[bpf-next,1/2] bpf: Fail verification for sign-extension of packet data/data_end/data_meta

Commit Message

Yonghong Song July 15, 2024, 8:18 p.m. UTC

syzbot reported a kernel crash due to
  commit 1f1e864b6555 ("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/[e-mail]/

Reported-by: [e-mail]
Fixes: 1f1e864b6555 ("bpf: Handle sign-extenstin ctx member accesses")
Signed-off-by: Yonghong Song <[e-mail]>
---
 include/linux/bpf.h   |  1 +
 kernel/bpf/verifier.c |  5 +++--
 net/core/filter.c     | 21 ++++++++++++++++-----
 3 files changed, 20 insertions(+), 7 deletions(-)

Comments

Eduard Zingerman July 16, 2024, 7:49 p.m. UTC | #1

On Mon, 2024-07-15 at 13:18 -0700, Yonghong Song wrote:
> syzbot reported a kernel crash due to
>   commit 1f1e864b6555 ("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.

[...]

> 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/[e-mail]/
> 
> Reported-by: [e-mail]
> Fixes: 1f1e864b6555 ("bpf: Handle sign-extenstin ctx member accesses")
> Signed-off-by: Yonghong Song <[e-mail]>
> ---

I looked through all context types and seems like only two types
identified in this patch use u32 values to obtain pointers:
- struct xdp_md       fields: data, data_end, data_meta
- struct __sk_buff    fields: data, data_end, data_meta

Double checked all locations where access to the above fields is
verified, every location is covered by is_ldsx check.

Acked-by: Eduard Zingerman <[e-mail]>

Patch

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 4f1d4a97b9d1..d7ca17bd314e 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -919,6 +919,7 @@  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 {
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 8da132a1ef28..3a04eab7a962 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5587,11 +5587,12 @@  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)
+			    struct btf **btf, u32 *btf_id, bool is_ldsx)
 {
 	struct bpf_insn_access_aux info = {
 		.reg_type = *reg_type,
 		.log = &env->log,
+		.is_ldsx = is_ldsx,
 	};
 
 	if (env->ops->is_valid_access &&
@@ -6891,7 +6892,7 @@  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);
+				       &btf_id, is_ldsx);
 		if (err)
 			verbose_linfo(env, insn_idx, "; ");
 		if (!err && t == BPF_READ && value_regno >= 0) {
diff --git a/net/core/filter.c b/net/core/filter.c
index 4cf1d34f7617..771872e43e3f 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -8572,13 +8572,16 @@  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;
@@ -9022,6 +9025,14 @@  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) {
@@ -9347,12 +9358,12 @@  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;