FAILED: patch "[PATCH] bpf, x86: Fix encoding for lower 8-bit registers in BPF_STX" failed to apply to 4.19-stable tree

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From aee194b14dd2b2bde6252b3acf57d36dccfc743a Mon Sep 17 00:00:00 2001
From: Luke Nelson <[email protected]>
Date: Sat, 18 Apr 2020 16:26:53 -0700
Subject: [PATCH] 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.

index 5ea7c2cf7ab4..42b6709e6dc7 100644
@@ -158,6 +158,19 @@ static bool is_ereg(u32 reg)
 			     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;

@@ -598,9 +611,8 @@ static void emit_stx(u8 **pprog, u32 size, u32 dst_reg, u32 src_reg, int off)
 	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);