[PATCH bpf v2 1/2] bpf: Fix same-register dst/src OOB read and pointer leak in sock_ops

Jiayuan Chen
2026-04-06T03:14:14Z

When a BPF sock_ops program accesses ctx fields with dst_reg == src_reg,
the SOCK_OPS_GET_SK() and SOCK_OPS_GET_FIELD() macros fail to zero the
destination register in the !fullsock / !locked_tcp_sock path.

Both macros borrow a temporary register to check is_fullsock /
is_locked_tcp_sock when dst_reg == src_reg, because dst_reg holds the
ctx pointer. When the check is false (e.g., TCP_NEW_SYN_RECV state with
a request_sock), dst_reg should be zeroed but is not, leaving the stale
ctx pointer:

- SOCK_OPS_GET_SK: dst_reg retains the ctx pointer, passes NULL checks
as PTR_TO_SOCKET_OR_NULL, and can be used as a bogus socket pointer,
leading to stack-out-of-bounds access in helpers like
bpf_skc_to_tcp6_sock().

- SOCK_OPS_GET_FIELD: dst_reg retains the ctx pointer which the
verifier believes is a SCALAR_VALUE, leaking a kernel pointer.

Fix both macros by:
- Changing JMP_A(1) to JMP_A(2) in the fullsock path to skip the
added instruction.
- Adding BPF_MOV64_IMM(si->dst_reg, 0) after the temp register
restore in the !fullsock path, placed after the restore because
dst_reg == src_reg means we need src_reg intact to read ctx->temp.

Fixes: 84f44df664e9 ("bpf: sock_ops sk access may stomp registers when dst_reg = src_reg")
Reported-by: Quan Sun <2022090917019@std.uestc.edu.cn>
Reported-by: Yinhao Hu <dddddd@hust.edu.cn>
Reported-by: Kaiyan Mei <M202472210@hust.edu.cn>
Reported-by: Dongliang Mu <dzm91@hust.edu.cn>
Closes: https://lore.kernel.org/bpf/6fe1243e-149b-4d3b-99c7-fcc9e2f75787@std.uestc.edu.cn/T/#u
Suggested-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
---
net/core/filter.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 78b548158fb05..53ce06ed4a88e 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -10581,10 +10581,11 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
si->dst_reg, si->dst_reg, \
offsetof(OBJ, OBJ_FIELD)); \
if (si->dst_reg == si->src_reg) { \
- *insn++ = BPF_JMP_A(1); \
+ *insn++ = BPF_JMP_A(2); \
*insn++ = BPF_LDX_MEM(BPF_DW, reg, si->src_reg, \
offsetof(struct bpf_sock_ops_kern, \
temp)); \
+ *insn++ = BPF_MOV64_IMM(si->dst_reg, 0); \
} \
} while (0)

@@ -10618,10 +10619,11 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
si->dst_reg, si->src_reg, \
offsetof(struct bpf_sock_ops_kern, sk));\
if (si->dst_reg == si->src_reg) { \
- *insn++ = BPF_JMP_A(1); \
+ *insn++ = BPF_JMP_A(2); \
*insn++ = BPF_LDX_MEM(BPF_DW, reg, si->src_reg, \
offsetof(struct bpf_sock_ops_kern, \
temp)); \
+ *insn++ = BPF_MOV64_IMM(si->dst_reg, 0); \
} \
} while (0)

--
2.43.0