[PATCH 5.15 824/917] bpf, sockmap: sk_skb data_end access incorrect when src_reg = dst_reg

Greg Kroah-Hartman
2021-11-16T00:36:35Z

From: Jussi Maki <joamaki@gmail.com>

[ Upstream commit b2c4618162ec615a15883a804cce7e27afecfa58 ]

The current conversion of skb->data_end reads like this:

; data_end = (void*)(long)skb->data_end;
559: (79) r1 = *(u64 *)(r2 +200) ; r1 = skb->data
560: (61) r11 = *(u32 *)(r2 +112) ; r11 = skb->len
561: (0f) r1 += r11
562: (61) r11 = *(u32 *)(r2 +116)
563: (1f) r1 -= r11

But similar to the case in 84f44df664e9 ("bpf: sock_ops sk access may stomp
registers when dst_reg = src_reg"), the code will read an incorrect skb->len
when src == dst. In this case we end up generating this xlated code:

; data_end = (void*)(long)skb->data_end;
559: (79) r1 = *(u64 *)(r1 +200) ; r1 = skb->data
560: (61) r11 = *(u32 *)(r1 +112) ; r11 = (skb->data)->len
561: (0f) r1 += r11
562: (61) r11 = *(u32 *)(r1 +116)
563: (1f) r1 -= r11

... where line 560 is the reading 4B of (skb->data + 112) instead of the
intended skb->len Here the skb pointer in r1 gets set to skb->data and the
later deref for skb->len ends up following skb->data instead of skb.

This fixes the issue similarly to the patch mentioned above by creating an
additional temporary variable and using to store the register when dst_reg =
src_reg. We name the variable bpf_temp_reg and place it in the cb context for
sk_skb. Then we restore from the temp to ensure nothing is lost.

Fixes: 16137b09a66f2 ("bpf: Compute data_end dynamically with JIT code")
Signed-off-by: Jussi Maki <joamaki@gmail.com>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20211103204736.248403-6-john.fastabend@gmail.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/net/strparser.h | 4 ++++
net/core/filter.c | 36 ++++++++++++++++++++++++++++++------
2 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/include/net/strparser.h b/include/net/strparser.h
index bec1439bd3be6..732b7097d78e4 100644
--- a/include/net/strparser.h
+++ b/include/net/strparser.h
@@ -66,6 +66,10 @@ struct sk_skb_cb {
#define SK_SKB_CB_PRIV_LEN 20
unsigned char data[SK_SKB_CB_PRIV_LEN];
struct _strp_msg strp;
+ /* temp_reg is a temporary register used for bpf_convert_data_end_access
+ * when dst_reg == src_reg.
+ */
+ u64 temp_reg;
};

static inline struct strp_msg *strp_msg(struct sk_buff *skb)
diff --git a/net/core/filter.c b/net/core/filter.c
index 23a9bf92b5bb9..f4a63af45f008 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -9735,22 +9735,46 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
static struct bpf_insn *bpf_convert_data_end_access(const struct bpf_insn *si,
struct bpf_insn *insn)
{
- /* si->dst_reg = skb->data */
+ int reg;
+ int temp_reg_off = offsetof(struct sk_buff, cb) +
+ offsetof(struct sk_skb_cb, temp_reg);
+
+ if (si->src_reg == si->dst_reg) {
+ /* We need an extra register, choose and save a register. */
+ reg = BPF_REG_9;
+ if (si->src_reg == reg || si->dst_reg == reg)
+ reg--;
+ if (si->src_reg == reg || si->dst_reg == reg)
+ reg--;
+ *insn++ = BPF_STX_MEM(BPF_DW, si->src_reg, reg, temp_reg_off);
+ } else {
+ reg = si->dst_reg;
+ }
+
+ /* reg = skb->data */
*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, data),
- si->dst_reg, si->src_reg,
+ reg, si->src_reg,
offsetof(struct sk_buff, data));
/* AX = skb->len */
*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, len),
BPF_REG_AX, si->src_reg,
offsetof(struct sk_buff, len));
- /* si->dst_reg = skb->data + skb->len */
- *insn++ = BPF_ALU64_REG(BPF_ADD, si->dst_reg, BPF_REG_AX);
+ /* reg = skb->data + skb->len */
+ *insn++ = BPF_ALU64_REG(BPF_ADD, reg, BPF_REG_AX);
/* AX = skb->data_len */
*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, data_len),
BPF_REG_AX, si->src_reg,
offsetof(struct sk_buff, data_len));
- /* si->dst_reg = skb->data + skb->len - skb->data_len */
- *insn++ = BPF_ALU64_REG(BPF_SUB, si->dst_reg, BPF_REG_AX);
+
+ /* reg = skb->data + skb->len - skb->data_len */
+ *insn++ = BPF_ALU64_REG(BPF_SUB, reg, BPF_REG_AX);
+
+ if (si->src_reg == si->dst_reg) {
+ /* Restore the saved register */
+ *insn++ = BPF_MOV64_REG(BPF_REG_AX, si->src_reg);
+ *insn++ = BPF_MOV64_REG(si->dst_reg, reg);
+ *insn++ = BPF_LDX_MEM(BPF_DW, reg, BPF_REG_AX, temp_reg_off);
+ }

return insn;
}
--
2.33.0