From: Luke Nelson <[email protected]>
commit 5fa9a98fb10380e48a398998cd36a85e4ef711d6 upstream.
The current JIT uses the following sequence to zero-extend into the
upper 32 bits of the destination register for BPF_LDX BPF_{B,H,W},
when the destination register is not on the stack:
EMIT3(0xC7, add_1reg(0xC0, dst_hi), 0);
The problem is that C7 /0 encodes a MOV instruction that requires a 4-byte
immediate; the current code emits only 1 byte of the immediate. This
means that the first 3 bytes of the next instruction will be treated as
the rest of the immediate, breaking the stream of instructions.
This patch fixes the problem by instead emitting "xor dst_hi,dst_hi"
to clear the upper 32 bits. This fixes the problem and is more efficient
than using MOV to load a zero immediate.
This bug may not be currently triggerable as BPF_REG_AX is the only
register not stored on the stack and the verifier uses it in a limited
way, and the verifier implements a zero-extension optimization. But the
JIT should avoid emitting incorrect encodings regardless.
Fixes: 03f5781be2c7b ("bpf, x86_32: add eBPF JIT compiler for ia32")
Signed-off-by: Xi Wang <[email protected]>
Signed-off-by: Luke Nelson <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Reviewed-by: H. Peter Anvin (Intel) <[email protected]>
Acked-by: Wang YanQing <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
arch/x86/net/bpf_jit_comp32.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
--- a/arch/x86/net/bpf_jit_comp32.c
+++ b/arch/x86/net/bpf_jit_comp32.c
@@ -1854,7 +1854,9 @@ static int do_jit(struct bpf_prog *bpf_p
STACK_VAR(dst_hi));
EMIT(0x0, 4);
} else {
- EMIT3(0xC7, add_1reg(0xC0, dst_hi), 0);
+ /* xor dst_hi,dst_hi */
+ EMIT2(0x33,
+ add_2reg(0xC0, dst_hi, dst_hi));
}
break;
case BPF_DW:
