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Message-Id: <20241105114522.285032152@infradead.org>
Date: Tue, 05 Nov 2024 12:39:06 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
peterz@...radead.org,
alyssa.milburn@...el.com,
scott.d.constable@...el.com,
joao@...rdrivepizza.com,
andrew.cooper3@...rix.com,
jpoimboe@...nel.org,
alexei.starovoitov@...il.com,
ebiggers@...nel.org,
samitolvanen@...gle.com,
kees@...nel.org
Subject: [PATCH 5/8] x86/traps: Cleanup and robustify decode_bug()
Notably, don't attempt to decode an immediate when MOD == 3.
Additionally have it return the instruction length, such that WARN
like bugs can more reliably skip to the correct instruction.
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
arch/x86/include/asm/bug.h | 5 +-
arch/x86/include/asm/ibt.h | 4 +-
arch/x86/kernel/cet.c | 18 +++++++---
arch/x86/kernel/traps.c | 77 ++++++++++++++++++++++++++++++++-------------
4 files changed, 73 insertions(+), 31 deletions(-)
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -22,8 +22,9 @@
#define SECOND_BYTE_OPCODE_UD2 0x0b
#define BUG_NONE 0xffff
-#define BUG_UD1 0xfffe
-#define BUG_UD2 0xfffd
+#define BUG_UD2 0xfffe
+#define BUG_UD1 0xfffd
+#define BUG_UD1_UBSAN 0xfffc
#ifdef CONFIG_GENERIC_BUG
--- a/arch/x86/include/asm/ibt.h
+++ b/arch/x86/include/asm/ibt.h
@@ -41,7 +41,7 @@
_ASM_PTR fname "\n\t" \
".popsection\n\t"
-static inline __attribute_const__ u32 gen_endbr(void)
+static __always_inline __attribute_const__ u32 gen_endbr(void)
{
u32 endbr;
@@ -56,7 +56,7 @@ static inline __attribute_const__ u32 ge
return endbr;
}
-static inline __attribute_const__ u32 gen_endbr_poison(void)
+static __always_inline __attribute_const__ u32 gen_endbr_poison(void)
{
/*
* 4 byte NOP that isn't NOP4 (in fact it is OSP NOP3), such that it
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -94,10 +94,17 @@ __always_inline int is_valid_bugaddr(uns
/*
* Check for UD1 or UD2, accounting for Address Size Override Prefixes.
- * If it's a UD1, get the ModRM byte to pass along to UBSan.
+ * If it's a UD1, further decode to determine its use:
+ *
+ * UBSan{0}: 67 0f b9 00 ud1 (%eax),%eax
+ * UBSan{10}: 67 0f b9 40 10 ud1 0x10(%eax),%eax
+ * static_call: 0f b9 cc ud1 %esp,%ecx
+ *
+ * Notably UBSAN uses EAX, static_call uses ECX.
*/
-__always_inline int decode_bug(unsigned long addr, u32 *imm)
+__always_inline int decode_bug(unsigned long addr, s32 *imm, int *len)
{
+ unsigned long start = addr;
u8 v;
if (addr < TASK_SIZE_MAX)
@@ -110,24 +117,37 @@ __always_inline int decode_bug(unsigned
return BUG_NONE;
v = *(u8 *)(addr++);
- if (v == SECOND_BYTE_OPCODE_UD2)
+ if (v == SECOND_BYTE_OPCODE_UD2) {
+ *len = addr - start;
return BUG_UD2;
+ }
- if (!IS_ENABLED(CONFIG_UBSAN_TRAP) || v != SECOND_BYTE_OPCODE_UD1)
+ if (v != SECOND_BYTE_OPCODE_UD1)
return BUG_NONE;
- /* Retrieve the immediate (type value) for the UBSAN UD1 */
- v = *(u8 *)(addr++);
- if (X86_MODRM_RM(v) == 4)
- addr++;
-
*imm = 0;
- if (X86_MODRM_MOD(v) == 1)
- *imm = *(u8 *)addr;
- else if (X86_MODRM_MOD(v) == 2)
- *imm = *(u32 *)addr;
- else
- WARN_ONCE(1, "Unexpected MODRM_MOD: %u\n", X86_MODRM_MOD(v));
+ v = *(u8 *)(addr++); /* ModRM */
+
+ /* Decode immediate, if present */
+ if (X86_MODRM_MOD(v) != 3) {
+ if (X86_MODRM_RM(v) == 4)
+ addr++; /* Skip SIB byte */
+
+ if (X86_MODRM_MOD(v) == 1) {
+ *imm = *(s8 *)addr;
+ addr += 1;
+
+ } else if (X86_MODRM_MOD(v) == 2) {
+ *imm = *(s32 *)addr;
+ addr += 4;
+ }
+ }
+
+ /* record instruction length */
+ *len = addr - start;
+
+ if (X86_MODRM_REG(v) == 0) /* EAX */
+ return BUG_UD1_UBSAN;
return BUG_UD1;
}
@@ -258,10 +278,10 @@ static inline void handle_invalid_op(str
static noinstr bool handle_bug(struct pt_regs *regs)
{
bool handled = false;
- int ud_type;
- u32 imm;
+ int ud_type, ud_len;
+ s32 ud_imm;
- ud_type = decode_bug(regs->ip, &imm);
+ ud_type = decode_bug(regs->ip, &ud_imm, &ud_len);
if (ud_type == BUG_NONE)
return handled;
@@ -281,15 +301,28 @@ static noinstr bool handle_bug(struct pt
*/
if (regs->flags & X86_EFLAGS_IF)
raw_local_irq_enable();
- if (ud_type == BUG_UD2) {
+
+ switch (ud_type) {
+ case BUG_UD2:
if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN ||
handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN) {
- regs->ip += LEN_UD2;
+ regs->ip += ud_len;
handled = true;
}
- } else if (IS_ENABLED(CONFIG_UBSAN_TRAP)) {
- pr_crit("%s at %pS\n", report_ubsan_failure(regs, imm), (void *)regs->ip);
+ break;
+
+ case BUG_UD1_UBSAN:
+ if (IS_ENABLED(CONFIG_UBSAN_TRAP)) {
+ pr_crit("%s at %pS\n",
+ report_ubsan_failure(regs, ud_imm),
+ (void *)regs->ip);
+ }
+ break;
+
+ default:
+ break;
}
+
if (regs->flags & X86_EFLAGS_IF)
raw_local_irq_disable();
instrumentation_end();
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