bpf: sock_ops sk access may stomp registers when dst_reg = src_reg

Similar to patch ("bpf: sock_ops ctx access may stomp registers") if the
src_reg = dst_reg when reading the sk field of a sock_ops struct we
generate xlated code,

  53: (61) r9 = *(u32 *)(r9 +28)
  54: (15) if r9 == 0x0 goto pc+3
  56: (79) r9 = *(u64 *)(r9 +0)

This stomps on the r9 reg to do the sk_fullsock check and then when
reading the skops->sk field instead of the sk pointer we get the
sk_fullsock. To fix use similar pattern noted in the previous fix
and use the temp field to save/restore a register used to do
sk_fullsock check.

After the fix the generated xlated code reads,

  52: (7b) *(u64 *)(r9 +32) = r8
  53: (61) r8 = *(u32 *)(r9 +28)
  54: (15) if r9 == 0x0 goto pc+3
  55: (79) r8 = *(u64 *)(r9 +32)
  56: (79) r9 = *(u64 *)(r9 +0)
  57: (05) goto pc+1
  58: (79) r8 = *(u64 *)(r9 +32)

Here r9 register was in-use so r8 is chosen as the temporary register.
In line 52 r8 is saved in temp variable and at line 54 restored in case
fullsock != 0. Finally we handle fullsock == 0 case by restoring at
line 58.

This adds a new macro SOCK_OPS_GET_SK it is almost possible to merge
this with SOCK_OPS_GET_FIELD, but I found the extra branch logic a
bit more confusing than just adding a new macro despite a bit of
duplicating code.

Fixes: 1314ef56 ("bpf: export bpf_sock for BPF_PROG_TYPE_SOCK_OPS prog type")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/159718349653.4728.6559437186853473612.stgit@john-Precision-5820-Tower

net/core/filter.c

@@ -8358,6 +8358,43 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
 		}							      \
 	} while (0)
 
+#define SOCK_OPS_GET_SK()							      \
+	do {								      \
+		int fullsock_reg = si->dst_reg, reg = BPF_REG_9, jmp = 1;     \
+		if (si->dst_reg == reg || si->src_reg == reg)		      \
+			reg--;						      \
+		if (si->dst_reg == reg || si->src_reg == reg)		      \
+			reg--;						      \
+		if (si->dst_reg == si->src_reg) {			      \
+			*insn++ = BPF_STX_MEM(BPF_DW, si->src_reg, reg,	      \
+					  offsetof(struct bpf_sock_ops_kern,  \
+					  temp));			      \
+			fullsock_reg = reg;				      \
+			jmp += 2;					      \
+		}							      \
+		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(			      \
+						struct bpf_sock_ops_kern,     \
+						is_fullsock),		      \
+				      fullsock_reg, si->src_reg,	      \
+				      offsetof(struct bpf_sock_ops_kern,      \
+					       is_fullsock));		      \
+		*insn++ = BPF_JMP_IMM(BPF_JEQ, fullsock_reg, 0, jmp);	      \
+		if (si->dst_reg == si->src_reg)				      \
+			*insn++ = BPF_LDX_MEM(BPF_DW, reg, si->src_reg,	      \
+				      offsetof(struct bpf_sock_ops_kern,      \
+				      temp));				      \
+		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(			      \
+						struct bpf_sock_ops_kern, sk),\
+				      si->dst_reg, si->src_reg,		      \
+				      offsetof(struct bpf_sock_ops_kern, sk));\
+		if (si->dst_reg == si->src_reg)	{			      \
+			*insn++ = BPF_JMP_A(1);				      \
+			*insn++ = BPF_LDX_MEM(BPF_DW, reg, si->src_reg,	      \
+				      offsetof(struct bpf_sock_ops_kern,      \
+				      temp));				      \
+		}							      \
+	} while (0)
+
 #define SOCK_OPS_GET_TCP_SOCK_FIELD(FIELD) \
 		SOCK_OPS_GET_FIELD(FIELD, FIELD, struct tcp_sock)
 
@@ -8642,17 +8679,7 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
 		SOCK_OPS_GET_TCP_SOCK_FIELD(bytes_acked);
 		break;
 	case offsetof(struct bpf_sock_ops, sk):
-		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(
-						struct bpf_sock_ops_kern,
-						is_fullsock),
-				      si->dst_reg, si->src_reg,
-				      offsetof(struct bpf_sock_ops_kern,
-					       is_fullsock));
-		*insn++ = BPF_JMP_IMM(BPF_JEQ, si->dst_reg, 0, 1);
-		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(
-						struct bpf_sock_ops_kern, sk),
-				      si->dst_reg, si->src_reg,
-				      offsetof(struct bpf_sock_ops_kern, sk));
+		SOCK_OPS_GET_SK();
 		break;
 	}
 	return insn - insn_buf;