[tip: x86/urgent] x86/bug: Handle __WARN_printf() trap in early_fixup_exception()

[tip-bot2 for Hou Wenlong]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     a0cb371b521dde44f32cfe954b6ef6f82b407393
Gitweb:        https://git.kernel.org/tip/a0cb371b521dde44f32cfe954b6ef6f82b407393
Author:        Hou Wenlong <houwenlong.hwl@xxxxxxxxxxxx>
AuthorDate:    Sat, 10 Jan 2026 11:47:37 +08:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 23 Feb 2026 11:19:11 +01:00

x86/bug: Handle __WARN_printf() trap in early_fixup_exception()

The commit 5b472b6e5bd9 ("x86_64/bug: Implement __WARN_printf()")
implemented __WARN_printf(), which changed the mechanism to use UD1
instead of UD2. However, it only handles the trap in the runtime IDT
handler, while the early booting IDT handler lacks this handling. As a
result, the usage of WARN() before the runtime IDT setup can lead to
kernel crashes. Since KMSAN is enabled after the runtime IDT setup, it
is safe to use handle_bug() directly in early_fixup_exception() to
address this issue.

Fixes: 5b472b6e5bd9 ("x86_64/bug: Implement __WARN_printf()")
Signed-off-by: Hou Wenlong <houwenlong.hwl@xxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://patch.msgid.link/c4fb3645f60d3a78629d9870e8fcc8535281c24f.1768016713.git.houwenlong.hwl@xxxxxxxxxxxx
---
 arch/x86/include/asm/traps.h | 2 ++
 arch/x86/kernel/traps.c      | 2 +-
 arch/x86/mm/extable.c        | 7 ++-----
 3 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index 869b880..3f24cc4 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -25,6 +25,8 @@ extern int ibt_selftest_noendbr(void);
 void handle_invalid_op(struct pt_regs *regs);
 #endif
 
+noinstr bool handle_bug(struct pt_regs *regs);
+
 static inline int get_si_code(unsigned long condition)
 {
 	if (condition & DR_STEP)
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 5a6a772..4dbff8e 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -397,7 +397,7 @@ static inline void handle_invalid_op(struct pt_regs *regs)
 		      ILL_ILLOPN, error_get_trap_addr(regs));
 }
 
-static noinstr bool handle_bug(struct pt_regs *regs)
+noinstr bool handle_bug(struct pt_regs *regs)
 {
 	unsigned long addr = regs->ip;
 	bool handled = false;
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 2fdc1f1..6b9ff1c 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -411,14 +411,11 @@ void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
 		return;
 
 	if (trapnr == X86_TRAP_UD) {
-		if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
-			/* Skip the ud2. */
-			regs->ip += LEN_UD2;
+		if (handle_bug(regs))
 			return;
-		}
 
 		/*
-		 * If this was a BUG and report_bug returns or if this
+		 * If this was a BUG and handle_bug returns or if this
 		 * was just a normal #UD, we want to continue onward and
 		 * crash.
 		 */