[PATCH] riscv: Only consider swbp/ss handlers for correct privileged mode

[Björn Töpel]

From: Björn Töpel <bjorn at rivosinc.com>

RISC-V software breakpoint trap handlers are used for {k,u}probes.

When trapping from kernelmode, only the kernelmode handlers should be
considered. Vice versa, only usermode handlers for usermode
traps. This is not the case on RISC-V, which can trigger a bug if a
userspace process uses uprobes, and a WARN() is triggered from
kernelmode (which is implemented via {c.,}ebreak).

The kernel will trap on the kernelmode {c.,}ebreak, look for uprobes
handlers, realize incorrectly that uprobes need to be handled, and
exit the trap handler early. The trap returns to re-executing the
{c.,}ebreak, and enter an infinite trap-loop.

The issue was found running the BPF selftest [1].

Fix this issue by only considering the swbp/ss handlers for
kernel/usermode respectively. Also, move CONFIG ifdeffery from traps.c
to the asm/{k,u}probes.h headers.

Note that linux/uprobes.h only include asm/uprobes.h if CONFIG_UPROBES
is defined, which is why asm/uprobes.h needs to be unconditionally
included in traps.c

Link: https://lore.kernel.org/linux-riscv/87v8d19aun.fsf@all.your.base.are.belong.to.us/ # [1]
Fixes: 74784081aac8 ("riscv: Add uprobes supported")
Signed-off-by: Björn Töpel <bjorn at rivosinc.com>
---
 arch/riscv/include/asm/kprobes.h | 11 ++++++++++-
 arch/riscv/include/asm/uprobes.h | 13 ++++++++++++-
 arch/riscv/kernel/traps.c        | 28 ++++++++++++++++++----------
 3 files changed, 40 insertions(+), 12 deletions(-)

diff --git a/arch/riscv/include/asm/kprobes.h b/arch/riscv/include/asm/kprobes.h
index e7882ccb0fd4..89fbd90f16a2 100644
--- a/arch/riscv/include/asm/kprobes.h
+++ b/arch/riscv/include/asm/kprobes.h
@@ -40,6 +40,15 @@ void arch_remove_kprobe(struct kprobe *p);
 int kprobe_fault_handler(struct pt_regs *regs, unsigned int trapnr);
 bool kprobe_breakpoint_handler(struct pt_regs *regs);
 bool kprobe_single_step_handler(struct pt_regs *regs);
-
+#else
+static inline bool kprobe_breakpoint_handler(struct pt_regs *)
+{
+	return false;
+}
+
+static inline bool kprobe_single_step_handler(struct pt_regs *)
+{
+	return false;
+}
 #endif /* CONFIG_KPROBES */
 #endif /* _ASM_RISCV_KPROBES_H */
diff --git a/arch/riscv/include/asm/uprobes.h b/arch/riscv/include/asm/uprobes.h
index f2183e00fdd2..3fc7deda9190 100644
--- a/arch/riscv/include/asm/uprobes.h
+++ b/arch/riscv/include/asm/uprobes.h
@@ -34,7 +34,18 @@ struct arch_uprobe {
 	bool simulate;
 };
 
+#ifdef CONFIG_UPROBES
 bool uprobe_breakpoint_handler(struct pt_regs *regs);
 bool uprobe_single_step_handler(struct pt_regs *regs);
-
+#else
+static inline bool uprobe_breakpoint_handler(struct pt_regs *regs)
+{
+	return false;
+}
+
+static inline bool uprobe_single_step_handler(struct pt_regs *regs)
+{
+	return false;
+}
+#endif /* CONFIG_UPROBES */
 #endif /* _ASM_RISCV_UPROBES_H */
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f798c853bede..cd6f10c73a16 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -13,6 +13,8 @@
 #include <linux/kdebug.h>
 #include <linux/uaccess.h>
 #include <linux/kprobes.h>
+#include <linux/uprobes.h>
+#include <asm/uprobes.h>
 #include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/irq.h>
@@ -246,22 +248,28 @@ static inline unsigned long get_break_insn_length(unsigned long pc)
 	return GET_INSN_LENGTH(insn);
 }
 
+static bool probe_single_step_handler(struct pt_regs *regs)
+{
+	bool user = user_mode(regs);
+
+	return user ? uprobe_single_step_handler(regs) : kprobe_single_step_handler(regs);
+}
+
+static bool probe_breakpoint_handler(struct pt_regs *regs)
+{
+	bool user = user_mode(regs);
+
+	return user ? uprobe_breakpoint_handler(regs) : kprobe_breakpoint_handler(regs);
+}
+
 void handle_break(struct pt_regs *regs)
 {
-#ifdef CONFIG_KPROBES
-	if (kprobe_single_step_handler(regs))
+	if (probe_single_step_handler(regs))
 		return;
 
-	if (kprobe_breakpoint_handler(regs))
-		return;
-#endif
-#ifdef CONFIG_UPROBES
-	if (uprobe_single_step_handler(regs))
+	if (probe_breakpoint_handler(regs))
 		return;
 
-	if (uprobe_breakpoint_handler(regs))
-		return;
-#endif
 	current->thread.bad_cause = regs->cause;
 
 	if (user_mode(regs))

base-commit: 7d2f353b2682dcfe5f9bc71e5b61d5b61770d98e
-- 
2.39.2