[PATCH v2 1/3] powerpc: sstep: Fix load and update emulation

[Sandipan Das]

The Power ISA says that the fixed-point load and update
instructions must neither use R0 for the base address (RA)
nor have the destination (RT) and the base address (RA) as
the same register. In these cases, the instruction is
invalid. This applies to the following instructions.
  * Load Byte and Zero with Update (lbzu)
  * Load Byte and Zero with Update Indexed (lbzux)
  * Load Halfword and Zero with Update (lhzu)
  * Load Halfword and Zero with Update Indexed (lhzux)
  * Load Halfword Algebraic with Update (lhau)
  * Load Halfword Algebraic with Update Indexed (lhaux)
  * Load Word and Zero with Update (lwzu)
  * Load Word and Zero with Update Indexed (lwzux)
  * Load Word Algebraic with Update Indexed (lwaux)
  * Load Doubleword with Update (ldu)
  * Load Doubleword with Update Indexed (ldux)

However, the following behaviour is observed using some
invalid opcodes where RA = RT.

An userspace program using an invalid instruction word like
0xe9ce0001, i.e. "ldu r14, 0(r14)", runs and exits without
getting terminated abruptly. The instruction performs the
load operation but does not write the effective address to
the base address register. Attaching an uprobe at that
instruction's address results in emulation which writes the
effective address to the base register. Thus, the final value
of the base address register is different.

To remove any inconsistencies, this adds an additional check
for the aforementioned instructions to make sure that they
are treated as unknown by the emulation infrastructure when
RA = 0 or RA = RT. The kernel will then fallback to executing
the instruction on hardware.

Fixes: 0016a4cf5582 ("powerpc: Emulate most Book I instructions in emulate_step()")
Reviewed-by: Ravi Bangoria <ravi.bangoria at linux.ibm.com>
Signed-off-by: Sandipan Das <sandipan at linux.ibm.com>
---
Previous versions can be found at:
v1: https://lore.kernel.org/linuxppc-dev/20201119054139.244083-1-sandipan@linux.ibm.com/

Changes in v2:
- Jump to unknown_opcode instead of returning -1 for invalid
  instruction forms.

---
 arch/powerpc/lib/sstep.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
index e96cff845ef7..db824fec6165 100644
--- a/arch/powerpc/lib/sstep.c
+++ b/arch/powerpc/lib/sstep.c
@@ -2232,11 +2232,15 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 
 		case 23:	/* lwzx */
 		case 55:	/* lwzux */
+			if (u && (ra == 0 || ra == rd))
+				goto unknown_opcode;
 			op->type = MKOP(LOAD, u, 4);
 			break;
 
 		case 87:	/* lbzx */
 		case 119:	/* lbzux */
+			if (u && (ra == 0 || ra == rd))
+				goto unknown_opcode;
 			op->type = MKOP(LOAD, u, 1);
 			break;
 
@@ -2290,6 +2294,8 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 #ifdef __powerpc64__
 		case 21:	/* ldx */
 		case 53:	/* ldux */
+			if (u && (ra == 0 || ra == rd))
+				goto unknown_opcode;
 			op->type = MKOP(LOAD, u, 8);
 			break;
 
@@ -2311,18 +2317,24 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 
 		case 279:	/* lhzx */
 		case 311:	/* lhzux */
+			if (u && (ra == 0 || ra == rd))
+				goto unknown_opcode;
 			op->type = MKOP(LOAD, u, 2);
 			break;
 
 #ifdef __powerpc64__
 		case 341:	/* lwax */
 		case 373:	/* lwaux */
+			if (u && (ra == 0 || ra == rd))
+				goto unknown_opcode;
 			op->type = MKOP(LOAD, SIGNEXT | u, 4);
 			break;
 #endif
 
 		case 343:	/* lhax */
 		case 375:	/* lhaux */
+			if (u && (ra == 0 || ra == rd))
+				goto unknown_opcode;
 			op->type = MKOP(LOAD, SIGNEXT | u, 2);
 			break;
 
@@ -2656,12 +2668,16 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 
 	case 32:	/* lwz */
 	case 33:	/* lwzu */
+		if (u && (ra == 0 || ra == rd))
+			goto unknown_opcode;
 		op->type = MKOP(LOAD, u, 4);
 		op->ea = dform_ea(word, regs);
 		break;
 
 	case 34:	/* lbz */
 	case 35:	/* lbzu */
+		if (u && (ra == 0 || ra == rd))
+			goto unknown_opcode;
 		op->type = MKOP(LOAD, u, 1);
 		op->ea = dform_ea(word, regs);
 		break;
@@ -2680,12 +2696,16 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 
 	case 40:	/* lhz */
 	case 41:	/* lhzu */
+		if (u && (ra == 0 || ra == rd))
+			goto unknown_opcode;
 		op->type = MKOP(LOAD, u, 2);
 		op->ea = dform_ea(word, regs);
 		break;
 
 	case 42:	/* lha */
 	case 43:	/* lhau */
+		if (u && (ra == 0 || ra == rd))
+			goto unknown_opcode;
 		op->type = MKOP(LOAD, SIGNEXT | u, 2);
 		op->ea = dform_ea(word, regs);
 		break;
@@ -2779,6 +2799,8 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 			op->type = MKOP(LOAD, 0, 8);
 			break;
 		case 1:		/* ldu */
+			if (ra == 0 || ra == rd)
+				goto unknown_opcode;
 			op->type = MKOP(LOAD, UPDATE, 8);
 			break;
 		case 2:		/* lwa */
-- 
2.25.1