bpf, x86: Fix encoding for lower 8-bit registers in BPF_STX BPF_B

This patch fixes an encoding bug in emit_stx for BPF_B when the source
register is BPF_REG_FP.

The current implementation for BPF_STX BPF_B in emit_stx saves one REX
byte when the operands can be encoded using Mod-R/M alone. The lower 8
bits of registers %rax, %rbx, %rcx, and %rdx can be accessed without using
a REX prefix via %al, %bl, %cl, and %dl, respectively. Other registers,
(e.g., %rsi, %rdi, %rbp, %rsp) require a REX prefix to use their 8-bit
equivalents (%sil, %dil, %bpl, %spl).

The current code checks if the source for BPF_STX BPF_B is BPF_REG_1
or BPF_REG_2 (which map to %rdi and %rsi), in which case it emits the
required REX prefix. However, it misses the case when the source is
BPF_REG_FP (mapped to %rbp).

The result is that BPF_STX BPF_B with BPF_REG_FP as the source operand
will read from register %ch instead of the correct %bpl. This patch fixes
the problem by fixing and refactoring the check on which registers need
the extra REX byte. Since no BPF registers map to %rsp, there is no need
to handle %spl.

Fixes: 622582786c9e0 ("net: filter: x86: internal BPF JIT")
Signed-off-by: Xi Wang <[e-mail]>
Signed-off-by: Luke Nelson <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

arch/x86/net/bpf_jit_comp.c

···
 			     BIT(BPF_REG_AX));
 }
 
+/*
+ * is_ereg_8l() == true if BPF register 'reg' is mapped to access x86-64
+ * lower 8-bit registers dil,sil,bpl,spl,r8b..r15b, which need extra byte
+ * of encoding. al,cl,dl,bl have simpler encoding.
+ */
+static bool is_ereg_8l(u32 reg)
+{
+	return is_ereg(reg) ||
+	    (1 << reg) & (BIT(BPF_REG_1) |
+			  BIT(BPF_REG_2) |
+			  BIT(BPF_REG_FP));
+}
+
 static bool is_axreg(u32 reg)
 {
 	return reg == BPF_REG_0;
···
 	switch (size) {
 	case BPF_B:
 		/* Emit 'mov byte ptr [rax + off], al' */
-		if (is_ereg(dst_reg) || is_ereg(src_reg) ||
-		    /* We have to add extra byte for x86 SIL, DIL regs */
-		    src_reg == BPF_REG_1 || src_reg == BPF_REG_2)
+		if (is_ereg(dst_reg) || is_ereg_8l(src_reg))
+			/* Add extra byte for eregs or SIL,DIL,BPL in src_reg */
 			EMIT2(add_2mod(0x40, dst_reg, src_reg), 0x88);
 		else
 			EMIT1(0x88);