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

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

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