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From:   Luke Nelson <lukenels@cs.washington.edu>
To:     bpf@vger.kernel.org
Cc:     Brian Gerst <brgerst@gmail.com>,
        Luke Nelson <luke.r.nels@gmail.com>,
        Xi Wang <xi.wang@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Wang YanQing <udknight@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH bpf v2 2/2] bpf, x86_32: Fix clobbering of dst for BPF_JSET
Date:   Wed, 22 Apr 2020 10:36:30 -0700
Message-Id: <20200422173630.8351-2-luke.r.nels@gmail.com>
In-Reply-To: <20200422173630.8351-1-luke.r.nels@gmail.com>
References: <20200422173630.8351-1-luke.r.nels@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

The current JIT clobbers the destination register for BPF_JSET BPF_X
and BPF_K by using "and" and "or" instructions. This is fine when the
destination register is a temporary loaded from a register stored on
the stack but not otherwise.

This patch fixes the problem (for both BPF_K and BPF_X) by always loading
the destination register into temporaries since BPF_JSET should not
modify the destination register.

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.

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>
---
v1 -> v2: No changes.
---
 arch/x86/net/bpf_jit_comp32.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp32.c b/arch/x86/net/bpf_jit_comp32.c
index cc9ad3892ea6..ba7d9ccfc662 100644
--- a/arch/x86/net/bpf_jit_comp32.c
+++ b/arch/x86/net/bpf_jit_comp32.c
@@ -2015,8 +2015,8 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
 		case BPF_JMP | BPF_JSET | BPF_X:
 		case BPF_JMP32 | BPF_JSET | BPF_X: {
 			bool is_jmp64 = BPF_CLASS(insn->code) == BPF_JMP;
-			u8 dreg_lo = dstk ? IA32_EAX : dst_lo;
-			u8 dreg_hi = dstk ? IA32_EDX : dst_hi;
+			u8 dreg_lo = IA32_EAX;
+			u8 dreg_hi = IA32_EDX;
 			u8 sreg_lo = sstk ? IA32_ECX : src_lo;
 			u8 sreg_hi = sstk ? IA32_EBX : src_hi;
 
@@ -2028,6 +2028,13 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
 					      add_2reg(0x40, IA32_EBP,
 						       IA32_EDX),
 					      STACK_VAR(dst_hi));
+			} else {
+				/* mov dreg_lo,dst_lo */
+				EMIT2(0x89, add_2reg(0xC0, dreg_lo, dst_lo));
+				if (is_jmp64)
+					/* mov dreg_hi,dst_hi */
+					EMIT2(0x89,
+					      add_2reg(0xC0, dreg_hi, dst_hi));
 			}
 
 			if (sstk) {
@@ -2052,8 +2059,8 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
 		case BPF_JMP | BPF_JSET | BPF_K:
 		case BPF_JMP32 | BPF_JSET | BPF_K: {
 			bool is_jmp64 = BPF_CLASS(insn->code) == BPF_JMP;
-			u8 dreg_lo = dstk ? IA32_EAX : dst_lo;
-			u8 dreg_hi = dstk ? IA32_EDX : dst_hi;
+			u8 dreg_lo = IA32_EAX;
+			u8 dreg_hi = IA32_EDX;
 			u8 sreg_lo = IA32_ECX;
 			u8 sreg_hi = IA32_EBX;
 			u32 hi;
@@ -2066,6 +2073,13 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
 					      add_2reg(0x40, IA32_EBP,
 						       IA32_EDX),
 					      STACK_VAR(dst_hi));
+			} else {
+				/* mov dreg_lo,dst_lo */
+				EMIT2(0x89, add_2reg(0xC0, dreg_lo, dst_lo));
+				if (is_jmp64)
+					/* mov dreg_hi,dst_hi */
+					EMIT2(0x89,
+					      add_2reg(0xC0, dreg_hi, dst_hi));
 			}
 
 			/* mov ecx,imm32 */
-- 
2.17.1