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authorLuke Nelson <lukenels@cs.washington.edu>2020-04-22 10:36:29 -0700
committerAlexei Starovoitov <ast@kernel.org>2020-04-24 17:11:46 -0700
commit5fa9a98fb10380e48a398998cd36a85e4ef711d6 (patch)
tree4b5bd1281976009db88b347a0cc7007ddd26919d /arch/x86/net
parenta33d3147945543f9ded67a052f358a75595f1ecb (diff)
bpf, x86_32: Fix incorrect encoding in BPF_LDX zero-extension
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 <xi.wang@gmail.com> Signed-off-by: Luke Nelson <luke.r.nels@gmail.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com> Acked-by: Wang YanQing <udknight@gmail.com> Link: https://lore.kernel.org/bpf/20200422173630.8351-1-luke.r.nels@gmail.com
Diffstat (limited to 'arch/x86/net')
-rw-r--r--arch/x86/net/bpf_jit_comp32.c4
1 files changed, 3 insertions, 1 deletions
diff --git a/arch/x86/net/bpf_jit_comp32.c b/arch/x86/net/bpf_jit_comp32.c
index 4d2a7a764602..cc9ad3892ea6 100644
--- 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_prog, int *addrs, u8 *image,
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: