From: Luke Nelson lukenels@cs.washington.edu
[ Upstream commit aee194b14dd2b2bde6252b3acf57d36dccfc743a ]
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 xi.wang@gmail.com Signed-off-by: Luke Nelson luke.r.nels@gmail.com Signed-off-by: Alexei Starovoitov ast@kernel.org Link: https://lore.kernel.org/bpf/20200418232655.23870-1-luke.r.nels@gmail.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/net/bpf_jit_comp.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index cdb386fa71013..0415c0cd4a191 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -153,6 +153,19 @@ 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)); +} + /* add modifiers if 'reg' maps to x64 registers r8..r15 */ static u8 add_1mod(u8 byte, u32 reg) { @@ -770,9 +783,8 @@ st: if (is_imm8(insn->off)) /* STX: *(u8*)(dst_reg + off) = src_reg */ case BPF_STX | BPF_MEM | BPF_B: /* emit 'mov byte ptr [rax + off], al' */ - if (is_ereg(dst_reg) || is_ereg(src_reg) || - /* 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);