1, fp, A64_SP), ctx);
>
>   	/* Set up function call stack */
> -	emit(A64_SUB_I(1, A64_SP, A64_SP, stack_size), ctx);
> +	emit(A64_SUB_I(1, A64_SP, A64_SP, STACK_SIZE), ctx);
>
>   	/* Clear registers A and X */
>   	emit_a64_mov_i64(ra, 0, ctx);
> @@ -213,13 +216,9 @@ static void build_epilogue(struct jit_ctx *ctx)
>   	const u8 fp = bpf2a64[BPF_REG_FP];
>   	const u8 tmp1 = bpf2a64[TMP_REG_1];
>   	const u8 tmp2 = bpf2a64[TMP_REG_2];
> -	int stack_size = MAX_BPF_STACK;
> -
> -	stack_size += 4; /* extra for skb_copy_bits buffer */
> -	stack_size = STACK_ALIGN(stack_size);
>
>   	/* We're done with BPF stack */
> -	emit(A64_ADD_I(1, A64_SP, A64_SP, stack_size), ctx);
> +	emit(A64_ADD_I(1, A64_SP, A64_SP, STACK_SIZE), ctx);
>
>   	/* Restore fs (x25) and x26 */
>   	emit(A64_POP(fp, A64_R(26), A64_SP), ctx);
> @@ -658,7 +657,7 @@ emit_cond_jmp:
>   			return -EINVAL;
>   		}
>   		emit_a64_mov_i64(r3, size, ctx);
> -		emit(A64_ADD_I(1, r4, fp, MAX_BPF_STACK), ctx);
> +		emit(A64_SUB_I(1, r4, fp, STACK_SIZE), ctx);

Should not it sub MAX_BPF_STACK?

If you sub STACK_SIZE here, the buffer pointer will point to bottom of 
the reserved area.

You stack layout change also shows this:

+	 *                        +-----+ <= (BPF_FP - MAX_BPF_STACK)
+	 *                        |RSVD | JIT scratchpad
+	 * current A64_SP =>      +-----+ <= (BPF_FP - STACK_SIZE)

Thanks,
Yang

>   		emit_a64_mov_i64(r5, (unsigned long)bpf_load_pointer, ctx);
>   		emit(A64_PUSH(A64_FP, A64_LR, A64_SP), ctx);
>   		emit(A64_MOV(1, A64_FP, A64_SP), ctx);
>