[tip: x86/entry] x86/entry: Fix #UD vs WARN more

tip-bot2 for Peter Zijlstra
2020-06-25T11:54:39Z

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

Commit-ID: 145a773aef83181d47ebab21bb33c89233aadb1e
Gitweb: https://git.kernel.org/tip/145a773aef83181d47ebab21bb33c89233aadb1e
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Tue, 16 Jun 2020 13:28:36 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 25 Jun 2020 13:45:40 +02:00

x86/entry: Fix #UD vs WARN more

vmlinux.o: warning: objtool: exc_invalid_op()+0x47: call to probe_kernel_read() leaves .noinstr.text section

Since we use UD2 as a short-cut for 'CALL __WARN', treat it as such.
Have the bare exception handler do the report_bug() thing.

Fixes: 15a416e8aaa7 ("x86/entry: Treat BUG/WARN as NMI-like entries")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200622114713.GE577403@hirez.programming.kicks-ass.net
---
arch/x86/kernel/traps.c | 72 +++++++++++++++++++++-------------------
1 file changed, 38 insertions(+), 34 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index a7d1570..1d9ea21 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -84,17 +84,16 @@ static inline void cond_local_irq_disable(struct pt_regs *regs)
local_irq_disable();
}

-int is_valid_bugaddr(unsigned long addr)
+__always_inline int is_valid_bugaddr(unsigned long addr)
{
- unsigned short ud;
-
if (addr < TASK_SIZE_MAX)
return 0;

- if (probe_kernel_address((unsigned short *)addr, ud))
- return 0;
-
- return ud == INSN_UD0 || ud == INSN_UD2;
+ /*
+ * We got #UD, if the text isn't readable we'd have gotten
+ * a different exception.
+ */
+ return *(unsigned short *)addr == INSN_UD2;
}

static nokprobe_inline int
@@ -216,40 +215,45 @@ static inline void handle_invalid_op(struct pt_regs *regs)
ILL_ILLOPN, error_get_trap_addr(regs));
}

-DEFINE_IDTENTRY_RAW(exc_invalid_op)
+static noinstr bool handle_bug(struct pt_regs *regs)
{
- bool rcu_exit;
+ bool handled = false;
+
+ if (!is_valid_bugaddr(regs->ip))
+ return handled;

/*
- * Handle BUG/WARN like NMIs instead of like normal idtentries:
- * if we bugged/warned in a bad RCU context, for example, the last
- * thing we want is to BUG/WARN again in the idtentry code, ad
- * infinitum.
+ * All lies, just get the WARN/BUG out.
*/
- if (!user_mode(regs) && is_valid_bugaddr(regs->ip)) {
- enum bug_trap_type type;
+ instrumentation_begin();
+ /*
+ * Since we're emulating a CALL with exceptions, restore the interrupt
+ * state to what it was at the exception site.
+ */
+ if (regs->flags & X86_EFLAGS_IF)
+ raw_local_irq_enable();
+ if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
+ regs->ip += LEN_UD2;
+ handled = true;
+ }
+ if (regs->flags & X86_EFLAGS_IF)
+ raw_local_irq_disable();
+ instrumentation_end();

- nmi_enter();
- instrumentation_begin();
- trace_hardirqs_off_finish();
- type = report_bug(regs->ip, regs);
- if (regs->flags & X86_EFLAGS_IF)
- trace_hardirqs_on_prepare();
- instrumentation_end();
- nmi_exit();
+ return handled;
+}

- if (type == BUG_TRAP_TYPE_WARN) {
- /* Skip the ud2. */
- regs->ip += LEN_UD2;
- return;
- }
+DEFINE_IDTENTRY_RAW(exc_invalid_op)
+{
+ bool rcu_exit;

- /*
- * Else, if this was a BUG and report_bug returns or if this
- * was just a normal #UD, we want to continue onward and
- * crash.
- */
- }
+ /*
+ * We use UD2 as a short encoding for 'CALL __WARN', as such
+ * handle it before exception entry to avoid recursive WARN
+ * in case exception entry is the one triggering WARNs.
+ */
+ if (!user_mode(regs) && handle_bug(regs))
+ return;

rcu_exit = idtentry_enter_cond_rcu(regs);
instrumentation_begin();