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Subject[RFC PATCH v6 2/3] arm64: Introduce stack trace reliability checks in the unwinder
Date
From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>

The unwinder should check for the presence of various features and
conditions that can render the stack trace unreliable. Introduce a
function unwind_check_frame() for this purpose.

Introduce the first reliability check in unwind_check_frame() - If
a return PC is not a valid kernel text address, consider the stack
trace unreliable. It could be some generated code.

Other reliability checks will be added in the future.

If a reliability check fails, it is a non-fatal error. Introduce a new
return code, UNWIND_CONTINUE_WITH_RISK, for non-fatal errors.

Call unwind_check_frame() from unwind_frame(). Also, call it from
start_backtrace() to remove the current assumption that the starting
frame is reliable.

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
---
arch/arm64/include/asm/stacktrace.h | 4 +++-
arch/arm64/kernel/stacktrace.c | 17 ++++++++++++++++-
2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
index 6fcd58553fb1..d1625d55b980 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h
@@ -32,6 +32,7 @@ struct stack_info {

enum unwind_rc {
UNWIND_CONTINUE, /* No errors encountered */
+ UNWIND_CONTINUE_WITH_RISK, /* Non-fatal errors encountered */
UNWIND_ABORT, /* Fatal errors encountered */
UNWIND_FINISH, /* End of stack reached successfully */
};
@@ -73,6 +74,7 @@ extern void walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
bool (*fn)(void *, unsigned long), void *data);
extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
const char *loglvl);
+extern enum unwind_rc unwind_check_frame(struct stackframe *frame);

DECLARE_PER_CPU(unsigned long *, irq_stack_ptr);

@@ -176,7 +178,7 @@ static inline enum unwind_rc start_backtrace(struct stackframe *frame,
bitmap_zero(frame->stacks_done, __NR_STACK_TYPES);
frame->prev_fp = 0;
frame->prev_type = STACK_TYPE_UNKNOWN;
- return UNWIND_CONTINUE;
+ return unwind_check_frame(frame);
}

#endif /* __ASM_STACKTRACE_H */
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index e9c2c1fa9dde..ba7b97b119e4 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -18,6 +18,21 @@
#include <asm/stack_pointer.h>
#include <asm/stacktrace.h>

+/*
+ * Check the stack frame for conditions that make unwinding unreliable.
+ */
+enum unwind_rc unwind_check_frame(struct stackframe *frame)
+{
+ /*
+ * If the PC is not a known kernel text address, then we cannot
+ * be sure that a subsequent unwind will be reliable, as we
+ * don't know that the code follows our unwind requirements.
+ */
+ if (!__kernel_text_address(frame->pc))
+ return UNWIND_CONTINUE_WITH_RISK;
+ return UNWIND_CONTINUE;
+}
+
/*
* AArch64 PCS assigns the frame pointer to x29.
*
@@ -109,7 +124,7 @@ enum unwind_rc notrace unwind_frame(struct task_struct *tsk,

frame->pc = ptrauth_strip_insn_pac(frame->pc);

- return UNWIND_CONTINUE;
+ return unwind_check_frame(frame);
}
NOKPROBE_SYMBOL(unwind_frame);

--
2.25.1
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