bpf, x86: Fix BPF_FETCH atomic and/or/xor with r0 as src

This code generates a CMPXCHG loop in order to implement atomic_fetch
bitwise operations. Because CMPXCHG is hard-coded to use rax (which
holds the BPF r0 value), it saves the _real_ r0 value into the
internal "ax" temporary register and restores it once the loop is
complete.

In the middle of the loop, the actual bitwise operation is performed
using src_reg. The bug occurs when src_reg is r0: as described above,
r0 has been clobbered and the real r0 value is in the ax register.

Therefore, perform this operation on the ax register instead, when
src_reg is r0.

Fixes: 981f94c ("bpf: Add bitwise atomic instructions")
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: KP Singh <kpsingh@kernel.org>
Link: https://lore.kernel.org/bpf/20210216125307.1406237-1-jackmanb@google.com

arch/x86/net/bpf_jit_comp.c

@@ -1349,6 +1349,7 @@ st:			if (is_imm8(insn->off))
 			    insn->imm == (BPF_XOR | BPF_FETCH)) {
 				u8 *branch_target;
 				bool is64 = BPF_SIZE(insn->code) == BPF_DW;
+				u32 real_src_reg = src_reg;
 
 				/*
 				 * Can't be implemented with a single x86 insn.
@@ -1357,6 +1358,9 @@ st:			if (is_imm8(insn->off))
 
 				/* Will need RAX as a CMPXCHG operand so save R0 */
 				emit_mov_reg(&prog, true, BPF_REG_AX, BPF_REG_0);
+				if (src_reg == BPF_REG_0)
+					real_src_reg = BPF_REG_AX;
+
 				branch_target = prog;
 				/* Load old value */
 				emit_ldx(&prog, BPF_SIZE(insn->code),
@@ -1366,9 +1370,9 @@ st:			if (is_imm8(insn->off))
 				 * put the result in the AUX_REG.
 				 */
 				emit_mov_reg(&prog, is64, AUX_REG, BPF_REG_0);
-				maybe_emit_mod(&prog, AUX_REG, src_reg, is64);
+				maybe_emit_mod(&prog, AUX_REG, real_src_reg, is64);
 				EMIT2(simple_alu_opcodes[BPF_OP(insn->imm)],
-				      add_2reg(0xC0, AUX_REG, src_reg));
+				      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,
@@ -1381,7 +1385,7 @@ st:			if (is_imm8(insn->off))
 				 */
 				EMIT2(X86_JNE, -(prog - branch_target) - 2);
 				/* Return the pre-modification value */
-				emit_mov_reg(&prog, is64, src_reg, BPF_REG_0);
+				emit_mov_reg(&prog, is64, real_src_reg, BPF_REG_0);
 				/* Restore R0 after clobbering RAX */
 				emit_mov_reg(&prog, true, BPF_REG_0, BPF_REG_AX);
 				break;

tools/testing/selftests/bpf/verifier/atomic_and.c

@@ -75,3 +75,26 @@
 	},
 	.result = ACCEPT,
 },
+{
+	"BPF_ATOMIC_AND with fetch - r0 as source reg",
+	.insns = {
+		/* val = 0x110; */
+		BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0x110),
+		/* old = atomic_fetch_and(&val, 0x011); */
+		BPF_MOV64_IMM(BPF_REG_0, 0x011),
+		BPF_ATOMIC_OP(BPF_DW, BPF_AND | BPF_FETCH, BPF_REG_10, BPF_REG_0, -8),
+		/* if (old != 0x110) exit(3); */
+		BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0x110, 2),
+		BPF_MOV64_IMM(BPF_REG_0, 3),
+		BPF_EXIT_INSN(),
+		/* if (val != 0x010) exit(2); */
+		BPF_LDX_MEM(BPF_DW, BPF_REG_1, BPF_REG_10, -8),
+		BPF_JMP_IMM(BPF_JEQ, BPF_REG_1, 0x010, 2),
+		BPF_MOV64_IMM(BPF_REG_1, 2),
+		BPF_EXIT_INSN(),
+		/* exit(0); */
+		BPF_MOV64_IMM(BPF_REG_0, 0),
+		BPF_EXIT_INSN(),
+	},
+	.result = ACCEPT,
+},