Subject: [PATCH bpf v2 5/5] bpf, sockmap: sk_skb data_end access incorrect when src_reg = dst_reg

From: John Fastabend <[email-masked]>
Date: Wed, 3 Nov 2021 13:47:36 -0700

From: Jussi Maki <[email-masked]>

The current conversion of skb->data_end reads like this,

  ; data_end = (void*)(long)skb->data_end;
   559: (79) r1 = *(u64 *)(r2 +200)   ; r1  = skb->data
   560: (61) r11 = *(u32 *)(r2 +112)  ; r11 = skb->len
   561: (0f) r1 += r11
   562: (61) r11 = *(u32 *)(r2 +116)
   563: (1f) r1 -= r11

But similar to the case

 ("bpf: sock_ops sk access may stomp registers when dst_reg = src_reg"),

the code will read an incorrect skb->len when src == dst. In this case we
end up generating this xlated code.

  ; data_end = (void*)(long)skb->data_end;
   559: (79) r1 = *(u64 *)(r1 +200)   ; r1  = skb->data
   560: (61) r11 = *(u32 *)(r1 +112)  ; r11 = (skb->data)->len
   561: (0f) r1 += r11
   562: (61) r11 = *(u32 *)(r1 +116)
   563: (1f) r1 -= r11

where line 560 is the reading 4B of (skb->data + 112) instead of the
intended skb->len Here the skb pointer in r1 gets set to skb->data and
the later deref for skb->len ends up following skb->data instead of skb.

This fixes the issue similarly to the patch mentioned above by creating
an additional temporary variable and using to store the register when
dst_reg = src_reg. We name the variable bpf_temp_reg and place it in the
cb context for sk_skb. Then we restore from the temp to ensure nothing
is lost.

Fixes: 16137b09a66f2 ("bpf: Compute data_end dynamically with JIT code")
Reviewed-by: Jakub Sitnicki <[email-masked]>
Signed-off-by: Jussi Maki <[email-masked]>
Signed-off-by: John Fastabend <[email-masked]>
---
 include/net/strparser.h |  4 ++++
 net/core/filter.c       | 36 ++++++++++++++++++++++++++++++------
 2 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/include/net/strparser.h b/include/net/strparser.h
index bec1439bd3be..732b7097d78e 100644
--- a/include/net/strparser.h
+++ b/include/net/strparser.h
@@ -66,6 +66,10 @@ struct sk_skb_cb {
 #define SK_SKB_CB_PRIV_LEN 20
 	unsigned char data[SK_SKB_CB_PRIV_LEN];
 	struct _strp_msg strp;
+	/* temp_reg is a temporary register used for bpf_convert_data_end_access
+	 * when dst_reg == src_reg.
+	 */
+	u64 temp_reg;
 };
 
 static inline struct strp_msg *strp_msg(struct sk_buff *skb)
diff --git a/net/core/filter.c b/net/core/filter.c
index c3936d0724b8..e471c9b09670 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -9756,22 +9756,46 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
 static struct bpf_insn *bpf_convert_data_end_access(const struct bpf_insn *si,
 						    struct bpf_insn *insn)
 {
-	/* si->dst_reg = skb->data */
+	int reg;
+	int temp_reg_off = offsetof(struct sk_buff, cb) +
+			   offsetof(struct sk_skb_cb, temp_reg);
+
+	if (si->src_reg == si->dst_reg) {
+		/* We need an extra register, choose and save a register. */
+		reg = BPF_REG_9;
+		if (si->src_reg == reg || si->dst_reg == reg)
+			reg--;
+		if (si->src_reg == reg || si->dst_reg == reg)
+			reg--;
+		*insn++ = BPF_STX_MEM(BPF_DW, si->src_reg, reg, temp_reg_off);
+	} else {
+		reg = si->dst_reg;
+	}
+
+	/* reg = skb->data */
 	*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, data),
-			      si->dst_reg, si->src_reg,
+			      reg, si->src_reg,
 			      offsetof(struct sk_buff, data));
 	/* AX = skb->len */
 	*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, len),
 			      BPF_REG_AX, si->src_reg,
 			      offsetof(struct sk_buff, len));
-	/* si->dst_reg = skb->data + skb->len */
-	*insn++ = BPF_ALU64_REG(BPF_ADD, si->dst_reg, BPF_REG_AX);
+	/* reg = skb->data + skb->len */
+	*insn++ = BPF_ALU64_REG(BPF_ADD, reg, BPF_REG_AX);
 	/* AX = skb->data_len */
 	*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, data_len),
 			      BPF_REG_AX, si->src_reg,
 			      offsetof(struct sk_buff, data_len));
-	/* si->dst_reg = skb->data + skb->len - skb->data_len */
-	*insn++ = BPF_ALU64_REG(BPF_SUB, si->dst_reg, BPF_REG_AX);
+
+	/* reg = skb->data + skb->len - skb->data_len */
+	*insn++ = BPF_ALU64_REG(BPF_SUB, reg, BPF_REG_AX);
+
+	if (si->src_reg == si->dst_reg) {
+		/* Restore the saved register */
+		*insn++ = BPF_MOV64_REG(BPF_REG_AX, si->src_reg);
+		*insn++ = BPF_MOV64_REG(si->dst_reg, reg);
+		*insn++ = BPF_LDX_MEM(BPF_DW, reg, BPF_REG_AX, temp_reg_off);
+	}
 
 	return insn;
 }
-- 
2.33.0