It is possible we reach bpf_convert_ctx_access() with
si->dst_reg == si->src_reg
Therefore, we need to load BPF_REG_AX before eventually
mangling si->src_reg.
syzbot generated this x86 code :
3: 55 push %rbp
4: 48 89 e5 mov %rsp,%rbp
7: 48 81 ec 00 00 00 00 sub $0x0,%rsp // Might be avoided ?
e: 53 push %rbx
f: 41 55 push %r13
11: 41 56 push %r14
13: 41 57 push %r15
15: 6a 00 pushq $0x0
17: 31 c0 xor %eax,%eax
19: 48 8b bf c0 00 00 00 mov 0xc0(%rdi),%rdi
20: 44 8b 97 bc 00 00 00 mov 0xbc(%rdi),%r10d
27: 4c 01 d7 add %r10,%rdi
2a: 48 0f b7 7f 06 movzwq 0x6(%rdi),%rdi // Crash
2f: 5b pop %rbx
30: 41 5f pop %r15
32: 41 5e pop %r14
34: 41 5d pop %r13
36: 5b pop %rbx
37: c9 leaveq
38: c3 retq
Fixes: d9ff286a0f59 ("bpf: allow BPF programs access skb_shared_info->gso_segs
field")
Signed-off-by: Eric Dumazet <eduma...@google.com>
Reported-by: syzbot <syzkal...@googlegroups.com>
---
net/core/filter.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/net/core/filter.c b/net/core/filter.c
index
4e2a79b2fd77f36ba2a31e9e43af1abc1207766e..7878f918b8c057b7b90ca0afcf2d5773cfb55e15
100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -7455,12 +7455,12 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type
type,
case offsetof(struct __sk_buff, gso_segs):
/* si->dst_reg = skb_shinfo(SKB); */
#ifdef NET_SKBUFF_DATA_USES_OFFSET
- *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, head),
- si->dst_reg, si->src_reg,
- offsetof(struct sk_buff, head));
*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, end),
BPF_REG_AX, si->src_reg,
offsetof(struct sk_buff, end));
+ *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, head),
+ si->dst_reg, si->src_reg,
+ offsetof(struct sk_buff, head));
*insn++ = BPF_ALU64_REG(BPF_ADD, si->dst_reg, BPF_REG_AX);
#else
*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, end),
--
2.22.0.657.g960e92d24f-goog
