bpf, x86_32: Fix clobbering of dst for BPF_JSET

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

diff --git a/arch/x86/net/bpf_jit_comp32.c b/arch/x86/net/bpf_jit_comp32.c
index cc9ad3892ea6bde7bd79acfd68c5b9e723325ae9..ba7d9ccfc66263c21c3a50c8af948f330c0ad7c3 100644 (file)
--- 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 */