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

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/000000000000c90eee061d236d37@google.com/

Reported-by: syzbot+ad9ec60c8eaf69e6f99c@syzkaller.appspotmail.com
Fixes: 1f1e864b6555 ("bpf: Handle sign-extenstin ctx member accesses")
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240723153439.2429035-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

1 files changed, 16 insertions, 5 deletions

diff --git a/net/core/filter.c b/net/core/filter.c
index f3c72cf86099..78a6f746ea0b 100644
--- a/net/core/filter.c
+++ b/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;