[PATCH] ARM: alignment: advance IT state after emulating Thumb instruction

[PATCH] ARM: alignment: advance IT state after emulating Thumb instruction

[Ard Biesheuvel]

After emulating a misaligned load or store issued in Thumb mode, we have
to advance the IT state by hand, or it will get out of sync with the
actual instruction stream, which means we'll end up applying the wrong
condition code to subsequent instructions. This might corrupt the
program state rather catastrophically.

So borrow the it_advance() helper from the probing code, and use it on
CPSR if the emulated instruction is Thumb.

Cc: <stable@vger.kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm/include/asm/ptrace.h | 26 ++++++++++++++++++++++++++
 arch/arm/mm/alignment.c       |  3 +++
 arch/arm/probes/decode.h      | 26 +-------------------------
 3 files changed, 30 insertions(+), 25 deletions(-)

diff --git a/arch/arm/include/asm/ptrace.h b/arch/arm/include/asm/ptrace.h
index 93051e2f402c..1408a6a15d0e 100644
--- a/arch/arm/include/asm/ptrace.h
+++ b/arch/arm/include/asm/ptrace.h
@@ -163,5 +163,31 @@ static inline unsigned long user_stack_pointer(struct pt_regs *regs)
 		((current_stack_pointer | (THREAD_SIZE - 1)) - 7) - 1;	\
 })
 
+
+/*
+ * Update ITSTATE after normal execution of an IT block instruction.
+ *
+ * The 8 IT state bits are split into two parts in CPSR:
+ *	ITSTATE<1:0> are in CPSR<26:25>
+ *	ITSTATE<7:2> are in CPSR<15:10>
+ */
+static inline unsigned long it_advance(unsigned long cpsr)
+{
+	if ((cpsr & 0x06000400) == 0) {
+		/* ITSTATE<2:0> == 0 means end of IT block, so clear IT state */
+		cpsr &= ~PSR_IT_MASK;
+	} else {
+		/* We need to shift left ITSTATE<4:0> */
+		const unsigned long mask = 0x06001c00;  /* Mask ITSTATE<4:0> */
+		unsigned long it = cpsr & mask;
+		it <<= 1;
+		it |= it >> (27 - 10);  /* Carry ITSTATE<2> to correct place */
+		it &= mask;
+		cpsr &= ~mask;
+		cpsr |= it;
+	}
+	return cpsr;
+}
+
 #endif /* __ASSEMBLY__ */
 #endif
diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
index 6f499559d193..f8dd0b3cc8e0 100644
--- a/arch/arm/mm/alignment.c
+++ b/arch/arm/mm/alignment.c
@@ -935,6 +935,9 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 	if (type == TYPE_LDST)
 		do_alignment_finish_ldst(addr, instr, regs, offset);
 
+	if (thumb_mode(regs))
+		regs->ARM_cpsr = it_advance(regs->ARM_cpsr);
+
 	return 0;
 
  bad_or_fault:
diff --git a/arch/arm/probes/decode.h b/arch/arm/probes/decode.h
index 973173598992..facc889d05ee 100644
--- a/arch/arm/probes/decode.h
+++ b/arch/arm/probes/decode.h
@@ -14,6 +14,7 @@
 #include <linux/types.h>
 #include <linux/stddef.h>
 #include <asm/probes.h>
+#include <asm/ptrace.h>
 #include <asm/kprobes.h>
 
 void __init arm_probes_decode_init(void);
@@ -35,31 +36,6 @@ void __init find_str_pc_offset(void);
 #endif
 
 
-/*
- * Update ITSTATE after normal execution of an IT block instruction.
- *
- * The 8 IT state bits are split into two parts in CPSR:
- *	ITSTATE<1:0> are in CPSR<26:25>
- *	ITSTATE<7:2> are in CPSR<15:10>
- */
-static inline unsigned long it_advance(unsigned long cpsr)
-	{
-	if ((cpsr & 0x06000400) == 0) {
-		/* ITSTATE<2:0> == 0 means end of IT block, so clear IT state */
-		cpsr &= ~PSR_IT_MASK;
-	} else {
-		/* We need to shift left ITSTATE<4:0> */
-		const unsigned long mask = 0x06001c00;  /* Mask ITSTATE<4:0> */
-		unsigned long it = cpsr & mask;
-		it <<= 1;
-		it |= it >> (27 - 10);  /* Carry ITSTATE<2> to correct place */
-		it &= mask;
-		cpsr &= ~mask;
-		cpsr |= it;
-	}
-	return cpsr;
-}
-
 static inline void __kprobes bx_write_pc(long pcv, struct pt_regs *regs)
 {
 	long cpsr = regs->ARM_cpsr;
-- 
2.35.1


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Re: [PATCH] ARM: alignment: advance IT state after emulating Thumb instruction

[Linus Walleij]

On Tue, Jun 28, 2022 at 5:13 PM Ard Biesheuvel <ardb@kernel.org> wrote:

> After emulating a misaligned load or store issued in Thumb mode, we have
> to advance the IT state by hand, or it will get out of sync with the
> actual instruction stream, which means we'll end up applying the wrong
> condition code to subsequent instructions. This might corrupt the
> program state rather catastrophically.
>
> So borrow the it_advance() helper from the probing code, and use it on
> CPSR if the emulated instruction is Thumb.
>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

A genuine piece of art to track this down. Thanks!
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

Yours,
Linus Walleij

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