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@xxxxxxxxxx/
Reported-by: syzbot+ad9ec60c8eaf69e6f99c@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 1f1e864b6555 ("bpf: Handle sign-extenstin ctx member accesses")
Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
include/linux/bpf.h | 1 +
kernel/bpf/verifier.c | 5 +++--
net/core/filter.c | 21 ++++++++++++++++-----
3 files changed, 20 insertions(+), 7 deletions(-)
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, ®_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;
--
2.43.0
