[PATCH 2/6] arm64: probes: Fix simulate_ldr*_literal()

[Mark Rutland]

The simulate_ldr_literal() code always loads a 64-bit quantity, and when
simulating a 32-bit load into a 'W' register, it discards the most
significant 32 bits. For big-endian kernels this means that the relevant
bits are discarded, and the value returned is the the subsequent 32 bits
in memory (i.e. the value at addr + 4).

Additionally, simulate_ldr_literal() and simulate_ldrsw_literal() use a
plain C load, which the compiler may tear or elide (e.g. if the target
is the zero register). Today this doesn't happen to matter, but it may
matter in future if trampoline code uses a LDR (literal) or LDRSW
(literal).

Update simulate_ldr_literal() and simulate_ldrsw_literal() to use an
appropriately-sized READ_ONCE() to perform the access, which avoids
these problems.

Fixes: 39a67d49ba353630 ("arm64: kprobes instruction simulation support")
Cc: stable at vger.kernel.org
Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will at kernel.org>
---
 arch/arm64/kernel/probes/simulate-insn.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/kernel/probes/simulate-insn.c b/arch/arm64/kernel/probes/simulate-insn.c
index 22d0b32524763..b65334ab79d2b 100644
--- a/arch/arm64/kernel/probes/simulate-insn.c
+++ b/arch/arm64/kernel/probes/simulate-insn.c
@@ -171,17 +171,15 @@ simulate_tbz_tbnz(u32 opcode, long addr, struct pt_regs *regs)
 void __kprobes
 simulate_ldr_literal(u32 opcode, long addr, struct pt_regs *regs)
 {
-	u64 *load_addr;
+	unsigned long load_addr;
 	int xn = opcode & 0x1f;
-	int disp;
 
-	disp = ldr_displacement(opcode);
-	load_addr = (u64 *) (addr + disp);
+	load_addr = addr + ldr_displacement(opcode);
 
 	if (opcode & (1 << 30))	/* x0-x30 */
-		set_x_reg(regs, xn, *load_addr);
+		set_x_reg(regs, xn, READ_ONCE(*(u64 *)load_addr));
 	else			/* w0-w30 */
-		set_w_reg(regs, xn, *load_addr);
+		set_w_reg(regs, xn, READ_ONCE(*(u32 *)load_addr));
 
 	instruction_pointer_set(regs, instruction_pointer(regs) + 4);
 }
@@ -189,14 +187,12 @@ simulate_ldr_literal(u32 opcode, long addr, struct pt_regs *regs)
 void __kprobes
 simulate_ldrsw_literal(u32 opcode, long addr, struct pt_regs *regs)
 {
-	s32 *load_addr;
+	unsigned long load_addr;
 	int xn = opcode & 0x1f;
-	int disp;
 
-	disp = ldr_displacement(opcode);
-	load_addr = (s32 *) (addr + disp);
+	load_addr = addr + ldr_displacement(opcode);
 
-	set_x_reg(regs, xn, *load_addr);
+	set_x_reg(regs, xn, READ_ONCE(*(s32 *)load_addr));
 
 	instruction_pointer_set(regs, instruction_pointer(regs) + 4);
 }
-- 
2.30.2