[PATCH bpf v1 1/2] bpf: Fix SOCK_OPS_GET_SK same-register OOB read in sock_ops

[Jiayuan Chen <jiayuan.chen@linux.dev>]

When a BPF sock_ops program reads ctx->sk with dst_reg == src_reg
(e.g., r1 = *(u64 *)(r1 + offsetof(sk))), the SOCK_OPS_GET_SK() macro
fails to zero the destination register in the is_fullsock == 0 path.

The macro saves/restores a temporary register and checks is_fullsock.
When is_fullsock == 0 (e.g., TCP_NEW_SYN_RECV state with a request_sock),
it should set dst_reg = 0 (NULL) so the verifier's PTR_TO_SOCKET_OR_NULL
type is correct at runtime. Instead, dst_reg retains the original ctx
pointer, which passes subsequent NULL checks and can be used as a bogus
socket pointer, leading to stack-out-of-bounds access in helpers like
bpf_skc_to_tcp6_sock().

Fix 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
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
Apologies for the Easter timing!
---
 net/core/filter.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 78b548158fb05..8fee00e6adef4 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -10618,10 +10618,11 @@ 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