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

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syzbot reported a kernel crash due to
  commit 1f1e864b65 ("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: 1f1e864b65 ("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>
This commit is contained in:
Yonghong Song 2024-07-23 08:34:39 -07:00 committed by Andrii Nakryiko
parent f86601c366
commit 92de36080c
3 changed files with 20 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/bpf.h
View File

@ -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 {

5
kernel/bpf/verifier.c
View File

@ -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) {

21
net/core/filter.c
View File

@ -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;