This is a note to let you know that I've just added the patch titled
bpf, x86_32: Fix incorrect encoding in BPF_LDX zero-extension
to the 5.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
bpf-x86_32-fix-incorrect-encoding-in-bpf_ldx-zero-ex.patch
and it can be found in the queue-5.6 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 512494b0ca46b4b302443abb0dafbe317183a2ae
Author: Luke Nelson <lukenels@xxxxxxxxxxxxxxxxx>
Date: Wed Apr 22 10:36:29 2020 -0700
bpf, x86_32: Fix incorrect encoding in BPF_LDX zero-extension
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 <xi.wang@xxxxxxxxx>
Signed-off-by: Luke Nelson <luke.r.nels@xxxxxxxxx>
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Reviewed-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
Acked-by: Wang YanQing <udknight@xxxxxxxxx>
Link: https://lore.kernel.org/bpf/20200422173630.8351-1-luke.r.nels@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
diff --git a/arch/x86/net/bpf_jit_comp32.c b/arch/x86/net/bpf_jit_comp32.c
index 4d2a7a7646026..cc9ad3892ea6b 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:
