[RFC PATCH 11/14] khwasan: add brk handler for inline instrumentation

[Andrey Konovalov <andreyknvl@google.com>]

KHWASAN inline instrumentation mode (which embeds checks of shadow memory
into the generated code, instead of inserting a callback) generates a brk
instruction when a tag mismatch is detected.

This commit add a KHWASAN brk handler, that decodes the immediate value
passed to the brk instructions (to extract information about the memory
access that triggered the mismatch), reads the register values (x0 contains
the guilty address) and reports the bug.
---
 arch/arm64/include/asm/brk-imm.h |  2 ++
 arch/arm64/kernel/traps.c        | 40 ++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/arch/arm64/include/asm/brk-imm.h b/arch/arm64/include/asm/brk-imm.h
index ed693c5bcec0..e4a7013321dc 100644
--- a/arch/arm64/include/asm/brk-imm.h
+++ b/arch/arm64/include/asm/brk-imm.h
@@ -16,10 +16,12 @@
  * 0x400: for dynamic BRK instruction
  * 0x401: for compile time BRK instruction
  * 0x800: kernel-mode BUG() and WARN() traps
+ * 0x9xx: KHWASAN trap (allowed values 0x900 - 0x9ff)
  */
 #define FAULT_BRK_IMM			0x100
 #define KGDB_DYN_DBG_BRK_IMM		0x400
 #define KGDB_COMPILED_DBG_BRK_IMM	0x401
 #define BUG_BRK_IMM			0x800
+#define KHWASAN_BRK_IMM			0x900
 
 #endif
diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index eb2d15147e8d..5df8cdf5af13 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -35,6 +35,7 @@
 #include <linux/sizes.h>
 #include <linux/syscalls.h>
 #include <linux/mm_types.h>
+#include <linux/kasan.h>
 
 #include <asm/atomic.h>
 #include <asm/bug.h>
@@ -771,6 +772,38 @@ static struct break_hook bug_break_hook = {
 	.fn = bug_handler,
 };
 
+#ifdef CONFIG_KASAN_TAGS
+static int khwasan_handler(struct pt_regs *regs, unsigned int esr)
+{
+	bool recover = esr & 0x20;
+	bool write = esr & 0x10;
+	size_t size = 1 << (esr & 0xf);
+	u64 addr = regs->regs[0];
+	u64 pc = regs->pc;
+
+	if (user_mode(regs))
+		return DBG_HOOK_ERROR;
+
+	khwasan_report(addr, size, write, pc);
+
+	if (!recover)
+		die("Oops - KHWASAN", regs, 0);
+
+	/* If thread survives, skip over the BUG instruction and continue: */
+	arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
+	return DBG_HOOK_HANDLED;
+}
+
+#define KHWASAN_ESR_VAL (0xf2000000 | KHWASAN_BRK_IMM)
+#define KHWASAN_ESR_MASK 0xffffff00
+
+static struct break_hook khwasan_break_hook = {
+	.esr_val = KHWASAN_ESR_VAL,
+	.esr_mask = KHWASAN_ESR_MASK,
+	.fn = khwasan_handler,
+};
+#endif
+
 /*
  * Initial handler for AArch64 BRK exceptions
  * This handler only used until debug_traps_init().
@@ -778,6 +811,10 @@ static struct break_hook bug_break_hook = {
 int __init early_brk64(unsigned long addr, unsigned int esr,
 		struct pt_regs *regs)
 {
+#ifdef CONFIG_KASAN_TAGS
+	if ((esr & KHWASAN_ESR_MASK) == KHWASAN_ESR_VAL)
+		return khwasan_handler(regs, esr) != DBG_HOOK_HANDLED;
+#endif
 	return bug_handler(regs, esr) != DBG_HOOK_HANDLED;
 }
 
@@ -785,4 +822,7 @@ int __init early_brk64(unsigned long addr, unsigned int esr,
 void __init trap_init(void)
 {
 	register_break_hook(&bug_break_hook);
+#ifdef CONFIG_KASAN_TAGS
+	register_break_hook(&khwasan_break_hook);
+#endif
 }
-- 
2.16.2.395.g2e18187dfd-goog

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