bpf: Fail verification for sign-extension of packet data/data_end/dat…

…a_meta

syzbot reported a kernel crash due to
  commit 1f1e864 ("bpf: Handle sign-extenstin ctx member accesses").
The reason is due to sign-extension of 32-bit load for
packet data/data_end/data_meta uapi field.

The original code looks like:
        r2 = *(s32 *)(r1 + 76) /* load __sk_buff->data */
        r3 = *(u32 *)(r1 + 80) /* load __sk_buff->data_end */
        r0 = r2
        r0 += 8
        if r3 > r0 goto +1
        ...
Note that __sk_buff->data load has 32-bit sign extension.

After verification and convert_ctx_accesses(), the final asm code looks like:
        r2 = *(u64 *)(r1 +208)
        r2 = (s32)r2
        r3 = *(u64 *)(r1 +80)
        r0 = r2
        r0 += 8
        if r3 > r0 goto pc+1
        ...
Note that 'r2 = (s32)r2' may make the kernel __sk_buff->data address invalid
which may cause runtime failure.

Currently, in C code, typically we have
        void *data = (void *)(long)skb->data;
        void *data_end = (void *)(long)skb->data_end;
        ...
and it will generate
        r2 = *(u64 *)(r1 +208)
        r3 = *(u64 *)(r1 +80)
        r0 = r2
        r0 += 8
        if r3 > r0 goto pc+1

If we allow sign-extension,
        void *data = (void *)(long)(int)skb->data;
        void *data_end = (void *)(long)skb->data_end;
        ...
the generated code looks like
        r2 = *(u64 *)(r1 +208)
        r2 <<= 32
        r2 s>>= 32
        r3 = *(u64 *)(r1 +80)
        r0 = r2
        r0 += 8
        if r3 > r0 goto pc+1
and this will cause verification failure since "r2 <<= 32" is not allowed
as "r2" is a packet pointer.

To fix this issue for case
  r2 = *(s32 *)(r1 + 76) /* load __sk_buff->data */
this patch added additional checking in is_valid_access() callback
function for packet data/data_end/data_meta access. If those accesses
are with sign-extenstion, the verification will fail.

  [1] https://lore.kernel.org/bpf/[email protected]/

Reported-by: [email protected]
Fixes: 1f1e864 ("bpf: Handle sign-extenstin ctx member accesses")
Acked-by: Eduard Zingerman <[email protected]>
Signed-off-by: Yonghong Song <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>

include/linux/bpf.h

@@ -920,6 +920,7 @@ static_assert(__BPF_REG_TYPE_MAX <= BPF_BASE_TYPE_LIMIT);
  */
 struct bpf_insn_access_aux {
 	enum bpf_reg_type reg_type;
+	bool is_ldsx;
 	union {
 		int ctx_field_size;
 		struct {

kernel/bpf/verifier.c

@@ -5626,12 +5626,13 @@ static int check_packet_access(struct bpf_verifier_env *env, u32 regno, int off,
 /* check access to 'struct bpf_context' fields.  Supports fixed offsets only */
 static int check_ctx_access(struct bpf_verifier_env *env, int insn_idx, int off, int size,
 			    enum bpf_access_type t, enum bpf_reg_type *reg_type,
-			    struct btf **btf, u32 *btf_id, bool *is_retval)
+			    struct btf **btf, u32 *btf_id, bool *is_retval, bool is_ldsx)
 {
 	struct bpf_insn_access_aux info = {
 		.reg_type = *reg_type,
 		.log = &env->log,
 		.is_retval = false,
+		.is_ldsx = is_ldsx,
 	};
 
 	if (env->ops->is_valid_access &&
@@ -6945,7 +6946,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 			return err;
 
 		err = check_ctx_access(env, insn_idx, off, size, t, &reg_type, &btf,
-				       &btf_id, &is_retval);
+				       &btf_id, &is_retval, is_ldsx);
 		if (err)
 			verbose_linfo(env, insn_idx, "; ");
 		if (!err && t == BPF_READ && value_regno >= 0) {

net/core/filter.c

@@ -8579,13 +8579,16 @@ static bool bpf_skb_is_valid_access(int off, int size, enum bpf_access_type type
 		if (off + size > offsetofend(struct __sk_buff, cb[4]))
 			return false;
 		break;
+	case bpf_ctx_range(struct __sk_buff, data):
+	case bpf_ctx_range(struct __sk_buff, data_meta):
+	case bpf_ctx_range(struct __sk_buff, data_end):
+		if (info->is_ldsx || size != size_default)
+			return false;
+		break;
 	case bpf_ctx_range_till(struct __sk_buff, remote_ip6[0], remote_ip6[3]):
 	case bpf_ctx_range_till(struct __sk_buff, local_ip6[0], local_ip6[3]):
 	case bpf_ctx_range_till(struct __sk_buff, remote_ip4, remote_ip4):
 	case bpf_ctx_range_till(struct __sk_buff, local_ip4, local_ip4):
-	case bpf_ctx_range(struct __sk_buff, data):
-	case bpf_ctx_range(struct __sk_buff, data_meta):
-	case bpf_ctx_range(struct __sk_buff, data_end):
 		if (size != size_default)
 			return false;
 		break;
@@ -9029,6 +9032,14 @@ static bool xdp_is_valid_access(int off, int size,
 			}
 		}
 		return false;
+	} else {
+		switch (off) {
+		case offsetof(struct xdp_md, data_meta):
+		case offsetof(struct xdp_md, data):
+		case offsetof(struct xdp_md, data_end):
+			if (info->is_ldsx)
+				return false;
+		}
 	}
 
 	switch (off) {
@@ -9354,12 +9365,12 @@ static bool flow_dissector_is_valid_access(int off, int size,
 
 	switch (off) {
 	case bpf_ctx_range(struct __sk_buff, data):
-		if (size != size_default)
+		if (info->is_ldsx || size != size_default)
 			return false;
 		info->reg_type = PTR_TO_PACKET;
 		return true;
 	case bpf_ctx_range(struct __sk_buff, data_end):
-		if (size != size_default)
+		if (info->is_ldsx || size != size_default)
 			return false;
 		info->reg_type = PTR_TO_PACKET_END;
 		return true;