[PATCH] bpf, x86_32: Fix clobbering of dst for BPF_JSET

[Wang YanQing <udknight@gmail.com>]

commit 50fe7ebb6475711c15b3397467e6424e20026d94 upstream.

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>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Wang YanQing <udknight@gmail.com>
Link: https://lore.kernel.org/bpf/20200422173630.8351-2-luke.r.nels@gmail.com
Signed-off-by: Wang YanQing <udknight@gmail.com>
---
 arch/x86/net/bpf_jit_comp32.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp32.c b/arch/x86/net/bpf_jit_comp32.c
index 24d573b..7e7ca5d 100644
--- a/arch/x86/net/bpf_jit_comp32.c
+++ b/arch/x86/net/bpf_jit_comp32.c
@@ -1968,8 +1968,8 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
 			goto emit_cond_jmp_signed;
 		}
 		case BPF_JMP | BPF_JSET | BPF_X: {
-			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;
 
@@ -1978,6 +1978,12 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
 				      STACK_VAR(dst_lo));
 				EMIT3(0x8B, 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));
+				/* mov dreg_hi,dst_hi */
+				EMIT2(0x89,
+				      add_2reg(0xC0, dreg_hi, dst_hi));
 			}
 
 			if (sstk) {
@@ -1996,8 +2002,8 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
 		}
 		case BPF_JMP | BPF_JSET | BPF_K: {
 			u32 hi;
-			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;
 
@@ -2006,6 +2012,12 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
 				      STACK_VAR(dst_lo));
 				EMIT3(0x8B, 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));
+				/* mov dreg_hi,dst_hi */
+				EMIT2(0x89,
+				      add_2reg(0xC0, dreg_hi, dst_hi));
 			}
 			hi = imm32 & (1<<31) ? (u32)~0 : 0;
 
-- 
1.8.5.6.2.g3d8a54e.dirty