[PATCH v15 5/6] arm64: Create a list of SYM_CODE functions, check return PC against list

madvenka
2022-06-17T21:19:50Z

From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>

SYM_CODE functions don't follow the usual calling conventions. Check if the
return PC in a stack frame falls in any of these. If it does, consider the
stack trace unreliable.

Define a special section for unreliable functions
=================================================

Define a SYM_CODE_END() macro for arm64 that adds the function address
range to a new section called "sym_code_functions".

Linker file
===========

Include the "sym_code_functions" section under read-only data in
vmlinux.lds.S.

Initialization
==============

Define an early_initcall() to create a sym_code_functions[] array from
the linker data.

Unwinder check
==============

Add a reliability check in unwind_check_reliability() that compares a
return PC with sym_code_functions[]. If there is a match, then return
failure.

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
---
arch/arm64/include/asm/linkage.h | 11 +++++++
arch/arm64/include/asm/sections.h | 1 +
arch/arm64/kernel/stacktrace.c | 55 +++++++++++++++++++++++++++++++
arch/arm64/kernel/vmlinux.lds.S | 10 ++++++
4 files changed, 77 insertions(+)

diff --git a/arch/arm64/include/asm/linkage.h b/arch/arm64/include/asm/linkage.h
index 43f8c25b3fda..d4058de4af78 100644
--- a/arch/arm64/include/asm/linkage.h
+++ b/arch/arm64/include/asm/linkage.h
@@ -39,4 +39,15 @@
SYM_START(name, SYM_L_WEAK, SYM_A_NONE) \
bti c ;

+/*
+ * Record the address range of each SYM_CODE function in a struct code_range
+ * in a special section.
+ */
+#define SYM_CODE_END(name) \
+ SYM_END(name, SYM_T_NONE) ;\
+99: .pushsection "sym_code_functions", "aw" ;\
+ .quad name ;\
+ .quad 99b ;\
+ .popsection
+
#endif
diff --git a/arch/arm64/include/asm/sections.h b/arch/arm64/include/asm/sections.h
index 40971ac1303f..50cfd1083563 100644
--- a/arch/arm64/include/asm/sections.h
+++ b/arch/arm64/include/asm/sections.h
@@ -22,6 +22,7 @@ extern char __irqentry_text_start[], __irqentry_text_end[];
extern char __mmuoff_data_start[], __mmuoff_data_end[];
extern char __entry_tramp_text_start[], __entry_tramp_text_end[];
extern char __relocate_new_kernel_start[], __relocate_new_kernel_end[];
+extern char __sym_code_functions_start[], __sym_code_functions_end[];

static inline size_t entry_tramp_text_size(void)
{
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index 5ef2ce217324..eda8581f7dbe 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -62,6 +62,31 @@ struct unwind_state {
bool reliable;
};

+struct code_range {
+ unsigned long start;
+ unsigned long end;
+};
+
+static struct code_range *sym_code_functions;
+static int num_sym_code_functions;
+
+int __init init_sym_code_functions(void)
+{
+ size_t size = (unsigned long)__sym_code_functions_end -
+ (unsigned long)__sym_code_functions_start;
+
+ sym_code_functions = (struct code_range *)__sym_code_functions_start;
+ /*
+ * Order it so that sym_code_functions is not visible before
+ * num_sym_code_functions.
+ */
+ smp_mb();
+ num_sym_code_functions = size / sizeof(struct code_range);
+
+ return 0;
+}
+early_initcall(init_sym_code_functions);
+
static void unwind_init_common(struct unwind_state *state,
struct task_struct *task)
{
@@ -251,6 +276,10 @@ NOKPROBE_SYMBOL(unwind_next);
*/
static void unwind_check_reliability(struct unwind_state *state)
{
+ const struct code_range *range;
+ unsigned long pc;
+ int i;
+
if (state->fp == state->final_fp) {
/* Final frame; no more unwind, no need to check reliability */
return;
@@ -263,6 +292,32 @@ static void unwind_check_reliability(struct unwind_state *state)
*/
if (!__kernel_text_address(state->pc))
state->reliable = false;
+
+ /*
+ * Check the return PC against sym_code_functions[]. If there is a
+ * match, then the consider the stack frame unreliable.
+ *
+ * As SYM_CODE functions don't follow the usual calling conventions,
+ * we assume by default that any SYM_CODE function cannot be unwound
+ * reliably.
+ *
+ * Note that this includes:
+ *
+ * - Exception handlers and entry assembly
+ * - Trampoline assembly (e.g., ftrace, kprobes)
+ * - Hypervisor-related assembly
+ * - Hibernation-related assembly
+ * - CPU start-stop, suspend-resume assembly
+ * - Kernel relocation assembly
+ */
+ pc = state->pc;
+ for (i = 0; i < num_sym_code_functions; i++) {
+ range = &sym_code_functions[i];
+ if (pc >= range->start && pc < range->end) {
+ state->reliable = false;
+ return;
+ }
+ }
}

static bool notrace unwind(struct unwind_state *state,
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 2d4a8f995175..414dbc82d0a6 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -120,6 +120,14 @@ jiffies = jiffies_64;
#define TRAMP_TEXT
#endif

+#define SYM_CODE_FUNCTIONS \
+ . = ALIGN(16); \
+ .symcode : AT(ADDR(.symcode) - LOAD_OFFSET) { \
+ __sym_code_functions_start = .; \
+ KEEP(*(sym_code_functions)) \
+ __sym_code_functions_end = .; \
+ }
+
/*
* The size of the PE/COFF section that covers the kernel image, which
* runs from _stext to _edata, must be a round multiple of the PE/COFF
@@ -212,6 +220,8 @@ SECTIONS
swapper_pg_dir = .;
. += PAGE_SIZE;

+ SYM_CODE_FUNCTIONS
+
. = ALIGN(SEGMENT_ALIGN);
__init_begin = .;
__inittext_begin = .;
--
2.25.1