bpf, sockmap: sk_skb data_end access incorrect when src_reg = dst_reg

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 in 84f44df664e9 ("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")
Signed-off-by: Jussi Maki <joamaki@gmail.com>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20211103204736.248403-6-john.fastabend@gmail.com

diff --git a/include/net/strparser.h b/include/net/strparser.h
index bec1439..732b709 100644 (file)
--- 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 c3936d0..e471c9b 100644 (file)
--- 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;
 }