From ced185824c89b60e65b5a2606954c098320cdfb8 Mon Sep 17 00:00:00 2001 From: Johan Almbladh Date: Mon, 27 Sep 2021 13:11:57 +0000 Subject: [PATCH] bpf, x86: Fix bpf mapping of atomic fetch implementation Fix the case where the dst register maps to %rax as otherwise this produces an incorrect mapping with the implementation in 981f94c3e921 ("bpf: Add bitwise atomic instructions") as %rax is clobbered given it's part of the cmpxchg as operand. The issue is similar to b29dd96b905f ("bpf, x86: Fix BPF_FETCH atomic and/or/ xor with r0 as src") just that the case of dst register was missed. Before, dst=r0 (%rax) src=r2 (%rsi): [...] c5: mov %rax,%r10 c8: mov 0x0(%rax),%rax <---+ (broken) cc: mov %rax,%r11 | cf: and %rsi,%r11 | d2: lock cmpxchg %r11,0x0(%rax) <---+ d8: jne 0x00000000000000c8 | da: mov %rax,%rsi | dd: mov %r10,%rax | [...] | | After, dst=r0 (%rax) src=r2 (%rsi): | | [...] | da: mov %rax,%r10 | dd: mov 0x0(%r10),%rax <---+ (fixed) e1: mov %rax,%r11 | e4: and %rsi,%r11 | e7: lock cmpxchg %r11,0x0(%r10) <---+ ed: jne 0x00000000000000dd ef: mov %rax,%rsi f2: mov %r10,%rax [...] The remaining combinations were fine as-is though: After, dst=r9 (%r15) src=r0 (%rax): [...] dc: mov %rax,%r10 df: mov 0x0(%r15),%rax e3: mov %rax,%r11 e6: and %r10,%r11 e9: lock cmpxchg %r11,0x0(%r15) ef: jne 0x00000000000000df _ f1: mov %rax,%r10 | (unneeded, but f4: mov %r10,%rax _| not a problem) [...] After, dst=r9 (%r15) src=r4 (%rcx): [...] de: mov %rax,%r10 e1: mov 0x0(%r15),%rax e5: mov %rax,%r11 e8: and %rcx,%r11 eb: lock cmpxchg %r11,0x0(%r15) f1: jne 0x00000000000000e1 f3: mov %rax,%rcx f6: mov %r10,%rax [...] The case of dst == src register is rejected by the verifier and therefore not supported, but x86 JIT also handles this case just fine. After, dst=r0 (%rax) src=r0 (%rax): [...] eb: mov %rax,%r10 ee: mov 0x0(%r10),%rax f2: mov %rax,%r11 f5: and %r10,%r11 f8: lock cmpxchg %r11,0x0(%r10) fe: jne 0x00000000000000ee 100: mov %rax,%r10 103: mov %r10,%rax [...] Fixes: 981f94c3e921 ("bpf: Add bitwise atomic instructions") Reported-by: Johan Almbladh Signed-off-by: Johan Almbladh Co-developed-by: Daniel Borkmann Signed-off-by: Daniel Borkmann Reviewed-by: Brendan Jackman Acked-by: Alexei Starovoitov --- arch/x86/net/bpf_jit_comp.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index d24a512fd6f3..9ea57389c554 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -1341,9 +1341,10 @@ st: if (is_imm8(insn->off)) if (insn->imm == (BPF_AND | BPF_FETCH) || insn->imm == (BPF_OR | BPF_FETCH) || insn->imm == (BPF_XOR | BPF_FETCH)) { - u8 *branch_target; bool is64 = BPF_SIZE(insn->code) == BPF_DW; u32 real_src_reg = src_reg; + u32 real_dst_reg = dst_reg; + u8 *branch_target; /* * Can't be implemented with a single x86 insn. @@ -1354,11 +1355,13 @@ st: if (is_imm8(insn->off)) emit_mov_reg(&prog, true, BPF_REG_AX, BPF_REG_0); if (src_reg == BPF_REG_0) real_src_reg = BPF_REG_AX; + if (dst_reg == BPF_REG_0) + real_dst_reg = BPF_REG_AX; branch_target = prog; /* Load old value */ emit_ldx(&prog, BPF_SIZE(insn->code), - BPF_REG_0, dst_reg, insn->off); + BPF_REG_0, real_dst_reg, insn->off); /* * Perform the (commutative) operation locally, * put the result in the AUX_REG. @@ -1369,7 +1372,8 @@ st: if (is_imm8(insn->off)) add_2reg(0xC0, AUX_REG, real_src_reg)); /* Attempt to swap in new value */ err = emit_atomic(&prog, BPF_CMPXCHG, - dst_reg, AUX_REG, insn->off, + real_dst_reg, AUX_REG, + insn->off, BPF_SIZE(insn->code)); if (WARN_ON(err)) return err; @@ -1383,11 +1387,10 @@ st: if (is_imm8(insn->off)) /* Restore R0 after clobbering RAX */ emit_mov_reg(&prog, true, BPF_REG_0, BPF_REG_AX); break; - } err = emit_atomic(&prog, insn->imm, dst_reg, src_reg, - insn->off, BPF_SIZE(insn->code)); + insn->off, BPF_SIZE(insn->code)); if (err) return err; break;